Songbirds Stressed in Winter Grounds

Minnesotans may dream of relaxing winter escapes to the Caribbean, but not the American redstart. Winters down south are a time of stress for this migratory songbird, and a lean winter can dampen reproductive success during the next breeding season up north. The finding, reported in tomorrow's Science, suggests that land managers must protect all the habitat of migratory birds.

Although researchers have long sought to examine the relative influence of northern and southern habitats on the breeding of migratory birds, it's tough to track songbirds. A new approach taken by the researchers--avian ecologists Peter Marra and Richard Holmes of Dartmouth College in Hanover, New Hampshire, and Keith Hobson of the Canadian Wildlife Service in Saskatchewan--relies on an isotopic marker, carbon-13, that varies with winter habitat type. Plants in wet mangrove or wet lowland forests have less C-13 than do plants typical of dry scrub, a poorer habitat. This isotopic signature is passed up the food chain and shows up in the birds' blood.

Marra captured, weighed, and bled redstarts at the beginning and end of the winter season in Jamaica and Honduras. He found that birds that had feasted on the teeming insects in mangroves and wet lowland forests had maintained or gained weight, while the scrub-dwellers had lost up to 11% of their body mass. Back in the New Hampshire forest, the researchers found that first arrivals carried the wet forest signature; they were mostly older males who had fueled up for an early departure from the south. This gave them first pick on the prime breeding spots and, as previous studies have shown, better success in mating.

"Before now, no one has ever even come close to linking up quality of habitat in winter with reproductive success in the breeding grounds," says avian ecologist Scott Robinson of the Illinois Natural History Survey. And there's a clear take-home message for conservation, adds behavioral ecologist Sidney Gauthreaux of Clemson University in South Carolina: "You may do everything on the breeding grounds to protect these Neotropical migrants, but we can still lose the species by having the winter habitat destroyed."