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Research From The Auk and The Condor

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The official blog of ornithology journals The Auk and The Condor
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Endangered Ibises Benefit from Joining Egret Flocks

Wed, 03/22/2017 - 10:58

A Little Egret (far left) forages with endangered Crested Ibises. Photo credit: N. Zhao

Birds benefit from flocking together—even when they’re not of a feather. According to a new study in The Auk: Ornithological Advances, China’s endangered Crested Ibises benefit from joining forces with other, more visually-oriented bird species while searching for food.

Joining mixed-species flocks can reduce birds’ risk of predation while boosting their foraging opportunities, but it can also expose them to competition and disease, and little research has been done on what this means for birds such as ibises that rely on their sense of touch to find food. Yuanxing Ye and Changqing Ding of the Beijing Forestry University and their colleagues studied the behavior of Crested Ibises foraging with and without Little Egrets in central China’s Shaanxi Province, recording the birds’ behavior with a digital video camera to determine whether they picked up on social cues from the other species. They found that ibises in mixed-species flocks became alert to threats sooner, suggesting they felt less at risk when mingling with the more visually-oriented egrets.

Crested Ibises were once believed to be extinct in the wild, until seven birds were discovered in a remote area of China in 1981. Ye and his colleagues believe this new information about their foraging behavior could benefit ibis conservation. “Developing habitat conditions that favor mixed-species flocks may reduce the perception of risk by ibises due to the early warning effects of egrets, particularly in habitats with high levels of predation or disturbance,” according to Ye.

“Mixed-species flocks are a common occurrence in birds, but little is known about the costs and benefits of joining such groups when species differ in their foraging tactics,” adds the University of Montreal’s Guy Beauchamp, an expert on group living in birds. “In this case, ibises benefitted from joining another more visually-oriented species in that they detected threats more quickly. This study shows how detailed behavioral observations can help us understand why species forage in groups and also join other species.”

What makes a tactile forager join mixed-species flocks? A case study with the endangered Crested Ibis (Nipponia nippon) is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-191.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


How Reliable Are Traditional Wildlife Surveys?

Wed, 03/22/2017 - 10:58

A Sandhill Crane from the Rocky Mountain population. Photo credit: T. Cacek

To effectively manage a wildlife species, one of the most basic things you need to know is how many of them are out there. However, it’s almost never feasible to count every single individual—so how do the results of wildlife surveys compare to true population size? A new study in The Condor: Ornithological Applications tests this using the results of more than thirty years of surveys of the Rocky Mountain population of Sandhill Cranes.

A three-year “moving average” is often used to smooth out year-to-year irregularities in survey results, but this approach has never been evaluated. Brian Gerber of Colorado State University and William Kendall of the U. S. Geological Survey assessed whether the annual population changes reported by these moving averages were realistic, based on what is known about crane demographics, and how they compared to the results of a more sophisticated statistical approach called a hierarchical Bayesian time series model. They found that while the moving average population estimates were reasonable, the more complex method performed better over a large number of scenarios.

Bayesian approaches offer a structured way to incorporate new information as it becomes available. “The model-based approach we looked at is very flexible and has some major advantages over other methods,” says Gerber. “By taking a Bayesian approach, we can include additional information about both the observation process and the true population to obtain more realistic estimates and predictions. Also, the model-based approach includes measures of uncertainty about our population estimates, which are not usually provided by more common approaches and are crucial for understanding the level of confidence we have about our estimates.”

Evidence suggests that management practices over the last twenty years have largely met the annual population objectives for the Rocky Mountain Sandhill Crane population. “Looking forward,” adds Gerber, “managers may still be interested in adopting our more robust modeling approach due to its flexible framework, which makes implementing any changes relevant to the survey easier.” The investment in collecting these long-term data may pay off not just for crane management, but for an advance in methods that can be applied to other species as well.

Evaluating and improving count-based population inference: A case study from 31 years of monitoring Sandhill Cranes is available at http://americanornithologypubs.org/doi/full/10.1650/CONDOR-16-137.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


AUTHOR BLOG: Bringing Mitochondria into Species Concepts

Wed, 03/08/2017 - 16:37

Geoffrey Hill

Linked paper: The mitonuclear compatibility species concept by G.E. Hill, The Auk: Ornithological Advances 134:2, April 2017.

A couple of years ago I was at the Bentsen State Park hawk watch tower with a dozen other birders when the topic of species boundaries came up. No one knew me or that I was a professional ornithologist—I was deep undercover as a mediocre birder—so I just listened to the conversation. Someone suggested that the “experts” changed species boundaries periodically so that new field guides would have to be printed and bought. The not-so-subtle suggestion was that the checklist committee was in the pockets of powerful publishers. Then it was suggested that researcher elites manipulated the system to get government money. Everyone quickly reached consensus that both of these views were correct. From that conversation, it would seem that scientists have not been very effective at conveying to the public the rationale for changes to the checklist. But maybe a skeptical public is not surprising given that some professional ornithologists are also not enamored with how species boundaries of birds are demarcated. This uncertainty regarding what is or is not a species could simply reflect the reality that at the tips of the branches of the tree of life, species boundaries are nebulous. But I don’t think that is correct. I agree with Ernst Mayr’s conclusions that species are real and fundamental biological entities. I’m convinced that the problem with the current approach to assessing species boundaries lies in a poor understanding of the process of speciation leading to an incomplete and confused species concept.

I lived the first fifty years of my life without offering any opinion about the process of speciation or species boundaries, and I had no plans to ever wade into the morass. It was therefore quite unexpected when I fell head first into the deep end of the speciation controversy by proposing a new definition for how to define an avian species, published as an opinion piece in The Auk. This new speciation concept arose not from phylogeographic studies—the genesis of all previous species concepts—but from the study of mating patterns in relation to colorful feathers (see Hill and Johnson 2012). Sexual selection and speciation have always been closely associated topics in evolutionary biology, but what I realized about five years ago is that both of these fundamental evolutionary processes emerge as a necessary outcome of the critical need for coadaptation between mitochondrial and nuclear genes and that mitonuclear coadaptation is the foundation of any comprehensive species concept. Before I explain what I mean by “mitonuclear coadaptation,” however, I need explain current dogma for recognizing species.

