Wintering warblers choose agriculture over forest

Large areas of natural forest in the Mexican lowlands have been converted to agriculture, and without enough habitat to go around, researchers speculate that the biggest, oldest birds might claim the choicest spots. To find out, Simon Fraser University's Simón Valdez-Juárez and his colleagues studied Yellow Warblers wintering in western Mexico, counting how many birds were using each of three different types of habitat -- riparian gallery forest, scrub mangrove forest, and agricultural land -- as well as capturing birds to determine their age, sex, size, and likely point of origin. To their surprise, they found the highest density of warblers in agricultural habitat rather than either type of natural forest. There was also little evidence that a bird's body size, sex, or age influenced where it ended up, although females' habitat use differed depending on where they had spent the breeding season.

Irrigated agriculture may be attractive to Yellow Warblers as an alternative to the naturally dry forest habitat, which becomes even drier as winter goes on, or the limited availability of natural land cover may force most birds to occupy agricultural areas. Either way, this preference for agriculture could lead to problems if stressors such as pesticide use reduce the birds' survival. "The implications for migratory bird populations depend on whether the condition and survival of birds wintering in agriculture is lower than that of birds wintering in natural habitats or not," says Valdez-Juárez. "If it is lower, it might cause localized declines, as females from the contiguous U.S. and western Alaska were more likely to use agricultural habitats and lower female survival has been implicated in the population declines of other warblers."

"With increasing agricultural intensification across the ranges of many long-distance migratory songbirds, it is critical to determine if they are using these new habitats. This new study shows that yellow warblers overwintering in western Mexico not only use agriculturally dominated habitats but are more common there than nearby sites with more natural landcover," adds University of Manitoba Assistant Professor Kevin Fraser, an avian behavior and conservation expert who was not involved in this research. "They also report that more southern breeding females are more likely to use agricultural habitats than more northern breeding females, suggesting any impacts of overwintering habitat use may differ by breeding latitude. These results are important, as they highlight the need to investigate whether regions undergoing agricultural intensification are providing viable habitat for overwintering migrants, and how their use may carry-over to impact survival or fecundity in subsequent seasons."