Macho Birds vs Gentle Dads

The Birds: western bluebirds

• Hannah Schardt
• Feb 01, 2008

The Back Story: Western bluebirds lost much of their Montana habitat to logging and farming in the 1930s. Now that nesting boxes are encouraging the birds to recolonize their former range, researchers from Harvard University and the University of Arizona set out to band and follow the colonizers. They noted whether individual birds exhibited more or less aggressive behavior, then tracked the birds’ success in a newly established population. Their findings: Unsurprisingly, aggressive male bluebirds are the first to colonize the new area. But less aggressive males, which spend more time parenting their young, actually have greater long-term breeding success. The researchers found that this led to a rapid decrease in aggression across five generations of birds.

The Big Picture: Study coauthor Renee Duckworth of Harvard says that for western bluebirds, aggression has both benefits (faster colonization) and costs (poor parenting). This helps explain why neither aggressive nor passive males have won out completely. Less aggressive bluebirds have a harder time acquiring territories, but, says Duckworth, “once they do get territories, they tend to be more successful.”