Cliff swallows do not put all their eggs in one basket, a study of the bird's nesting behavior has found.

The birds commonly take an egg into their bills from their own nest and fly it to another swallow nest. Two Yale University biologists working in Nebraska reported in last week's Nature that this was the first clear evidence of birds moving their eggs to "parasitize" other nests. It is well known that many birds, including cliff swallows, lay eggs in other birds' nests.

The biologists, Charles R. and Mary Bomberger Brown, saw birds moving their own eggs and established that the practice was common by marking eggs in nests throughout a colony and then checking on later days to find that marked eggs had been moved. Of nearly 5,000 nests observed, more than 6 percent had been parasitized.

The behavior is explainable through the theories of sociobiology, the study of how evolution has shaped the instinctual behavior of social species. Natural selection favors the survival of individuals whose behavior improves the survival chances of their own offspring. To the extent that such behavior is governed by inherited "programming" of the brain, the offspring inherit the programming.

Thus, if the survival of cliff swallows is threatened by destruction of the nests, which may well be the case when the birds build their mud nests on windy cliff walls where rockfalls are common, parent birds may well improve the chances of having some surviving offspring by putting eggs in several nests.