Fragmentation May Limit Songbird Sex Lives

"Our results suggest a new link between social behavior and habitat choice in fragmented landscapes," say Ryan Norris, who did this work at York University in North York, Ontario, Canada, and is now at Queen's University in Kingston, Ontario, and Bridget Stutchbury of York University. This study is in the June issue of Conservation Biology.

Norris and Stutchbury studied the effects of fragmentation on male hooded warblers, small yellow songbirds with black caps and throats. To see if fragmentation restricts their movements, the researchers radiotracked 20 of the warblers in forest fragments separated by agricultural fields in northwestern Pennsylvania. Ranging from 1.7 to 6 acres, these isolated fragments were similar in size to the warblers' breeding territories in continuous forest.

Unexpectedly, Norris and Stutchbury found that these low levels of isolation did not restrict the birds' movements. Rather, males in fragments spent far more time out of their territories than those in continuous forest (about 16% vs. 5%) and also flew farther. These extra costs of living in fragments could explain why males occupy on about a fifth of those that are big enough for territories.

Norris and Stutchbury got another unexpected result that could explain why male hooded warblers living in fragments put so much effort into leaving them. About 60% of their forays were to woodlots occupied by another pair, suggesting that they travelled between fragments primarily to mate with other males' females.

"This 'need' for sex may explain why long-distance forest migrants actually avoid settling in extremely isolated forest fragments. In other words, if there are no opportunities to cheat on your mate then it's not worth settling in certain highly fragmented areas," says Norris.