Farmland would seem to be a bad neighborhood for forest-dwelling birds, because nest predators easily infiltrate scraps of forest that border fields. And yet, these woody strips may actually be safer than thick forests for many songbirds. In new results reported in the December Ecology, researchers have found that birds in river habitats of the western United States lose fewer young to predators when they nest near agricultural landscapes than near dense forests.
Narrow ribbons of deciduous riparian forests are prime breeding habitat for many nesting birds in the West, such as the American robin and yellow warbler. Ecologists expected that when adjoining coniferous forests were replaced by agricultural fields, the fields would attract magpies and other generalist predators. To test this assumption, ecologist Joshua Tewksbury and colleagues at the University of Montana, Missoula, spent two breeding seasons watching 1916 nests at 16 sites in western Montana. Half the sites were in primarily agricultural areas, while the others were in forests.
The team found that the intensity of nest predation was greatest for sites where the riparian forest was nestled in a larger pine forest. Those nests lost up to 40% more eggs and young to predation than those in riparian forests surrounded by agricultural land. "I was definitely surprised," says Tewksbury. One reason for the higher mortality: As forest cover thickened, so did the abundance of red squirrels, a common arboreal nest predator.
"The discovery that certain agricultural landscapes can be better for nesting success is definitely new," says Scott Robinson, an avian ecologist with the Illinois Natural History Survey in Champaign. And the implication that fragmentation affects birds by shifting the demographics of their predators, he adds, "is an incredibly interesting result."