Among western bluebirds, the scrappier males push into new territory first. But mild-mannered dads eventually take over, a long-term analysis finds.
Western bluebirds (Sialia mexicana) are recolonizing their former range in valleys of western Montana, say Renée 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 set up 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.