Wanted: secluded, shady nest, streamside view; for a small forest bird, real estate may boost sex appeal

Natural History, May, 2002 by Peter J. Marchand

I followed quickly along the stream bank as the lanky figure ahead of me skillfully negotiated the undergrowth and low branches with the awkward tools of his trade. Carrying long bamboo poles over his right shoulder, with bundled mist nets hanging from them, the man looked like the classic hobo. But vagabond he was not. Bob Mulvihill is an accomplished ornithologist, and on that spring afternoon he was pursuing his newest discovery, charged with energy and anticipation.

Suddenly Bob stopped. Eyes fixed on a bird, he slowly lowered his gear to the ground with one hand while raising his binoculars with the other. In an instant the silence exploded. "Black-white-metal-black-orange," Bob shouted back to an assistant, reeling off the band colors on the bird's legs. "That's the polygynous male!" Banded the previous year, the bird had only recently returned from wintering grounds in Central America, and already it had been seen at an upstream nest. But here it was in the vicinity of a second nest, moving defensively and carrying an insect in its bill, as if it had fledglings in the immediate area.

The bird was a Louisiana waterthrush. Unusual among warblers in its preference for streams, this waterthrush is not restricted to the South, as its name might suggest, but can be found breeding throughout eastern U.S. deciduous forests. This species was thought to be strictly monogamous, as are the majority of perching birds, until Bob and his assistants at the Powdermill Biological Station in western Pennsylvania discovered the polygynous male. (Tim O'Connell, of Penn State University, made a similar observation in central Pennsylvania at the same time.) And like most such discoveries, this one was serendipitous. When the banded male was seen near a nest in what was thought to be another male's territory, Bob was immediately suspicious and began watching it closely. Within several days, he witnessed the bird setting up housekeeping with a second female downstream, so he and his assistants erected observation blinds near each nest. Soon they saw the male and the first female feeding young at the upstream nest, but a few days after these fledged, the same male turned up at the downstream nest to help a second female with newly hatched chicks there. Bob estimated that this male provided about 60 percent of the food for the upstream nestlings and about 25 percent for the downstream chicks.

By most counts, only about fifteen species of North American perching birds are regularly polygynous, with an equal number occasionally so. Unlike waterthrushes, most birds known to be polygynous breed in marshes, prairies, or savanna-like habitats. Red-winged and yellow-headed blackbirds are among the most familiar; males of both species maintain harems of up to eight females in prime wetland territories. In marshy habitat, boat-tailed grackles, sharp-tailed sparrows, and marsh wrens, in addition to other blackbirds, are also commonly polygynous. Grasslands and prairies support a number of polygynous species as well, including meadowlarks, bobolinks, dickcissels, lark buntings, and great-tailed grackles.

For all their apparent differences, marshes and prairies share physical characteristics that appear to favor polygynous mating systems among perching birds. In both of these treeless habitats, not only food but also good nest sites are concentrated near ground level, where the territorial males can defend them more easily. A male controlling an especially desirable territory often attracts more than one female, even though other, still-unmated males may be available in neighboring territories. In sharing a mate rather than having one all to herself, the female may sacrifice some of the male's assistance in feeding their offspring. This often results in smaller fledglings. But the trade-offs--which might include safer nest sites or more abundant food--can mean that more of the chicks survive. (In a recent study of great reed warblers in northern Europe, polygyny was common in areas of low nest predation, while males holding territories that were more at risk often went unmated.)

Polygyny is uncommon in forests. On rare occasions, birds such as American redstarts, hooded warblers, and black-throated blue warblers engage in polygyny. But because food, such as insect prey, is less concentrated in a forest than in a marsh (being distributed from the ground to the treetops), females and young benefit when males can give them full attention. Why, then, would a female Louisiana waterthrush on this densely wooded stream choose a partner already mated with another female and whose time would have to be divided among the young of two broods?

Watching the male at the Powdermill reserve gave me a hint at the answer. As it picked through the leaf litter for insects--its dietary staple--the bird soon worked its way down to the stream, where it hopped from rock to rock and occasionally flipped a leaf out of the water, all the while bobbing its tail. As I followed it with my binoculars, the male suddenly darted for a salamander, and then struggled with it momentarily before the amphibian broke free. Louisiana waterthrushes feed almost entirely on the ground and in eddies of flowing water, and though not often reported, salamanders, along with crustaceans and freshwater mollusks, may be among the more exotic food items of this species.