It can be very challenging for humans to determine the age and sex of certain birds, even in the hand. Sometimes the presence or absence during the breeding season of a cloacal protruberance (in males) or an edematous brood patch (females) is the only way to distinguish the sexes.

Wrentit, photo by Rich Hoyer.

But not even that works for the Wrentit. This odd bird of North America’s Pacific Coast is anomalous in many ways, not least in the fact that males and females alike develop both sexual characteristics while breeding. There are no known plumage characters that distinguish the sexes, making this the most difficult of all North American birds to sex without dissection.