Whether it is explicitly stated or not, decisions regarding species boundaries are based on a species concept, and the Biological Species Concept currently being followed by the American Ornithological Society checklist committee was proposed by Ernst Mayr about 75 years ago, long before the age of modern genomics. As a matter of fact, the current species concept was written before biologists even knew that mitochondria have DNA that is inherited independently from the nuclear DNA.

It turns out the knowing about mitochondrial DNA is very important because mitochondrial DNA codes for components of the electron transport system, which is the biochemical machine that generates most of the energy for birds, mammals, and other complex life. But mitochondria only carry a few genes—not nearly enough to code for the entire electron-transport system—so much of the system is encoded by nuclear DNA, with the products moving over to the mitochondria to co-function with the products of mitochondrial genes. Having two distinct genomes coding for components that have to work together in a fully coordinated manner means that these genomes have to be coadapted; put another way, the genes in the mitochondria and nucleus have to coevolve so that their products are matched in physical shape and complementary of function. When populations become isolated, their mitochondrial and nuclear genes can coevolve to be different than the coevolved mitochondrial and nuclear genes of any other population such that they cannot be mixed without incompatibilities and fitness loss. What I propose in my opinion paper is that it is uniquely coadapted sets of mitochondrial and nuclear genes that define a species and that make hybrid offspring resulting from pairings between species less viable, thereby maintaining species boundaries.

Currently, it is not possible to assess the mitonuclear compatibility of any two populations of birds, although by simple extrapolation of advancing technology we will soon have such capability. What we currently have is information on the mitochondrial genotype of many species of birds, including many populations that are potentially distinct species. I propose that uniquely co-adapted mitonuclear genotypes necessarily involve unique mitochondrial genotypes. Thus, mitochondrial genotype becomes an excellent and available proxy for species boundaries.

Many biologists already look to mitochondrial genotypes when assessing species status.  My theory simply provides stronger justification for placing more emphasis on mitochondrial genotypes and less emphasis on nuclear genotypes. Indeed, advocates of DNA barcoding assert that most species can be unambiguously diagnosed by mitochondrial gene sequence, and my theory potentially explains why.

As with any new idea, this hypothesis for the process of speciation and nature of species boundaries must be tested. Whether the mitochondrial compatibility species concept proves to be accurate or an overstatement, ornithology will benefit by considering mitonuclear interactions when they ponder species boundaries.


Redefining “Species”: New Species Concept Based on Mitochondrial & Nuclear DNA Coadaptation

Wed, 03/08/2017 - 15:13

A hybrid “Brewster’s” warbler. Photo credit: L. Spitalnik

What is a species? Biologists—and ornithologists in particular—have been debating the best definition for a very long time. A new commentary published in The Auk: Ornithological Advances proposes a novel concept: that species can be defined based on the unique coadaptations between their two genomes, one in the nuclei of their cells and the other in their mitochondria.

All animals have two sets of genes, one in the cell nucleus and one in organelles called mitochondria, and these two sets of DNA work together to enable cellular respiration and energy production. If they’re mismatched, the result is reduced energy output and increased production of damaging free radicals. While the most commonly used species definition is based on the idea that isolated populations slowly accumulate changes in their nuclear genes that make interbreeding impossible, Auburn University’s Geoffrey Hill proposes a new twist on the species concept—that speciation is really the divergence of sets of coadapted mitochondrial and nuclear genes. Interspecies hybrids, his theory suggests, have reduced fitness due their mismatched genomes’ reduced ability to work together in the cell.

Past studies have shown that mitochondrial genotype tends to be very good at showing species boundaries between birds. This “mitonuclear compatibility species concept” helps explain the fact that the abrupt transitions between mitochondrial genotypes at species boundaries correspond with abrupt transitions in songs, plumage patterns, and female mating preferences. Interestingly, two closely related species that have recently been documented to have extensively intermingled nuclear genes—Blue-winged and Golden-winged warblers—also show an abrupt transition in mitochondrial genes.

“Almost all ornithologists who write and think about avian speciation study phylogeography—the geographical distribution and genetic structure of bird populations,” says Hill. “In contrast, I study bird ornamentation and, particularly, bird coloration. It was the discovery that ornaments signal mitochondrial type that led to my sudden realization that mitochondrial type—or, more accurately, coadapted sets of mitochondrial and nuclear genes—define species boundaries. I don’t think I would have ever seen the pattern if I had come at the question from a phylogeographic perspective.”

“This is an intriguing and controversial idea—that mitonuclear incompatibilities could be so central to generating new avian species—and I see this as a call for more research into how these incompatibilities might manifest themselves in young species,” says avian evolutionary biologist David Toews of Cornell University. “The functional aspects of mitochondrial genes have, in particular, received little attention from the ornithological community, and it will be interesting to see how these ideas play with additional empirical studies going forward.”

The mitonuclear compatibility species concept is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-201.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


Monitoring Birds by Drone

Wed, 02/15/2017 - 10:15

Researchers test the bird monitoring drone. Photo credit: A. Wilson

Forget delivering packages or taking aerial photographs—drones can even count small birds! A new study from The Auk: Ornithological Advances tests this new approach to wildlife monitoring and concludes that despite some drawbacks, the method has the potential to become an important tool for ecologists and land managers.

Bird surveys provide crucial data for environmental management, but they have limitations—some areas are difficult to access, and surveyors vary in their skills at identifying birds. Using audio recordings made by unmanned aerial vehicles (UAVs) can help to combat both of these pitfalls, as hard-to-reach sites can be flown over and multiple people can review the resulting recordings. Andrew Wilson of Gettysburg College and his colleagues tested the feasibility of this approach by using fishing line to suspend an audio recorder from a simple “drone,” first in trial runs on college athletic fields and then in real bird surveys on Pennsylvania State Game Lands.

The experiments on state game lands directly compared UAV data with traditional ground-based surveys of the same areas. A few bird species were undercounted by the UAV technique, such as Mourning Doves, whose extremely low-frequency calls weren’t picked up by the recorder, and Gray Catbirds, which occurred at such high densities that counting individual birds in the recordings was difficult. Overall, however, there were few significant differences between the results produced by the two methods.

