Urbanization, scientists say, can be either good or bad for birds.
It can be beneficial because suburban areas typically provide abundant food resources, including backyard feeders, which help seed-eaters, and they are dominated by ornamental and invasive plantings, which have been found to be essential for fruit-eaters, especially in winter.
But urban and suburban regions also harbor free-roaming cats and other mammalian predators, as well as raptors and crows, meaning city birds may face higher rates of predation than their country cousins. Worse, collisions with buildings and automobiles, higher rates of disease transmission (especially around feeders), and exposure to elevated levels of pollutants such as lead are all costs of living in urban areas.
So which effects dictate avian survival more, the good or bad?
To find out, Brian S. Evans and other researchers from the University of North Carolina at Chapel Hill and the Smithsonian Migratory Bird Center studied seven common species in the metropolitan Washington, D.C., area for over a decade and then compared where on the rural-to-urban gradient they fared best.
Four of the seven — American Robin, Gray Catbird, Northern Cardinal, and Song Sparrow — survived better in suburban and urban areas than at the rural end of the spectrum. All are habitat generalists whose diets consist largely of fruit and invertebrate resources.
The three others — Carolina Chickadee, Carolina Wren, and House Wren — had low survival rates everywhere but appeared to do best in rural areas. All three nest in cavities. Those in urban and suburban areas, suggest the researchers, may have found fewer cavity trees or experienced greater competition for them from starlings or House Sparrows.
“Cumulatively, our results suggest that the effect of urbanization influences avian vital rates,” write the researchers, “but that the regulatory mechanisms may be species-specific.”
The area of developed land in the United States is projected to nearly double between 2000 and 2025.
A version of this article will appear in the February 2015 issue of BirdWatching. Subscribe.
Read the abstract
Brian S. Evans, Thomas Brandt Ryder, Robert Reitsma, Allen H. Hurlbert, and Peter P. Marra. In press. Characterizing avian survival along a rural-to-urban land use gradient. Ecology. Abstract.