Female Cavity-Nesting Birds Show Higher Aggression in Battle for Nest Sites.

When it comes to defending precious territory, some birds are as fierce as any human shouting "Get off my lawn!" An international study led by Sara Lipshutz, assistant professor of Biology at Duke University, has found that female birds dependent on pre-existing cavities for nesting show significantly higher levels of aggression than those with more flexible nesting habits.

These birds, known as obligate secondary cavity nesters, must find suitable holes in trees, fence posts, or rocky areas to lay their eggs because they cannot create these cavities themselves. "They can't excavate that cavity themselves, and they can't just build a nest anywhere," Lipshutz explained. "They have to find a hole in a tree, and this is the only way they can reproduce." Because these nesting sites are scarce, defending one becomes a critical priority, leading to fierce behaviors such as biting, clawing, and loud vocal warnings.

The research, published in Nature Ecology & Evolution, examined five bird families—swallows, wood warblers, sparrows, thrushes, and wrens—comparing an obligate cavity nester and a more adaptable species from each family. Using decoys and Bluetooth speakers that played recorded calls, the researchers simulated intrusions near active nests and observed the defensive responses of resident birds.

Surprisingly, the results showed that nesting strategy played a stronger role in predicting aggression than evolutionary lineage. While sparrows generally remained passive and wrens consistently aggressive regardless of nesting habits, the study found that cavity-nesting females were far more defensive than their close relatives who nest more flexibly. Females, in particular, exhibited greater aggression than males, possibly because the cost of losing a nest was higher for them.

"It was a really striking behavioral pattern," noted Lipshutz. Kimberly Rosvall, a Duke alumna and researcher at Indiana University, added, "The pressure to compete led to higher aggressiveness, and this was especially strong for the females."

Interestingly, the team found no evidence that higher testosterone levels explained this aggressive behavior. Female cavity-nesters did not have elevated testosterone compared to other species. This led researchers to investigate whether genetic factors might underlie the observed differences.

Their genetic analysis showed a mixed picture. Although cavity-nesting species shared many similarly expressed genes related to aggression, no consistent, easily identifiable set of genes stood out across all species. "We always want to find the genes, a handful of genes that make a lot of sense," said Lipshutz. "And we didn't find any of those usual suspects."

Despite the complexity, Lipshutz finds the results exciting. She suggests that aggression likely involves hundreds of genes working in intricate, species-specific combinations. "It shows that there are independent ways to get to the same behavioral outcome," she said.

As the research team likes to summarize it: "There are many possible routes to build an aggressive bird."

Source:https://phys.org/news/2025-04-paths-angry-bird-female-cavity.html

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