“The inspiration for the study came while I was surveying forests in the Appalachian Mountain in Pennsylvania for Cerulean Warblers,” says Wilson. “All of our survey work was done from roadsides or hiking trails, for logistical reasons and to maximize survey efficiency, but I was always aware that our sample locations were very biased and that we were missing key areas such as steep forested slopes.” He notes that the drone and audio recorder used in this experiment were inexpensive, commercially available models, making this technique within reach even for those with limited funding.

“I recall my vocal reaction upon hearing their oral presentation during a session I chaired on emerging technologies to study birds at the 2016 North American Ornithology Conference last August, where I exclaimed, ‘What an amazingly simplistic but useful application of a drone for bird research—I wish I had thought of it!’” says McGill University’s David Bird, founding editor of the Journal of Unmanned Vehicle Systems. “This unique study provides a significant first step toward the inevitable common use of unmanned vehicle systems for monitoring songbird populations both during the breeding season and on migration.”

The feasibility of counting songbirds using unmanned aerial vehicles is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-216.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


AUTHOR BLOG: How Will an Arctic-Breeding Songbird Respond to Taller Shrubs and Warmer Temperatures?

Wed, 02/08/2017 - 10:11

A female Smith’s Longspur solicits a mate. Photo credit: J. Hughey

Heather McFarland

Linked paper: Nest-site selection and nest success of an Arctic-breeding passerine, Smith’s Longspurs, in a changing climate by H.R. McFarland, S. Kendall, and A.N. Powell, The Condor: Ornithological Applications 119:1, February 2017.

How will songbirds that nest in tundra respond as the Arctic transforms into a warmer and shrubbier environment? This is the question that drove us to study a small songbird known as the Smith’s Longspur. Endemic to North America, this songbird breeds in only a few remote mountain valleys in Canada and Alaska, making it particularly susceptible to changes at northern latitudes. Smith’s Longspur’s are also unique in that they are polygynandrous. This is a rare mating strategy where both sexes are polygamous, and birds of either sex may mate with up to three individuals each breeding season. Rather than a single male and female establishing a territory, Smith’s Longspurs usually form larger groups called neighborhoods which contain many inter-mated individuals. Since this mating strategy is poorly understood and so different from other tundra nesting songbirds, it is difficult to predict how breeding Smith’s Longspurs may respond to climate change. Therefore, prior to further change, baseline information about breeding requirements is needed.

To fill this void, we monitored more than 250 Smith’s Longspur nests between 2007 and 2013 in the Brooks Range of Alaska. All of the nests were found in open tundra areas, and females never placed their nests in tall vegetation. Aside from a lack of tall shrubs, no specific habitat features that they measured influenced where females placed their nests. This finding is contrary to patterns commonly observed in monogamous ground-nesting birds where females tend nest near a specific habitat feature. We believe that Smith’s Longspurs may deviate from this pattern because of their unique breeding strategy. Females may benefit more by nesting near other females were the chance of “hooking up” with additional males is greater. If this is the case, nest site selection may occur at a larger neighborhood scale. Considering these findings, we’re concerned that future shrub growth in the Arctic could limit the amount open tundra areas available for breeding neighborhoods of Smith’s Longspurs.

Although there may be fewer available nest sites in the future, warmer temperatures could benefit breeding Smith’s Longspurs. In this study, nests survived best when there were more warm days during the nesting period. Cold temperatures appeared to have no impact on nest success, possibly because females were able to delay nesting until weather conditions were favorable. During these years, females usually began nesting within a few days of one another, compared to years with good conditions early in the season when egg laying was spread out over several weeks. Considering that Smith’s Longspurs breed in the Arctic, it is not surprising that they have adapted strategies to withstand harsh conditions. Because of this adaptive ability, as well as the predicted increase in temperature throughout the Arctic, we believes that breeding Smith’s Longspurs could become more productive in the future. Even so, the combined outcome of reduced suitable habitat but potentially higher breeding productivity is still unknown. Continued monitoring of Smith’s Longspurs is needed as northern regions continue to change.


AUTHOR BLOG: Population-Specific Migration Patterns of Golden-Winged Warblers

Wed, 02/08/2017 - 09:59

Gunnar Kramer

Linked paper: Nonbreeding isolation and population-specific migration patterns among three populations of Golden-winged Warblers by G.B. Kramer, H.M. Streby, Sean M. Peterson, J.A. Lehman, D.A. Buehler, P.B. Wood, D.J. McNeil, J.L. Larkin, and D.E. Andersen, The Condor: Ornithological Applications 119:1, February 2017.

Our study in The Condor: Ornithological Applications follows decades of research on the population dynamics of a declining songbird species, the Golden-winged Warbler.

Golden-winged Warblers have long been the focus of research and conservation interest due in part to sensationalized range-wide declines on the order of -2.5% per year; however, the true nature of these declines is more nuanced and complicated. When geographic populations are considered, Golden-winged Warblers demonstrate two very different stories. On one hand, the Appalachian population, breeding from Ontario through North Carolina and Georgia, experienced severe declines (up to -7% per year) over the past 50 years resulting in local extirpations and noticeable declines in the abundance and distributions of Golden-winged Warblers in the region. On the other hand, the western Great Lakes population, breeding largely in Minnesota, is home to ~50% of the global breeding population of Golden-winged Warblers and is maintaining historic population levels or possibly increasing over the past 50 years. When combined, these two very different stories blend into a general picture of overall population decline, spawning intensive efforts to identify breeding-grounds factors that might explain this decline, such as the loss of nesting habitat, poor nesting success, and competition with other closely related species (i.e., Blue-winged Warblers). Despite all the extensive research, no single breeding-grounds factor or combination of factors provides a parsimonious and consistent explanation for the differential population trends that exist in this species.

