for National Geographic News
Like some young adults, male western bluebirds sometimes stay with their parents even after they are grown, sharing their parents' food supplies.
But if the parents aren't wealthythat is, if they don't have plenty of foodthe sons leave home.
Apparently, only wealthy families stay together for what bird experts call "prolonged brood care."
Western bluebirds are one of the very few species of birds in which the youngest generation helps its parents at the nest the following year. The species is even more unusual in that this behavior occurs only rarely.
Researchers described the findings last month in the London-based journal, Proceedings of the Royal Society B.
In central California, western bluebirds live in stable groups that form when sons stay home over the winter. Daughters usually disperse in late summer and are replaced by female immigrants.
This makes evolutionary sense. "Most scientists studying dispersal believe that one sex disperses farther than the other to reduce the likelihood that close relatives will mate with each other," said Janis Dickinson, a behavioral ecologist at Cornell University and the University of California, Berkeley.
"Incest is thought to be costly because it unmasks harmful gene copies," Dickinson, the study's lead author, added.
But researchers wanted to know why sons sometimes stay home over the winter and sometimes depart.
Bluebirds depend on mistletoe berries as their main winter food. Families of birds hold territories with an abundance of the berries. Bluebird pairs are faithful and use the same territories year after year.
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