Press Releases

Freeloading Fledglings Mirror
Human Behavior

Study finds young male bluebirds
reluctant to leave home

A Western bluebird fledgling, color-banded at UC Berkeley's Hastings Reserve in Carmel Valley. (Copyright 2004 by Neil Losin)

October 25, 2005

October, Ithaca, N.Y.--Fascinating new research on the Western Bluebird reveals that human parents are not alone in having to contend with offspring unwilling to leave the nest.

Janis Dickinson, citizen science director at the Cornell Lab of Ornithology, lead the study while at the University of California, Berkeley. Dickinson says fledgling female bluebirds do fly off on their own in late summer. But their brothers typically hang around through the winter and into the next breeding season, living off the bounty of their parents' larder or, as Dickinson calls it, their "resource wealth." For Western Bluebirds that means mistletoe berries, an abundant winter food source on California oaks. Dickinson manipulated this food source to see what the filial hangers-on would do. Once berries got scarce, mother's love wasn't enough to keep the sons at home. Dickinson says, "What our experiment shows is that reducing resource wealth causes sons to leave their parents, even if it means giving up nepotistic benefits."

Dickinson found that while half of all male offspring normally stick around during the winter, only eight percent stayed when she removed half the mistletoe from trees in the parent bluebirds' territory. The results of her study with coauthor Andrew McGowan, a former Berkeley researcher now at the Marine Turtle Research Group in the United Kingdom, were reported online last month in The Proceedings of the Royal Society B and will appear in print later this year.

Dickinson notes that development along California's central coast is leading to clear-cutting of the oaks that support mistletoe and provide winter food for the Western Bluebird. Much of this land is being converted into vineyards, which may, in turn, accelerate the decline of Western Bluebirds. Dickinson added, "We have learned much at Berkeley about the potential cascading effects of removal of habitat on the social lives and winter survival of native birds, from Western Bluebirds to Acorn Woodpeckers. Our work suggests that the biodiversity of central coastal California will be determined largely by how we treat the old-growth oak forests in the next decade."

The research was funded by the National Science Foundation. The full news release may be found at http://www.berkeley.edu/news/media/releases/2005/10/24_blue.shtml