11-day-old baby nestling
Raising baby house wrens is hard work. After a ~7 day egg laying period and a ~12 day incubation, the house wrens hatch and start growing quickly. The babies gain around a gram each day, reaching adult weight in just 10 days! By day 18 they can fly and look just like adult wrens. This is a lot of work for the females. Not only does she help the male feed them every several minutes throughout the day, but she cuts down on her feeding time drastically when she is incubating and may lay 2 or sometimes almost 3 times her body weight in eggs during the season. Whew!
One of the explanations for why females are generally less aggressive than males is that aggression interferes with reproductive demands on females. This trade-off exists for some males. The Ketterson lab has found that male juncos given testosterone implants (the hormone linked to aggression in males) are not as good at feeding their nestlings. We don't know yet if testosterone is responsible for aggression in female wrens. Regardless of the hormonal mechanism, fighting with other females could take time and energy away from the busy work of raising babies.
I originally predicted that more aggressive females would have smaller babies. However, I was surprised to find that babies from aggressive moms actually end up bigger! In other migratory bird species, the size of the nestling at fledging has a big impact on it's chances for survival. If these babies have a survival advantage and aggression is passed down in house wren genes, female aggression will continue in the house wren population.