Northern fickers are woodpeckers of the eastern united States. Males, but not females, have black feathers that resemblie a moustaoeet ientists painted a head. Whereas aggression between males is common, male and female members moustache on female members of breeding pairs, their mates attacked them. This suggests that

1. the moustache is a releaser for aggression.
2. moustache extending from the bill along each side o of breeding pairs are not aggressive toward one another.
3. However, when scientists painted a aggression is
4. a learned behavior the moustache is a neutral stimulus.