In Nature Vs. Nurture Debate, Biologists Find That Genes Drive Cliff Swallows In Group Choice

The Browns say their study shows that genes guide cliff swallows when they select the size of colony in which to live. They say this the first report of heredity influencing an animal's choice of a social system.

The study, "Heritable Basis for Choice of Group Size in a Colonial Bird," appears in the Dec. 19 issue of Proceedings of the National Academy of Sciences.

"We clearly found that individuals have a genetic basis as to where they choose to live," says Charles Brown, a biology professor who has studied cliff swallows along the Platte River in Nebraska for 19 years.

In 1997 and 1998 the Browns switched nearly 2,000 very young birds from nests in big colonies to nests in small colonies, and vice versa. The two biologists discovered that when these birds returned about nine months later to settle into their own nests and start their own families, they chose the same colony size as that in which they were born.

"Our study suggests that there is a genetic difference between birds that choose to live in large groups versus birds that choose to live in small colonies," he says.

The cliff swallows in their study area nest in colonies that range in size from two nests to more than 3,000 nests. Mud nests are usually found on bridges and cliffs.

Each summer Charles Brown and his wife, Mary, a research associate in TU's biological sciences department, band birds and recapture previously banded swallows for many reasons: to determine life span, migration patterns, sex and health. Since 1982 they have banded about 120,000 birds.

For this study, they moved birds that were five days old, placing identification bands on their legs and exchanging half the brood in a nest in a big colony with half the brood in a nest in a small colony. A cliff swallow nest usually has from two to five hatchlings. The parents raised the foster hatchlings along with the other chicks.

Baby cliff swallows hatch in June. Later in the fall they migrate to South America and return north in May. Brown says 36 percent of the 1,968 birds that were banded were caught in the study area where they were born. The Browns had already observed that among the birds that were part of their routine yearly study, offspring tended to select colony sizes similar to those of their parents.

Birds were caught using mist nets, and the Browns recorded the nest preference of the cliff swallows which they had switched between colonies as babies in 1997-98. "Then we were able to see which size colony they settled in and compare their choice to the colony in which they were raised and to the colony in which they were born," says Brown.

They found that birds born and raised in a large colony return to a large colony, and birds born in a large colony -- but raised in a small colony -- also return to a large colony.

Similarly, birds born and raised in a small colony return to a small colony, and birds born in a small colony -- but raised in a large colony -- also return to a small colony.

"They return to where they were born irrespective of when they were raised," says Brown. "They are picking the colonies that their parents picked; so it is not environment, it is genes that appear to be dictating their choice."

He says the hatchling exchange approach rules out environmental effects, such as sites with better food sources or nesting materials, that one might assume bear on a bird's decision.

The Browns, authors of the book "Coloniality in the Cliff Swallow: The Effect of Group Size on Social Behavior," say the benefits and drawbacks of living in groups are well understood, but they wanted to begin to find the underlying basis for the birds' selection of a large or a small aggregation.