Three’s A Crowd

 Threes A CrowdClimate change is pushing birds of different species to nest in the same areas, making them more vulnerable to predators, scientists report in Global Change Biology.

The shift resulted from a cascade of events that started with global warming. Higher temperatures have reduced snowpack at a high-elevation site called Mogollon Rim in Arizona. Elk normally move downslope in the winter because it’s hard to look for plants to eat in the deep snow. Now more elk are staying put and continuing to gobble trees. As a result, deciduous trees in the area are declining.

The study authors wondered how these environmental changes were affecting three songbird species, which build nests on the ground by different types of trees. The orange-crowned warbler likes to nest by maple trees, the Virginia’s warbler prefers locust, and the red-faced warbler prefers fir.

The team recorded plant density at sites around Mogollon Rim from 1989 to 2009 (except in 1990). The researchers also searched for nests and noted whether each nest produced at least one successful fledgling or was attacked by predators. Over those two decades, the study covered 1,684 nests and 10,532 sites.

The density of trees favored by the three warbler species dropped during the study, the team found. As a result, the number of available nest sites for orange-crowned and Virginia’s warblers also decreased, and the three species’ nesting areas increasingly overlapped.

The more the nest sites overlapped, the more likely predators were to attack them. Predators might have an easier time finding the nests because they don’t have to search as many sites for their prey, the researchers say. Roberta Kwok | 2 November 2012

Source: Auer, S.K. and T.E. Martin. 2012. Climate change has indirect effects on resource use and overlap among coexisting bird species with negative consequences for their reproductive success. Global Change Biology doi: 10.1111/gcb.12062.

Image © Dan Pancamo | Wikimedia Commons

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