Flying flirt. Male hooded warblers travel a lot in search of extramarital sex.

Your Forest or Mine?

The males of a bird species thought to live exclusively within forests while breeding actually fly frequently across other kinds of habitat, researchers have found ... and they do so to search for extramarital sex. The finding shows how probing an animal's behavior can help determine its conservation needs.

Ornithologists have long voiced concern over human fragmentation of forests. They've found that birds that dwell inside a forest fragment often suffer from decreased availability of food, more attacks on their nests from predators, and increased egg parasitism. Such factors are thought to determine the minimum size of forest patches birds need.

Like many temperate-breeding migrant birds, hooded warblers in continuous forest cheat on their partners so often that fully one-third of their offspring result from extrapair copulations. You might expect their sex lives to be much duller when warbler pairs are isolated in remote fragments. "Wrong!" says behavioral ecologist Bridget Stutchbury of York University in Toronto; DNA fingerprinting has shown that 20% of the young in fragments are sired by intruding males. So Stutchbury and her student Ryan Norris, now at Queens University in Kingston, Ontario, tracked male warblers using radio transmitters and closely observed their behavior. As it turned out, the males left their home fragments up to several times a day, and what they were looking for were new females.

The good news for avian conservation is that male hooded warblers frequently cross gaps of up to 450 meters, and they don't seem to require corridors connecting forest patches. "These 'shy forest birds' are boldly flying across cornfields," Stutchbury says. However, shuttling among fragments takes energy, which is why larger tracts of forest are a more attractive place to live. Indeed, many small fragments lack any warbler pairs at all, even though in the warbler's case, they are no worse than continuous forest in terms of food, predation, or parasitism.

Other researchers praise the work for tying sexual behavior to habitat use. Biologists have made many assumptions about the effects of fragmentation without looking closely at animals' behavior, says behavioral ecologist Steven Lima of Indiana State University in Terre Haute, "so we can scarcely avoid being surprised when we start following animals around to see where they're going and what they're doing."

Related sites

Bridget Stutchbury's home page
Hooded warbler information from USGS
Primer on forest fragmentation