Cold-hearted though it may seem, a breeding bird is sometimes willing to sacrifice its young to save itself, perhaps to breed again. Now researchers report that birds take two factors into account when deciding whether to risk delivering food to the nest in the presence of a predator: the number of their young and their own likelihood of surviving the encounter.
The new study started with an earlier observation by physiological ecologist Cameron Ghalambor of the University of California, Riverside, and his colleague Thomas Martin at the U.S. Geological Survey in Missoula, Montana. They documented that birds in the Northern Hemisphere tend to lay more eggs than do similar species in the Southern Hemisphere. Then they examined if the number of eggs affected the parent birds' behavior, in particular, its willingness to return to the nest to feed their chicks when confronted with a predator.
Ghalambor and Martin compared the reactions of each of five Argentinian species--a flycatcher, a thrush, a wren, a sparrow, and a warbler--to their closest relatives in Arizona. For each species, they tested the parents' reactions to recordings of calls from a hawk, which attacks adults; a jay, which attacks chicks; or a nonthreatening stuffed tanager. In Arizona, the birds were spooked more by the chick-eating jay than the hawk, while in Argentina the reverse was true, they report in the 20 April issue of Science.
"There is a trade-off between survival and reproduction," explains Ghalambor. Northern birds are less likely to survive the winter and have put all their eggs in one nest, so to speak. They do everything they can to care for those eggs. Southern birds produce fewer eggs at one time but usually breed more than once and thus value their own survival more, he suggests.
The study is one of the first to show how animals make trade-offs in behavior, and is exciting in that it demonstrates "birds have the cognitive ability to react [differently] to certain kinds of predators," says Jeffrey Brawn, a population ecologist with the Illinois Natural History Survey in Champaign.