Bluebird moms inadvertently fuel wars between species
Extra hormones delivered to sons in tough times have far-reaching consequences
By Susan Milius
Bluebird wars begin with the power of vexed moms in their nests.
Experiments show that female western bluebirds (Sialia mexicana) harassed by other species competing for nesting cavities tend to lay eggs with abundant testosterone, says evolutionary ecologist Renée Duckworth of the University of Arizona in Tucson.
Sons from these eggs grow up to be especially aggressive and likely to leave their parents’ territories far behind in seeking breeding space of their own, she and colleagues say in the Feb. 20 Science. When these restless, aggressive males discover new swaths of burned-over land, a paradise for bluebirds, they often drive away the milder-mannered mountain bluebirds (S. currucoides) that settled there first. The invaders can take over in just five or six generations.