Nest birds fight for houses

Not all females show the same aggressiveness when defending their nests. According to groundbreaking research at Duke University, those who rely on ready-made tree cavity to build nests are more likely to attack the invaders than their relatives.

This study published in natural ecology and evolution shows that preexisting holes in trees must be found in female birds to reproduce are evolutionary aggressive-aggressive-developing this trait independently in what scientists call fusion evolution.

“It’s a very amazing pattern of behavior,” said Sara Lipshutz, assistant researcher at Duke University’s assistant professor of biology. “The pressure of competition leads to higher aggression, which is especially powerful for women.”

The International Research Team compared closely related species pairs among five bird families: swallows, wood warblers, sparrows, thrush and wr. In each pair, a species is a “mandatory cavity nest” – meaning it can only nest in existing cavity, which it cannot be created – and the related species have flexibility where nests can be established.

By placing the bait bird near the nest and playing the recorded bird call, the researchers measured how residents defend their territory. The results are obvious: the accentuated birds, especially women, are significantly more aggressive to the perceived invaders.

These findings are evolutionary. Birds that can only nest in a limited ready-made cavity face fierce competition from these precious resources. Especially for women, losing a good nesting site means losing the chance to breed the season.

Duke alumni Kimberly Rosvall, who contributed to the project at Indiana University, noted that the model is not limited to a family of birds, but is independent of the evolutionary tree. “The pressure of competition leads to higher aggression,” she explained. “This is especially powerful for women.”

Surprisingly, the researchers found that this aggression had no connection to testosterone levels. This hormone in female cavity nests is often associated with aggressive behavior, suggesting other biological mechanisms at work.

To reveal what might drive this pattern, the team analyzed brain gene expression in each species. Although they hope to find the “usually suspicious” genes that are persistently associated with the aggressiveness of all aggressive species, they discovered something more complicated.

“There are probably hundreds of genes associated with aggression, and they may work together in subtle ways. Each species has different combinations, and from an evolutionary perspective, it’s really interesting,” Lipshutz explained. “This suggests that there are independent ways to achieve the same behavioral outcome.”

In other words, nature has discovered different genetic pathways that can produce similar behaviors of aggression among different bird families, which is a significant situation in multiple evolutionary paths to the same destination.

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

This study provides valuable insights into how ecology shapes behavior and various genetic mechanisms that may lead to similar traits. It also highlights the importance of taking gender into consideration in evolutionary research, as women and men may face different selection pressures even in the same species.

For backyard birders who install nest boxes, these findings may explain why certain cavity nest species seem particularly territorial—they evolved to defend these precious spaces that determine their reproductive success.