Tropical Urbanization

How does it affect birds?

Many of us share our northern backyards with generalist birds such as jays, robins, finches, or doves. Some migratory birds are more specialized and need forested habitats to breed, such as the Acadian Flycatcher, Cerulean Warbler, and Bicknell’s Thrush. Population declines of many Neotropical birds have been linked to urbanization on their breeding grounds. A recent study by Amanda Rodewald and her colleagues found that within urban landscapes, Acadian Flycatchers nested later and less often, and suffered higher rates of brood parasitism, than those in rural settings.

Acadian Flycatchers, like other Neotropical migrants, spend about half their time in tropical regions, but little is known about how urbanization affects them there. This is of great concern given increasing rates of urbanization in developing countries.

The United Nations estimates that in 20 years, four out of five people in Latin America will live in urban areas, a 60 percent increase in just half a century. Much of this change will involve conversion of natural and agricultural land, where many species of migratory birds winter, into urban habitats. How will increasing urbanization affect these populations?

Many Neotropical migrants, and residents such as this Pale-billed Woodpecker, decline with tropical urbanization.

Chris Wood

One study has provided some clues. Ornithologist F. Gary Stiles surveyed the University of Costa Rica campus, a green zone within the capital of San Jose, in 1968 when the campus was surrounded by traditional shaded coffee and rural development. The survey was repeated 20 years later, after the campus had become surrounded by dense urban and suburban development, with increasing noise pollution, airborne contaminants, and altered habitat. Half of the migratory bird species decreased in abundance over the 20-year span. The birds that had experienced the biggest declines were habitat specialists associated with forested areas, shaded coffee farms, and streams and rivers. Some species associated with pastures and fallow fields also declined. The species that increased in abundance were wide-ranging generalists associated with urban and open areas. Recent work in Costa Rica by Jeffrey Norris found similar effects of urbanization on bird communities across cities, but these effects varied between geographical regions.

Despite these and other efforts, more research is needed on how urbanization affects the distribution as well as survival of migratory birds on their wintering grounds. Such information will help guide conservation action aimed at preventing further population declines of migratory birds.

What can you do to help? Whenever possible, choose coffee and cacao certified as “Bird Friendly,” “Shade-grown Organic,” or “Fair Trade,” from certification programs such as the Smithsonian Migratory Bird Center’s. This supports farmers using sustainable methods to grow their crops, which protects at least some habitat from encroaching urbanization. Your membership and support of the Cornell Lab of Ornithology helps expand our avian research and monitoring efforts in Latin America through our Neotropical Bird Conservation Program. When you visit the tropics, you can also help our scientists and colleagues monitor changes in bird distribution by entering your observations into eBird (www.ebird.org).

—Viviana Ruiz-Gutierrez, doctoral candidate,
Cornell University Department of Ecology and Evolutionary Biology

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu