Nest-Desperate Birds Battle Hardest for Homes

When it comes to defending their nests, not all female birds show equal aggression. Those that depend on ready-made tree cavities for nesting are far more likely to fiercely attack intruders than their relatives who can build nests almost anywhere, according to groundbreaking research from Duke University.

The study, published in Nature Ecology and Evolution, reveals that female birds who must find pre-existing holes in trees to reproduce are evolutionary hardwired for aggression – developing this trait independently across multiple bird families in what scientists call convergent evolution.

“It was a really striking behavioral pattern,” said lead researcher Sara Lipshutz, assistant professor of Biology at Duke University. “The pressure to compete led to higher aggressiveness, and this was especially strong for the females.”

The international research team compared closely related species pairs across five bird families: swallows, wood warblers, sparrows, thrushes, and wrens. In each pair, one species was an “obligate cavity nester” – meaning it can only nest in pre-existing cavities it cannot create itself – while the related species had flexibility in where it could build nests.

By placing decoy birds near nests and playing recorded bird calls, researchers measured how aggressively resident birds defended their territories. The results were clear: cavity-dependent birds, especially females, were significantly more aggressive toward perceived intruders.

The findings make evolutionary sense. Birds that can only nest in limited, ready-made cavities face intense competition for these precious resources. For females especially, losing a good nesting site means potentially losing their chance to reproduce that season.

Kimberly Rosvall, a Duke alumna whose lab at Indiana University contributed to the project, noted that this pattern wasn’t confined to one bird family but emerged independently across the evolutionary tree. “The pressure to compete led to higher aggressiveness,” she explained. “And this was especially strong for the females.”

Surprisingly, the researchers found that this aggression wasn’t linked to testosterone levels. Female cavity-nesters didn’t have higher levels of this hormone typically associated with aggressive behavior, suggesting other biological mechanisms at work.

To uncover what might be driving this pattern, the team analyzed brain gene expression in each species. Although they expected to find “usual suspect” genes consistently associated with aggression across all aggressive species, they discovered something more complex.

“There are probably several hundred genes associated with aggression and they’re probably all working together in subtle ways. Those combinations are not the same for each species, which is really interesting from an evolutionary perspective,” Lipshutz explained. “It shows that there are independent ways to get to the same behavioral outcome.”

In other words, nature found different genetic pathways to create similar aggressive behaviors in different bird families – a remarkable case of multiple evolutionary roads leading to the same destination.

“As our team likes to say, ‘There are many possible routes to build an aggressive bird,'” Lipshutz noted.

This research provides valuable insight into how ecology shapes behavior and the various genetic mechanisms that can lead to similar traits. It also highlights the importance of considering both sexes in evolutionary studies, as females and males may face different selective pressures even within the same species.

For backyard birdwatchers installing nest boxes, these findings might explain why some cavity-nesting species seem particularly territorial – they’re evolutionarily programmed to fiercely defend these precious spaces that determine their reproductive success.