This is where our research comes in. Compared to the amount of research carried out in breeding populations of Golden-winged Warblers, relatively little has been done to link breeding populations to nonbreeding sites and identify migration routes in geographically isolated populations of Golden-winged Warblers. We developed a study to attach the smallest geolocators available (at the time in 2013 this was just under 0.50 g) to 9-10 g Golden-winged Warblers at multiple sites throughout their breeding distribution where they were experiencing variable population trends. Our main goal was simply to find out where these different populations went after they left the breeding grounds in North America and determine if the populations overlapped or occurred together in Central and South America during the nonbreeding period. If the declining Appalachian populations spent the winter in a region that was isolated from the stable western Great Lakes population, it is logical that the breeding-grounds population trends we observe might be caused at least in part by nonbreeding factors.

After all the hard work of redesigning the marking methods, getting the geolocators deployed, stressing over whether it would work, and then retrieving the geolocators, it was exciting to analyze the data and see entire year-long tracks of individual Golden-winged Warblers and to think about the distances these birds traveled and the places they spent their time when away from our study sites. We found that Golden-winged Warblers from Appalachian breeding populations spent the nonbreeding period in South America, mostly in a relatively small region on the border of Columbia and Venezuela. In contrast, Golden-winged Warblers from the western Great Lakes breeding population occurred throughout northern Central America in countries like Honduras, Nicaragua, Guatemala, Belize, and Mexico.

In the end, this geolocator study demonstrates a clear difference in nonbreeding locations and migration strategies among these different populations of Golden-winged Warblers. These differences are urgently meaningful from a conservation and management standpoint as they highlight a potential cause for regional differences in population trends observed across the breeding distribution. If nonbreeding factors are limiting Golden-winged Warbler population growth in the Appalachians, perhaps the most important implication of our work is to provide information that might help conservationists revise and refocus current strategies to better and target declining populations that spend the nonbreeding period in northern Colombia and Venezuela.

Read more at henrystreby.com.


Combined Count Data Reveals Shifts in Hawks’ Migratory Behavior

Wed, 02/08/2017 - 09:48

Red-tailed Hawks’ migratory behavior appears to be changing. Photo credit: N. Paprocki, HawkWatch International

Bird species’ distributions and migratory behavior are shifting in response to changes in climate and land-use, but surveys that focus on a particular season can cause scientists to miss trends in the bigger picture. A new study from The Condor: Ornithological Applications tackles this problem by combining Red-tailed Hawk counts from both migration and winter, and finds that while the hawks’ numbers are stable overall, their migratory behavior is undergoing a change.

To get a more complete picture of Red-tailed Hawks’ population status, Neil Paprocki of HawkWatch International and his colleagues compared data from the Raptor Population Index, which is based on counts of raptors during migration, with Christmas Bird Counts, which are carried out by amateur birdwatchers in December and January. They found declining hawk numbers at 43% of the migration count sites, with increases at only a few sites located in the western flyway. Among the winter counts, however, 67% showed increases, spread across all regions surveyed. The greatest increases were reported from northern latitudes.

The migration data alone would suggest that Red-tailed Hawks are on the decline in North America—but the full picture actually indicates stable to increasing populations, with an ongoing shift in migratory behavior. The hawks appear to be wintering farther north than they have in the past, explaining the lower numbers seen on migration. “We wanted to develop a more comprehensive view of raptor population change, using data from multiple stages of the annual cycle,” says Paprocki. “What does this study tell us about how Red-tailed Hawks are adapting to environmental change? It doesn’t tell us anything directly, but the results suggest that Red-tailed Hawks may be responding to climate change, land use change, or other environmental changes by migrating shorter distances or becoming year-round residents.”

Hawks breed at low densities, including at high latitudes where they can be difficult to survey.   This makes migration and winter surveys especially important for monitoring these species. “Many North American hawks, eagles, and falcons are counted at watch sites during migration, and monitoring programs use counts as an index of population size. The interpretation that trends or lack thereof in count data are indicative of changes in populations assumes that the proportion of the population passing by the migration site remains consistent over time. Unfortunately, this assumption may be at risk, as there is strong evidence that raptor migration strategies are changing in response to climate and land use change,” according to Julie Heath of Boise State University, a raptor biologist who was not involved with the study. “Paprocki et al. do an excellent job highlighting how changes in annual cycles could affect our ability to monitor raptors and how the use of additional information can provide insight to trends in migration counts.”

Combining migration and wintering counts to enhance understanding of population change in a generalist raptor species, the North American Red-tailed Hawk is available at http://americanornithologypubs.org/doi/full/10.1650/CONDOR-16-132.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Allen’s Hummingbird Boom Missed by Breeding Bird Surveys

Wed, 02/08/2017 - 09:46

Migratory and nonmigratory Allen’s Hummingbirds appear to be on different population trajectories. Photo credit: A. Varma/varmaphoto.com

Allen’s Hummingbird has been placed on several conservation watchlists, as breeding bird surveys indicating population declines have spurred concerns that climate change may push it out of Southern California. However, local birdwatchers have reported at the same time that the non-migratory subspecies of Allen’s Hummingbird, once restricted to the Channel Islands, is now a common sight at feeders in Riverside and Los Angeles. Why the discrepancy? A new commentary published in The Condor: Ornithological Applications may provide answers.

The University of California–Riverside’s Chris Clark used data from eBird, an online platform where citizen scientist birdwatchers can submit their sightings, to reexamine Allen’s Hummingbird population trends in urban Southern California since 1990. He found a steep increase in the species’ prevalence in eBird checklists from the region, with Allen’s Hummingbirds reported in 20% of all checklists submitted from Southern California today. Because the pattern is consistent year-round, it cannot be driven by the migratory subspecies, which is only in the area for part of the year.

While it appears that urban landscaping has created new habitat and food supplies that are exploited year round by non-migratory Allen’s Hummingbirds, ecological differences between the two subspecies could also be helping to drive their different trajectories. “The non-migratory Allen’s Hummingbird seems to do better in parks and backyards than does the migratory subspecies,” says Clark. “It also produces more offspring during the breeding season. Either of these reasons might be why the non-migratory subspecies seems to be doing so well within urban areas of the greater L.A. area.”

