While female fledglings fly off on their own in late summer, their brothers typically hang around through the winter and into the next breeding season, living off the bounty of their parents' larder.
As with humans, though, as the money runs low, the kids split, according to a new study by scientists at the University of California, Berkeley.
Janis Dickinson, a research associate with the Museum of Vertebrate Zoology at UC Berkeley and a newly appointed associate professor of natural resources at Cornell University, discovered parallels between human and bird families while studying the evolution of delayed dispersal, or natal philopatry - the tendency for offspring to stay at or near home rather than look for a new place to live and breed. Such behavior is common among cooperatively breeding birds as well as humans, and, at least in birds, it leads to close-knit families.
"The idea that inherited wealth promotes family stability comes from studies of cooperatively breeding birds and helps expand our theoretical understanding of the evolution of human social behavior," said Dickinson, who also directs the Citizen Science program at the Cornell Laboratory of Ornithology. "What has been lacking is an experimental demonstration that resource wealth promotes delayed dispersal, leading to the continuation of relationships between offspring and parents prerequisite for cooperative family life."
For bluebirds, "wealth" consists of mistletoe berries, an abundant winter food source growing on California oaks. Since the bluebirds also could be staying at home because they're babied by their parents - they're treated far better there than they would be outside their parents' territory - Dickinson manipulated this food source to see what the filial hangers-on would do.