Tough-guy bluebirds need a frontier.

Among western bluebirds, the scrappier males push into new territory first. But mild-mannered dads eventually take over, a long-term analysis finds.

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Western bluebirds (Sialia mexicana) are recolonizing their former range in valleys of western Montana, say Renee Duckworth of Harvard University and Alex Badyaev of the University of Arizona in Tucson. Logging and farming in the late 1930s wiped out old trees with good nesting holes. In the past 40 years, though, people have setup nest boxes for bluebirds.

The first species to move in was the mountain bluebird (Sialia currucoides). Western bluebirds, however, have been rapidly kicking them out of the territory.

This ongoing bluebird switch offers a rare chance to study how behavior affects a species' range, says Duckworth. She and Badyaev traced bluebird history through 3 decades of records. And from 2001 through 2005, Duckworth tested male bluebirds in eight study areas for aggression.

Western males leading the takeover of nesting sites ranked high on aggression. Westerns that stayed near their birthplaces ranked low. However, in the 5 years after the westerns conquered an area, the researchers saw a decline in average male aggression.

Aggressive males make terrible dads, says Duckworth. They rarely feed chicks, and their offspring aren't as likely to survive as a less aggressive male's are. Hot-tempered birds are the ones that disperse readily and continue to press on into new territory. In their wake, the quieter, fatherly types settle in and establish a population, Duckworth and Badyaev report in the Sept. 18 Proceedings of the National Academy of Sciences.--S.M.