Bluebird moms inadvertently fuel wars between species

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.

In the roomy new habitat, western bluebird moms don’t juice up their eggs with extra hormones. In time, new generations of less aggressive, stay-nearby males replace the tough guys —until fights over nesting sites stress out the moms again.

Scientists have already determined that mothers matter, having profound influences over their offspring, says Ben Dantzer of the University of Michigan in Ann Arbor. “But we don’t know very much about their evolutionary and ecological consequences, especially in wild animals,” he says.  And, he adds, tracing any small, local process, mother-related or not, to its large-scale consequences for ecology is an achievement in itself.

Duckworth and her colleagues have been following large-scale bluebird wars between the western and mountain species in the northwestern United States for 15 years. Both species thrive where forest fires leave plenty of cavity-riddled damaged tree trunks for nesting and create new open space rustling with grasshoppers and caterpillars to eat.

Earlier research had hinted that western bluebird mothers’ hormone contributions to eggs might play a role in starting the waves of aggressive invaders. But to see what might cue that, Duckworth and colleagues set up extra nest boxes to take the pressure of competition off some females, but left others in a state of frequent squabbling.

The competition for the mothers doesn’t come from any bluebird species, Duckworth says. Blue-on-blue fights over nest cavities get settled before eggs start forming in the females’ bodies. Instead, the researchers saw other cavity-nesting species, particularly tree swallows, harass the western bluebirds during the egg-forming phase.

Hormone tests of eggs showed that western bluebird mothers facing more competition dosed their eggs with more testosterone and related compounds. In these clutches, the mothers laid eggs holding males earlier in the laying order, but all the offspring were hormonally nudged toward aggression.

The daughters tend to leave their home territory for new breeding spaces even without that hormone addition. So just what their mother’s tiffs with neighbors mean for females is something Duckworth hopes to find out.