“This commentary is an object lesson in the importance of considering all sources of data and all aspects of a species’ natural history when its range and trends are modeled. As Clark emphasizes, the accuracy of such models matters when they are the basis for setting conservation priorities,” according to Philip Unitt of the San Diego Natural History Museum. “The paper calls attention to the continuing dramatic increase in the range and numbers of Allen’s Hummingbird, bringing into contact two subspecies differing in multiple aspects of their biology, an opportunity for study of evolution in process.”

eBird records show substantial growth of the Allen’s Hummingbird (Selasphorus sasin sedentarius) population in urban Southern California is available at http://americanornithologypubs.org/doi/abs/10.1650/CONDOR-16-153.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Greater Sage-Grouse More Mobile Than Previously Suspected

Wed, 02/08/2017 - 09:42

Greater Sage-Grouse occasionally make long-distance movements between leks. Photo credit: J. Carlson

Greater Sage-Grouse are thought to return to the same breeding ground, or “lek,” every spring—but how do populations avoid becoming isolated and inbred? A new study from The Condor: Ornithological Applications used thousands of DNA samples collected at leks across four states to reveal that some sage-grouse travel more widely than anyone suspected and, in doing so, may temper inbreeding and isolation.

Using genetic markers in DNA extracted from feather and blood samples, Todd Cross of the U.S. Forest Service Rocky Mountain Research Station and his colleagues identified more than three thousand individual sage-grouse that visited leks across the northeastern portion of the birds’ range over seven years. Approximately two and one-half percent of birds in the sample turned up twice, a mixture of individuals returning to the same lek in different years, visiting different leks within one year, or visiting different leks in different years. Seven birds made movements of over 50 kilometers (over 30 miles), six of which occurred within a single breeding season.

The study used 7,629 samples collected from 835 leks in Idaho, Montana, and North and South Dakota between 2007 and 2013. While the results support the idea that most grouse are faithful to their chosen lek sites, some individuals clearly make long-distance movements, which could help prevent inbreeding within leks and expand the size of the genetic neighborhood. “Our research demonstrates that Greater Sage-Grouse are an even more mobile species than we had realized before, moving large distances of up to 194 kilometers—over 120 miles—in a single breeding season,” says Cross. “These findings highlight the importance of landscape-scale efforts that conserve movement corridors.”

“The use of genetic recapture information opens an exciting new door to understanding the landscape dynamics of the Greater Sage-Grouse,” according to Pat Deibert, National Sage-Grouse Conservation Coordinator for the U.S. Fish and Wildlife Service. “Species conservation is more than tracking the number of individuals: We also have to understand how a species uses the landscape and the associated impacts on their vital rates. The data presented in this paper provide additional insight to effective landscape management and conservation design for the Greater Sage-Grouse and will contribute to the continual improvement of management for this species.”

Genetic recapture identifies long-distance breeding dispersal in Greater Sage-Grouse (Centrocercus urophasianus) is available at http://americanornithologypubs.org/doi/abs/10.1650/CONDOR-16-178.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Swamphens Signal Dominance Through Fleshy Faces

Wed, 01/18/2017 - 15:48

Pūkeko’s facial shields convey their social status. Image credit: C. O’Connor

What’s in a face? In addition to their plumage, Pūkeko—large purple swamphens found in New Zealand—convey information about their status through their faces. A new study from The Auk: Ornithological Advances shows that the strongest predictor of male dominance in Pūkeko is the size of their frontal shield, a fleshy ornament on their bill that can change quickly.

Cody Dey of McMaster University (now at the Great Lakes Institute for Environmental Research) and his colleagues measured the plumage coloration and frontal shield width and coloration of 272 adult Pūkeko on New Zealand’s North Island, then tested how those traits were related to the dominance hierarchies they observed within groups. While shield color and plumage brightness and color were all related to social dominance, the strongest predictor was frontal shield width. Frontal shield width was also strongly correlated with testes mass, suggesting that it is the primary status signal for males.

Pūkeko shields can decrease in size in as little as a week in response to aggressive challenges from other males, thanks to hormonal changes. “The take-home message from this study is that birds use both feather coloration and bare-part coloration as signals, but these different types of signals have different properties, and bare-part signals in particular are probably more accurate because they can be updated,” says Dey. “Feather-based signals can only change when the individual molts, which usually only happens once or twice a year. Since we have mostly been studying feather coloration in birds, we might have been missing a great deal of information about the role of avian coloration.”

“This exciting study on the understudied Pūkeko adds strong support to our growing understanding of bare part signaling. In particular, the authors reveal the dynamic nature of pigmented bare in communicating status and condition information,” adds Franklin and Marshall College’s Daniel Ardia, an expert on signaling in birds who was not involved with the study. “The differences they find between frontal shield signaling and plumage reveal the complexity of signaling and demonstrates that bare parts are not simply redundant signals.”

A bare-part ornament is a stronger predictor of dominance than plumage ornamentation in the cooperatively breeding Australian Swamphen is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-119.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


Mitochondrial DNA Shows Past Climate Change Effects on Gulls

Wed, 01/18/2017 - 15:44

A nesting colony of Heermann’s Gulls. Photo credit: C. Contreras

To understand the present and future, we have to start with the past. A new study in The Auk: Ornithological Advances uses the mitochondrial DNA of Heermann’s Gulls to draw conclusions about how their population has expanded in the Gulf of California since the time of the glaciers—and, by extension, how human-caused climate change may affect them in the future.

Enrico Ruiz of the University of California, Merced, and his colleagues sequenced the mitochondrial DNA of 286 Heermann’s Gulls breeding in the Gulf of California in 2011 and 2012. Using a combination of statistical approaches, the researchers found that the pattern of genetic diversity among the birds suggests a period of population growth from roughly 100,000 years ago to 45,000 years ago, coinciding with the last glacial retreat in the region.

Ruiz and his colleagues believe that the gulls’ population increase was the result of large-scale climatic shifts, which would have allowed the marine species on which the gulls rely for food to expand their ranges. Though this is one of the first studies to examine how regional climate change affected vertebrates such as seabirds, past analyses have found evidence of increases in fish, mollusk, and crustacean populations during the same period.

The Heermann’s Gull population in the region has remained relatively stable since the end of this period of expansion, but Ruiz and his colleagues are concerned that this may change. “During the last 16 years, the warm oceanographic anomalies in the Gulf of California have increased in frequency from an average of one every six or seven years to six anomalies in the last 16 years,” says Ruiz. “The consensus among researchers now is that there is a general productivity decline across the trophic web, including the availability of the small pelagic fish on which the seabirds feed.” By learning how the ancient climates affected modern species’ population sizes and distribution in the past, he hopes we may better understand present changes in their distribution and abundance.

