White-Headed Nuns, like Frank and Sally, are sexually dimorphic. That is, the males and females look different from each other. In the case of White-Headed Nuns, this difference is very small. The males have heads that are slightly whiter than the females. In the big picture, Frank is on the right. When the two birds are next to each other, I can usually tell which is which, but if they're on opposite sides of the Finsterium, I can't always tell. Of course, individual birds will vary; both males and females will have slightly lighter or darker shades of tan on their heads. It's particularly hard to tell Frank and Sally apart when it's dark, the lights are out, and I'm trying to catch one of them. Click here for a close-up of this small photo; Sally is in front.