After a wildfire tears through a forest, life slowly starts to be restored. Insects make their homes in dead trees, attracting the woodpeckers who eat them. The woodpeckers leave holes in the trees where they hunt, which many other birds—including the Western bluebird and the mountain bluebird—use to make nests. But post-fire, competition for these nest sites can be fierce. Some birds are better equipped to handle this type of conflict, thanks to an edge given to them by their mothers before they're even born. 

Mountain bluebirds almost always arrive first to new nesting grounds, but aggressive Western bluebirds quickly show up and bully their cousins out of their nests. Before long, Western bluebird pioneers are joined by less aggressive latecomers; for a while, the Western bluebird chicks born in the new territory tend to be more docile than the founding birds. But as the population grows and nest cavities get filled, another generation of aggressive babies are born and boldly fly off to chase mountain bluebirds out of yet another scorched forest. 

Renee Duckworth, a biologist at the University of Arizona, has been following the bluebirds’ territorial battles for more than a decade and has seen the predictable pattern play out over and over. She wanted to get to the bottom of this cycle: What made the invading Western bluebirds so aggressive, then docile, and then aggressive again?

Research on other animals suggests that mothers can influence certain characteristics of their developing children via hormones, priming the kids for the environment they’ll be born into. Duckworth also knew that bluebird males who are hatched earlier tend to be more aggressive than their brothers who hatch later. She and her team decided to focus on bluebird moms to see if, and how, they might be creating winged conquerors who would go on to shape whole bird communities. 

The researchers studied hundreds of bluebird nests in Montana during different stages of their population cycle, keeping a close eye on the moms and measuring the hormone levels in their eggs. They found that when there were plenty of empty cavities around for mother birds to nest in—when the "slower" birds began to arrive in a new territory—they produced fewer early-hatching, aggressive males. When the area became more crowded and birds had to compete for scarce nesting spots, the mother birds became stressed out and deposited more hormones, called androgens, in their eggs. The researchers posit that this hormonal increase leads to a bigger population of aggressive offspring. (It should be noted that the hormone increase is not a conscious decision; it's likely a physiological reaction to extra stress.) 

Competition with other birds—such as swallows and wrens, which show up and harass bluebirds during their egg-forming phase—tends to cause the most stress for bluebird mothers. The more put-upon the moms are, the more aggressive their kids are likely to be (and the more likely they are to take over new forests).

“The process of colonization itself sets the stage for rapid changes in aggression over time,” the researchers say. “By dominating mountain bluebirds and acquiring large resource-rich territories, the colonizing generation creates the environment that induces rapid changes in offspring aggression and population growth.”

Which just goes to show that a little bit of stress can be good for you in the long run—if you're a bluebird.