Cowbirds get head start with egg tricks.

Even before breaking out of the egg, the brown-headed cowbird sabotages the nestmates whose home it has usurped.

Several tricks enable the cowbird to win or tie the race to be the first egg to hatch, report D. Glen McMaster of Saskatchewan Wetland Conservation Corp. in Regina and Spencer G. Sealy of the University of Manitoba in Winnipeg. In the February Condor, they analyze cowbird eggs laid in the nests of yellow warblers, who tend baby cowbirds even if in doing so their own young starve.

By hatching first, the cowbird gets a head start on feeding and becomes the biggest, grabbiest nestling. It overwhelms the young of its host not by pushing them overboard, but by stealing dinner.

Researchers have speculated that cowbirds get their head start by prolonging the incubation needed by eggs of smaller birds. McMaster and Sealy tested the idea by comparing the amount of time yellow warbler eggs took to hatch in more than 41 nests with a cowbird egg and 26 without. The cowbird egg added about a day and a half to the normal 11-day incubation of yellow warblers, the researchers report. Tests in incubators supported the conclusion.

Eric K. Bollinger of Eastern Illinois University in Charleston, who has also studied cowbirds, welcomes McMaster and Sealy's robust demonstration of the prolonged incubation. "They were the first to show it well," he says.

"It's simply a heat-shielding phenomenon," Bollinger speculates. Because the cowbird egg is roughly twice the size of the warbler eggs, it keeps the incubating parent from making optimal contact with those eggs.

The new study also suggests that brownheaded cowbirds' eggs develop unusually quickly for their volume. After testing the eggs in incubators, McMaster and Sealy calculate that the cowbird's development indeed outpaces the warbler's.

Cowbirds; might even be able to accelerate their hatching after picking up clues that nestmates are ready to break out, the researchers suggest. One hint might be the tiny clicking sounds an embryo makes as it nears hatching and starts to breathe. That clicking seems to help ducks hatch in unison.

McMaster and Sealy checked to see whether cowbird eggs in incubators hatched faster when nestled against warbler eggs. The difference didn't reach statistical significance. "I don't think it completely kills the idea," McMaster says, noting that incubation failures shaved his sample to just a few eggs. Bollinger isn't ready to dismiss the notion either. "The data are suggestive," he says. "Its certainly a clever idea."

Cowbirds are blamed for decreasing the populations of cherished species and threatening the survival of such rarities as the willow flycatcher. Yet the declines are hardly all the cowbirds' fault, McMaster observes. The willow flycatcher has only 10 percent of its original habitat, whereas the cowbird's habitat has expanded as a certain other species has cut down forests. "Is the cowbird really the villain?" he asks.