MADISON, WISCONSIN--As the Midwestern prairies were reduced to tiny islands in a sea of farms, grassland birds became scarce. Now a study shows the birds aren't simply lacking breeding territory: Predators from neighboring woods creep far into grasslands to prey on chicks. The best sanctuaries for such birds, the researchers suggest, may be large tracts of grassland bordered by farm fields and pastures.
When people settle in wilderness, they break up the habitat, either by cutting forests and plowing fields or by planting trees as windbreaks in grasslands. And fragmented habitat is tough on animals. Songbirds, for example, face more predators when their nests are near the edge of a forest. But similar studies on grassland bird species have been ambiguous.
To see which predators snack on birds nesting in grassland and to gauge the damage, ornithologist Rosalind Renfrew of the University of Wisconsin, Madison, and colleagues spent 3 years observing five bird species: bobolinks, savannah sparrows, grasshopper sparrows, and eastern and western meadowlarks. They filmed nests around the clock on 12 pastures in southwestern Wisconsin. Grassland predators such as snakes and badgers killed their share of chicks, but a full 44% of the baby birds were snatched by woodland predators like raccoons and opossums, Renfrew reported here 9 August at the annual meeting of the Ecological Society of America. Nests nearest the edge were most likely to be raided. The woodland predators ventured up to 190 meters into the pastures, so Renfrew says that large, open fields surrounded by grassland and farms offer the best opportunity to let grassland birds raise their young in peace.
"She took a really nice approach," says ecologist Paige Warren of Arizona State University. But Warren cautions that the researchers still need to show that birds near the edge are fledging fewer chicks.