Demographic history of Heermann’s Gull (Larus heermanni) from late Quaternary to present: Effects of past climate change in the Gulf of California is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-57.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


Cavity Nest Niche of the Endangered Vinaceous-Breasted Parrot: Sharing a Limited Resource Among Birds, Bees, and Opossums

Wed, 01/18/2017 - 10:34

Photo credit: M. Lammertink

Eugenia Bianca Bonaparte and Kristina L. Cockle

Linked paper: Nest niche overlap among the endangered Vinaceous-breasted Parrot (Amazona vinacea) and sympatric cavity-using birds, mammals, and social insects in the subtropical Atlantic Forest, Argentina by E.B. Bonaparte and K.L. Cockle, The Condor: Ornithological Applications 119:1, February 2017.

Have you ever seen two animals fighting over a tree hole? Around the world, more than a thousand species of birds use tree holes or cavities for nesting, and they have to somehow share this critical resource with other birds, mammals, social insects, lizards and snakes. If two or more species of animals need cavities with similar characteristics, in the same place and at the same time of year, they overlap in nest niche. Nest niche overlap can lead to fierce competition for nest sites, lowering reproductive output and preventing the recovery of threatened species.

We got interested in how birds share the cavity resource because of our long-term work to conserve Vinaceous-breasted Parrots (Amazona vinacea), endangered cavity-nesters found only in the subtropical Atlantic Forest of South America. About 90% of the Atlantic Forest has already been converted to farmland, and nearly all the rest has been selectively logged, resulting in an extreme scarcity of nesting cavities. Vinaceous-breasted Parrots have declined throughout their range over the last century. In Argentina, where they were reported by early naturalists to “darken the sky” in “flocks of thousands,” less than 300 remain today. And this remnant population must share a dwindling supply of tree cavities with more than 70 other cavity-nesting species.

Our initial field work suggested that Vinaceous-breasted Parrots have low reproductive output. We wondered whether they might compete for nest sites with other species of birds, mammals, and social insects. On the other hand, theory and data from temperate forests suggested that cavity-nesting species reduce competition by partitioning the nest niche. For example, one species might use cavities high in the tree canopy, while another species uses cavities close to the ground. We reasoned that if such niche partitioning occurred in the Atlantic forest, we could identify specific cavity characteristics that we could then target for conservation of Vinaceous-breasted Parrots.

Photo credit: M. Lammertink

To find out the extent of nest niche overlap among Vinaceous-breasted Parrots and co-occuring cavity-nesters, we studied timing of breeding, characteristics of cavities, trees, and habitat, and interspecific reuse of the same individual cavities, by large (> 140 g) birds and mammals (parrots, owls, toucans, forest-falcons and opossums) and social insects (bees and wasps). Every spring for 10 years, we shadowed adult birds, snooped on their eggs with our tiny pole-mounted spy cameras, and tested the limits of our tree-climbing equipment to access cavities up to 24 m high in the Atlantic Forest canopy.

Contrasting with reports from temperate forests, our data showed very little evidence of nest niche partitioning among cavity-nesters in a diverse subtropical community. Except for White-eyed Parakeets (Psittacara leucophthalmus), which nested later in the season, all bird species nested at the same time of year. Furthermore, and unfortunately for conservation efforts, no combination of cavity, tree and habitat characteristics was used exclusively by Vinaceous-breasted Parrots. Indeed, our models were unable to distinguish among the cavities used by the various species, and 8 of the 10 species reused each other’s cavities. The high level of overlap in nest niche, combined with previous evidence that cavities can limit bird density in our study area, support the idea that competition among species could play an important role in structuring the community of cavity nesters in the Atlantic Forest. Such competition could potentially inhibit the recovery of threatened species like Vinaceous-breasted Parrot.

What can be done for Vinaceous-breasted Parrots? Although we had little success classifying cavities by species, some characteristics of cavities, trees and habitat were selected more by Vinaceous-breasted Parrots than by other taxa, and we recommend targeting conservation efforts toward cavities and trees with these characteristics: >10 m high, entrance diameter 7–40 cm, tree diameter (DBH)>55 cm. Additionally, we found 62% of Vinaceous-breasted Parrot nests on farms (vs.≤50% for any other taxon), which highlights the importance of working with local farmers to conserve cavities in human-altered habitats as well as protected areas. As a start, through Proyecto Selva de Pino Paraná we organize outreach activities in schools and on farms, an annual parrot census, and a small scale reforestation program, all of which involve local farmers in the conservation of Vinaceous-breasted Parrots and their nest trees. We’re now busy interviewing farmers to study how they manage remnant native trees and what they think about cavity-nesting birds on their land. Stay tuned for the results!


Connectivity Is Key for Preserving Isolated Sage-Grouse Populations

Wed, 01/11/2017 - 11:15

Greater Sage-Grouse depend on large, intact tracts of the sagebrush habitat. Current sage-grouse conservation plans focus on protecting selected “priority areas,” but these areas vary in size and proximity to each other—will they be able to sustain thriving, interconnected populations over time? A new study from The Condor: Ornithological Applications evaluates this approach.

Small, isolated populations of sage-grouse are especially vulnerable to threats like wildfires and West Nile virus, and genetic diversity declines if birds don’t have the ability to occasionally interbreed with other groups. This study provides land managers with a new way to rank priority areas based on their contributions to connectivity.

Using a statistical technique known as graph theory, Michele Crist, Steven Knick, and Steven Hanser of the U.S. Geological Survey examined how the spatial arrangement of priority areas might affect their ability to function as an interconnected network of reserves. They found that of the three networks of sage-grouse priority areas—the Washington network, the Bi-State Network comprising California and Nevada, and the Central network, which is the largest and includes parts of ten states—only the priority areas of the Central network had a high degree of connectivity, and even there connectivity was dominated by a small number of large, centrally located sites.

