When a young cliff swallow begins to breed, it seeks a nesting colony similar in size to the one its parents lived in. Surprisingly, the preference for group size is largely determined by the birds' genes, according to a study in the 19 December Proceedings of the National Academy of Sciences. This is the first time a preference for group size has been shown to have such a strong genetic basis.
Most scientists thought that a bird's preference for a certain number of companions had to be determined at least in part by life experience or by ecological conditions such as food abundance or the threat of predators. But no such ecological factors turned out to be responsible during the 2 decades that behavioral ecologists Charles Brown and Mary Bomberger Brown of the University of Tulsa have been studying cliff swallows along Nebraska's Platte River. As a last resort, they examined inheritance. "Frankly, we didn't really expect to find a strong [genetic effect]," Charles Brown says.
The Browns leg-banded 2000 nest-bound baby swallows. In subsequent years, they were able to relocate as many as 700, and most were breeding in a colony about the same size as the one they were raised in. But that didn't yet clinch the case for a genetic basis. So the duo also ran experiments in which hundreds of banded chicks were swapped between colonies of different sizes, which range from just two nests to as many as 3700. When the fostered birds went to breed, they chose colonies that matched their birth colony in size, avoiding those the size of their foster colony.
It's not yet clear why the birds made these choices. Perhaps the preference is linked to other heritable traits, Charles Brown says; for instance, birds that are genetically susceptible to parasites may be selected to avoid large colonies, where the risk of parasitism is higher. The Browns are now studying whether hormonal differences may lead some birds to avoid the stress of large colonies.
The study is remarkable because it was carried out in a natural setting, yet boasts a huge sample size and good controls, says Lee Dugatkin of the University of Louisville. "It's phenomenal they were able to pull that off." While plenty of studies have addressed the consequences of group size preference, Dugatkin says, the Browns demonstrate a cause for such behavior, providing "one of the puzzle pieces that's been missing."