Science for a Blue Planet

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Desert Bird Nesting Success Greatly Reduced During Drought

Sonoran desert birds are bucking the trend when it comes to climate change.

A recent study following 13 Sonoran desert bird species, such as the Black-tailed Gnatcatcher and Greater Roadrunner, shows the birds are nesting as much as three weeks later in response to dry winters.

This is in contrast to most studies: the American robin is nesting two weeks earlier on average than it did two decades ago.

“Scientists are showing overall that climate change and warmer weather are leading birds to nest earlier,” says Chris McCreedy, desert ecologist for Point Blue Conservation Science. “The fact that we found the opposite to be true is striking to me.”

The study, by Point Blue and the U.S. Geological Survey, and just published in the journal The Auk, found that after winters of low rainfall, all 13 bird species whose nests they monitored in western Arizona and southeastern California significantly delayed nesting. As a result, these bird species produced fewer young.

Winter rains bring the desert alive, providing much-needed moisture for plants to grow and, in turn, creating habitat for the insects birds need for food. A lack of food most likely delays nesting beyond the usual February-March timeframe.

The Alamo Basin in western Arizona. Photo: Chris McCreedy
The Alamo Basin in western Arizona. Photo: Chris McCreedy

“Global climate change models are showing the Southwest will experience drier winters,” says Charles van Riper III, USGS research ecologist at the Sonoran Desert Research Station in Tucson. “In response, desert birds will nest later and later, their nests will be more vulnerable to predators, especially cowbirds, and they may not produce any young those years.”

From 2004-2010, McCreedy led a team monitoring hundreds of nests within two large wash systems in California’s eastern San Bernardino County and Arizona’s La Paz County. Although water only flows aboveground after heavy rains, desert washes are home to a high diversity of trees and shrubs, such as palo verde and mesquite, and a subsequent higher bird abundance than surrounding uplands.

Two winters during the study, 2006 and 2007, received only 10 percent of average rainfall, making them two of the driest on record. Like other low rainfall years, all 13 species delayed nesting. Loggerhead Shrike, Black-tailed Gnatcatcher and Phainopepla delayed nesting the most, over three weeks later, resulting in very little nestling survival due to predation.

Besides snakes and a few other bird species such as Cactus Wren, McCreedy found Brown-headed Cowbirds to be the primary nest predators. Over the past 100 years, cowbirds have expanded their range into the Southwest, attracted by agriculture and urban bird feeders. They now arrive in the desert in April, just in time to ransack the eggs from late nesters.

A broken Black-tailed Gnatcatcher egg due to predation. Photo: Chris McCreedy
A broken Black-tailed Gnatcatcher egg due to predation. Photo: Chris McCreedy

“Our research shows human impact—climate change, associated drought, subsidization of cowbirds with easy food sources—is impacting the longevity of desert birds,” McCreedy says. “The desert is fragile and it doesn’t take much to upset the balance, making it harder for the birds to cope.”While other studies have documented birds forgoing nesting entirely during drought, McCreedy and van Riper found the Sonoran desert birds did not give up easily and simply delayed nesting into the spring. However, the higher rates of predation later in the breeding season sometimes required pairs to rebuild their nests three or more times after repeated failures. The tenacity takes extra energy and may affect the birds’ health into the next year.

“There’s going to be winners and losers,” van Riper says. “Global climate change is complex and each bird species is going to be influenced differently. Hopefully, further study will provide more answers.”

Main photo: Black-tailed Gnatcatcher

Study Citation:  McCreedy C. & van Riper III, C. (2014). Drought-caused delay in nesting of Sonoran Desert birds and its facilitation of parasite- and predator-mediated variation in reproductive success, The Auk, 132 (1) 235-247.

Learn more:  USGS Report, Projecting Climate Effects on Birds and Reptiles of the Southwestern United States.

Learn more:  visit Point Blue’s press releases page.