“Graph theory is a way to describe a network based on sets of nodes and their connections with others. The network’s characteristics reveal a lot of information about how that network functions,” says Knick. “For example, importance within the network can be inferred from being large and having many connections or by connecting different groups within the network. The analysis is commonly applied to understand social networks.”

“Managing the differing ecologies of a landscape-scale species presents many challenges even in a perfect world of unlimited resources and complete agreement amongst all associated stakeholders. As that is almost never the case, having studies such as the one presented here is essential to inform sound, science-based decisions,” according to Pat Deibert, National Sage-Grouse Conservation Coordinator for the U.S. Fish and Wildlife Service. “This exploration of connectivity and inferences for the long-term viability of prioritizing landscapes for conservation of the Greater Sage-Grouse is extremely valuable for assessing the efficacy of the current management strategy and informing decisions and appropriate adjustments in the future.”

Range-wide connectivity of priority areas for Greater Sage-Grouse: Implications for long-term conservation from graph theory is available at http://americanornithologypubs.org/doi/full/10.1650/CONDOR-16-60.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Traffic Noise Reduces Birds’ Response to Alarm Calls

Wed, 12/28/2016 - 14:41

A Tufted Titmouse perches on a feeding platform during a playback experiment. Image credit: J. Damsky and M. Gall

Pollution can take many forms—including noise. Excess noise in the environment from sources such as traffic can have negative effects on animals that rely on sound to communicate and get information about their surroundings. A new study from The Condor: Ornithological Applications shows that traffic noise makes birds less responsive to alarm calls that would otherwise alert them to dangers such as predators.

Megan Gall and Jacob Damsky of New York’s Vassar College tested how traffic noise affected the reactions of Black-capped Chickadees and Tufted Titmice to titmouse alarm calls, which warn birds that a predator is nearby. Using speakers set up near feeding platforms baited with bird seed, they recorded the birds’ responses to three different recordings—alarm calls alone, traffic noise alone, and a combination of the two. The traffic noise didn’t deter the birds from feeding, but five times as many birds approached speakers when the researchers played alarm calls on their own compared with when traffic sounds were added.

“There has been lots of work on how anthropogenic noise affects vocal production, but much less on the response of animals to signals in the presence of noise,” says Gall. “Additionally, a lot of this work focuses on song, but we were interested in how noise might affect responses to an anti-predator vocalization.  These vocalizations are evoked by the presence of a predator and so are closely linked in time with a particular stimulus.”

The study’s results suggest that traffic noise can reduce birds’ ability to hear an alarm call, potentially increasing their vulnerability to predators. “Gall and Damsky’s experiment helps us understand how human-caused noise can interfere with the transfer of information among animals in social groups,” according Florida Atlantic University’s Rindy Anderson, an expert in vocal communication in birds who was not involved with the study. “It’s interesting that the birds’ foraging behavior was not affected under any of the playback conditions, which suggests that the behavioral effects were due to the call playbacks being masked by noise, rather than the noise being simply aversive.”

Anthropogenic noise reduces approach of Black-capped Chickadee (Poecile atricapillus) and Tufted Titmouse (Baeolophus bicolor) to Tufted Titmouse mobbing calls is available at http://americanornithologypubs.org/doi/abs/10.1650/CONDOR-16-146.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Declining Male Offspring Further Imperil Endangered Flycatchers in Southern California

Wed, 12/21/2016 - 10:02

A Southwestern Willow Flycatcher nest. Photo credit: S. Howell

A new study in The Condor: Ornithological Applications documents the steep decline of a population of endangered Southwestern Willow Flycatchers over 16 years—and the change in the sex ratio that has left the birds’ future hanging on a dwindling number of males.

Changes in sex ratios can cause problems in small, declining populations, reducing individuals’ ability to find mates and reproduce. From 2000 to 2015, Barbara Kus of the U.S. Geological Survey and her colleagues monitored federally endangered Southwestern Willow Flycatchers on Marine Corps Base Camp Pendleton in southern California, collecting data as the population declined from 40 individuals to only five. They found that the number of adult males was stable until 2004, but then began to decrease sharply until females outnumbered males at least two to one from 2012 on.

As the number of males plummeted, more and more of them became polygynous, mating with multiple females. Kus speculates that this may have prevented even faster declines. “It was particularly amazing to watch two or three males manage 10 or so females between them,” says Kus. “They seemed to be able to increase their individual efforts such that every female was mated.”

Sex ratios of small populations can become unbalanced through chance, but it appears that other forces were at work in this case—more female than male birds were hatched almost every year of the study. The sex ratio of birds’ offspring can be influenced by factors ranging from the parents’ size to land use practices to the threat of nest parasitism, but it’s unclear what caused the flycatchers in Kus’ study to start producing more female offspring.

“The fact that we found a female bias not only in the adult population, but also in the nestling population and among recruits, suggests that the adult bias is not simply a random outcome of a declining population,” says Kus. “Environmental or other factors may be influencing sex ratios at multiple stages of the life cycle, and more research on these potential factors would be useful. I think our findings indicate that it’s important to track sex ratios as well as numbers in populations of species of conservation concern, in order to detect shifts that could affect population dynamics.”

Female-biased sex ratio, polygyny, and persistence in the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) is available at http://americanornithologypubs.org/doi/full/10.1650/CONDOR-16-119.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Can Bird Feeders Do More Harm Than Good?

Thu, 12/08/2016 - 16:31

A predated robin nest. Photo credit: Jenn Malpass

Many bird lovers put out feeders full of seed for their feathered friends—but those feeders may also attract predators that eat eggs and nestlings. The researchers behind a new study in The Condor: Ornithological Applications tried to untangle these relationships through a four-year study of songbird nests, bird feeders, and predators in urban Central Ohio.

Feeders may attract more predators to an area, but the food can also satiate predators so that they’re less likely to target nests. To learn more, Jennifer Malpass of The Ohio State University and her colleagues tracked the relationships between the success of American Robin and Northern Cardinal nests, the presence of potential nest predators like squirrels, domesitc cats, and other birds, and the presence of bird feeders in the area. Relationships among feeders, predators and nest survival were complex —areas with lots of feeders had more cowbirds and crows, birds that are known to prey on songbird nests, but that didn’t generally affect the success of the nests the researchers monitored.

