re of remaining with parents that could cause offspring to stay at home," Dickinson said. "The Western bluebird fledglings stay on their parents' territory living in cohesive groups where they sleep together in a cavity or nest box or in the mistletoe, forage together, and the mothers are nicer to their sons than they are to other males of the same age, even if those birds are members of their winter group. Since mothers show nepotism towards their offspring, this could also play a role in why sons stay. 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 8 percent stayed when she removed half the mistletoe from trees in the parent bluebirds' territory.

"Animals, including humans, can accumulate resources by staying in one place, or they can stay in one place because of the benefits of access to accrued resources," she said. "This is a first demonstration that experimentally manipulating the resource itself reduces the phenomenon of delayed dispersal, which means that the wealth - the amount of resources the parents have - causes delayed dispersal."

The results of her study with coauthor Andrew McGowan, a former museum 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.

"This experiment supports the idea that relative wealth of resources is the starting point for the evolution of philopatry and kin-based sociality," as proposed a decade ago by Cornell biologist Steve Emlen, the authors wrote.

Dickinson conducted her studies at UC Berkeley's Hastings Reserve and nearby ranches in Carmel Valley, Calif., where for more than 20 years researchers have been banding Western bluebirds, Sialia mexicana, and trac
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Contact: Robert Sanders
rsanders@berkeley.edu
510-643-6998
University of California - Berkeley
24-Oct-2005

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