Response of breeding and migrating bird populations to climate change in the national parks of the U.S. Southwest

By Harrison Jones

Related paper: Breeding birds of high-elevation mixed-conifer forests have declined in national parks of the southwestern U.S. while lower-elevation species have increased, with responses to drought varying by habitat by Harrison H. Jones, Chris Ray, Matthew Johnson, and Rodney Siegel. Ornithology.

In spring, the low-elevation grasslands and shrublands of the U.S. Southwest ring with the tinkling songs of Black-throated Sparrows (Amphispiza bilineata) and Horned Larks (Eremophila alpestris), while in the mid-elevation pinyon pine and juniper woodlands Gray Vireos (Vireo vicinior) warble lazily and Juniper Titmice (Baeolophus ridgwayi) fire off their rapid syllables. In the cooler montane forests, Red-breasted Nuthatches (Sitta canadensis) sound their tiny horns, and Mountain Chickadees (Poecile gambeli) whistle fee-bee-bee. These species and others may have adapted to breed in one of the hottest and driest regions of North America, but they’re in a hotspot of another kind as well. 

The Southwest is also a climate change hotspot, and it is home to some of the U.S.’s most spectacular national parks and monuments. These protected lands are ideal places to study how climate change is affecting bird populations. Our study “Breeding birds of high-elevation mixed-conifer forests have declined in national parks of the southwestern U.S. while lower-elevation species have increased, with responses to drought varying by habitat” published in Ornithological Applications, analyzed Southwestern bird monitoring data collected in six parks between 2007 and 2018, and climate data to examine how drought and the timing of the North American Monsoon affect bird populations in the region. 

The six parks in the study—Canyon de Chelly National Monument, Grand Canyon National Park, Petrified Forest National Park, Wupatki National Monument, Mesa Verde National Park, and Bandelier National Monument—are part of the National Park Service’s Southern Colorado Plateau Inventory and Monitoring Network. Birds were monitored using point counts. We quantified drought using the Hargreaves climatic moisture deficit, and monsoon timing by calculating the ratio of early season (June and July) to late season (August and September) precipitation. 

The Southwest’s birds, especially species of montane forest and grassland habitats, are among the fastest declining in North America, according to a 2019 paper in the journal Science. As average temperatures rise and extreme heat waves become more common, precipitation is declining. For the last 23 years, the Southwest has been experiencing a “megadrought,” the driest multi-year period in the region since the year 800 C.E., driven in part by climate change. Drought can affect bird populations both directly and indirectly. When coupled with high heat, drought can kill birds via dehydration and heat exhaustion. It can also reduce insect populations or alter plant communities, leading to a loss of food resources, nesting sites, or habitat structure.  

The North American Monsoon, a significant precipitation phenomenon unique to the region, is also being affected by climate change. The monsoon occurs from July through October, when shifting wind patterns bring moisture from the Pacific, Gulf of California, and Gulf of Mexico over the U.S. southwest and northwestern Mexico, leading to frequent rain and often violent thunderstorms. Scientists are still uncertain how climate change will affect the monsoon precipitation totals, but many studies predict that climate change will delay the monsoon, such that it starts later in the summer and ends in the early fall.  

Less is known about how the timing of the monsoon affects Southwestern birds in the U.S. This late summer dose of precipitation leads to a boom in the insect population and food for millions of birds. Many species use the post-breeding season spike in resources by migrating to this area to fuel their energetically demanding annual molt. Because the southwest’s monsoon occurs when other parts of the western U.S. are getting drier and their insect populations are declining, many species breeding in these regions migrate into the monsoon region, particularly northwestern Mexico and southeastern Arizona, to undergo molt before they continue onto their wintering grounds farther south (a phenomenon known as ‘molt migration’).

We found that the effects of the drought on birds varied according to elevation. Birds that breed in the higher elevation conifer forests, like Red-breasted Nuthatch and Ruby-crowned Kinglet (Corthylio calendula), seemed to benefit from drier years in the short term, likely due to earlier snowmelt which allowed them to start breeding earlier and possibly produce more young. However, over the longer term, many of these same species are declining, most likely due to habitat loss as drought exacerbates tree death and severe, stand-replacing fires. For the most part, these declining population trends mirrored similar declines detected from Breeding Bird Survey data across the Colorado Plateau, suggesting that the protected areas in the study are being affected by tree death and wildfire similarly to how they are across the broader regional landscape. 

Dry years seemed to be hardest on lower-elevation grassland birds such as Horned Lark and Chihuahuan Meadowlark (Sturnella lilianae), likely because grass doesn’t grow as tall or as dense in dry years which provides less cover for ground nests. Less precipitation also means less insect prey to feed to nestlings. In contrast, later monsoons (which are predicted by many climate-change models) seemed to benefit grassland birds, possibly because fewer late summer nests were lost due to flooding and hypothermia caused by intense rainstorms. 

However, we found that species that rely on the monsoon’s food pulse to fuel their molt, “molt migrants” such as Black-headed Grosbeak (Pheucticus melanocephalus), Warbling Vireo (Vireo gilvus), and Lazuli Bunting (Passerina amoena), were negatively impacted by later monsoons compared to species that don’t molt in the monsoon region, perhaps because of a mismatch between the timing of their molt and the monsoon’s abundant resources. 

Our study shows that human-caused climate change in the Southwest is already affecting bird populations and, in many ways, we are entering a new era of climate—one in which we are only beginning to understand how wildlife is being affected. Long-term monitoring, like the data from the National Park Service’s Inventory and Monitoring Program that we used here, is more important than ever to act as a warning system about the changes we are seeing and to understand what is driving them.