“One key message of our work is that there may be species-specific responses to anthropogenic foods,” says Malpass. Robins did experience increased nest predation in one specific set of circumstances, when neighborhoods contained both high numbers of bird feeders and large concentrations of crows. Human-dominated environments, it turns out, are complicated and variable, not lending themselves to simple generalizations.

To collect their data, Malpass and her colleagues spent four years monitoring songbird nests and conducting surveys for potential nest predators in seven residential neighborhoods in Columbus, Ohio. “One of the challenges of conducting research in the suburban matrix is gaining access to private property,” says Malpass. “In our experience, most of the residents in our focal neighborhoods were willing to allow access to their yards and had a positive experience being part of the project.”

“Even with over 50 million Americans feeding wild birds and other wildlife around their homes, our scientific understanding of the practice continues to lag,” according to Millikin University’s David Horn, an expert on wild bird feeding who was not involved with the study. “The study by Malpass, Rodewald, and Matthews increases our understanding of how the provision of supplemental food may influence predator abundance and nest survival of two common songbird species.”

Species-dependent effects of bird feeders on nest predators and nest survival of urban American Robins and Northern Cardinals is available at http://americanornithologypubs.org/doi/full/10.1650/CONDOR-16-72.1.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.


Migrating Birds Pile Up Along Great Lakes’ Shores

Wed, 12/07/2016 - 11:18

A series of radar images shows changes in migrating birds’ activity at sunrise. Image credit: K. Archibald, with NOAA data

Birds prefer to migrate at night—so much so that if day breaks while they’re over water, they’ll turn back toward the nearest shore rather than pressing on. That’s the key finding of a new study in The Auk: Ornithological Advances, which used weather radar to examine the behavior of birds crossing the Great Lakes.

Kevin Archibald and Jeff Buler of the University of Delaware and their colleagues turned the U.S.’s powerful network of weather surveillance radar stations on birds heading north across the Great Lakes during their spring migration. As dawn approaches, their data show, birds caught over water increase their elevation and often turn back. This leads to a pileup of birds in near-shore stopover habitat—the density of birds taking off from the southern shores of the Great Lakes on subsequent spring evenings was 48% higher than on the northern shores.

Birds presumably increase their altitude at dawn to try to see how much farther they have to go; if they decide it’s too far, they go back to try again the next night, leading to higher concentrations of migrants on near shores. When birds are migrating south in the fall, these pile-ups would happen on the north side of the lakes rather than the south. “Our study justifies the high value of shoreline habitats for conservation of migratory bird populations in the Great Lakes region,” says Buler. “It also emphasizes that the extent of stopover use in shoreline habitats is context-dependent. We hope professionals charged with managing stopover habitats recognize that shoreline areas can receive high migrant use by virtue of the proximity to a lake and how many migrants are aloft at dawn from day to day, rather than [just] by the presence of abundant food sources in these habitats.”

The data used in the study came from radar stations in Cleveland, Ohio; Grand Rapids, Michigan; and Green Bay, Wisconsin, collected in spring 2010–2013. Cleveland was the only station that did not observe birds increasing their elevation at dawn, possibly because Lake Erie is narrow enough for them to see across without an increase in altitude.

“Nearshore areas of the Great Lakes are important to migrating landbirds. Archibald, Buler, and their colleagues further investigate this distributional pattern by analyzing the interaction between spring migratory flight behavior and the migrant exodus at nearshore stopover sites using NEXRAD radar,” according to The Nature Conservancy’s Dave Ewert. “Their research supports earlier work that migrants concentrate near Great Lakes shorelines, but with new perspectives.”

Migrating birds reorient toward land at dawn over the Great Lakes, USA is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-123.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


Foraging Differences Let Closely Related Seabirds Coexist

Wed, 12/07/2016 - 11:15

A Great Frigatebird wearing a GPS tracking device. Photo credit: R. Clarke

How do seabirds share habitat when food is limited? In the case of frigatebirds, size differences drive them to seek different prey. A study in The Auk: Ornithological Advances uses new technology to explore how closely related Great and Lesser frigatebirds manage to coexist at shared breeding colonies where the need to stick close to their nests prevents them from traveling far in search of food.

Past studies have suggested that Great and Lesser frigatebirds have very similar foraging habits, but ecology’s “competitive exclusion principle” suggests that they must not be going after exactly the same food, or one would eventually outcompete the other. New advances in GPS tracking and chemical analysis of birds’ tissues allowed Rowan Mott of Australia’s Monash University and his colleagues to tease out some of the subtle differences that let these species coexist, and they found that the smaller Lesser Frigatebirds were eating prey items from lower on the food chain than their larger relatives, allowing the two species to subsist on the same resource base.

“As I began reading about frigatebirds, it seemed as if Great Frigatebirds and Lesser Frigatebirds share a remarkably similar foraging strategy. This didn’t sit well with my understanding of the competitive exclusion principle and niche partitioning theory,” says Mott. “It turns out that the two species of frigatebirds seem to have slight differences in their diet that are related to body size. The larger-bodied Great Frigatebird eats more prey from higher levels of the food chain than the smaller-bodied Lesser Frigatebird, and this relationship is particularly strong when comparing the diet of females with the male diet, because females are considerably larger than males.”

Mott and his colleagues carried out their research on an island in the Timor Sea, where they captured birds on their nests during the breeding season, fitting them with GPS devices and collecting feathers and blood for isotope analysis. By analyzing the ratios of nitrogen and carbon isotopes in the birds’ tissues, the researchers could glean insights into the types of food they had been eating—for example, predatory fish have higher proportion of a certain nitrogen isotope in their bodies, allowing the researchers to make inferences about prey type.

“Ecologists argue that coexistence of similar species can only persist if populations have more resources than they need or if they use different subsets of resources,” according to Acadia University’s Dave Shutler, a seabird ecologist not involved in the study. “In the latter case, identifying where and how species partition resources can be challenging for mobile species such as birds; using tracking devices and chemical signatures in tissues, Mott and his collaborators have evaluated differences in diets and spatial use of two species of frigatebirds.”

Resource partitioning between species and sexes in Great Frigatebirds and Lesser Frigatebirds is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-16-184.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.


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