In species that are sexually dimorphic, it’s pretty easy!  The Common Yellowthroat, for example.  Remember mother and baby from a previous post?

Now here’s dad:

In Common Yellowthroats, the boys and girls look quite different.

Now here’s a pair that is NOT dimorphic (at least not visually – from a distance):

Red-eyed Vireo

Can you tell the boy from the girl?  I’ll give you a hint… I’m holding the boy and Tom is holding the girl.  (What, you can’t tell my hand from his?)  OK, I’ll tell you:  The boy is on the left and the girl is on the right.  I only know this, because we examined them carefully while banding them.  The male Red-eyed Vireo is supposed to be slightly larger than the female, though they look pretty close in size here.

At a previous banding session, Tom had mentioned something about how in some species of birds, the boy will help incubate eggs.  In species that do that, the plumage of males and females is often quite similar.  The Red-eyed Vireo proved that out.  The boy had a partial brood patch.  The female’s belly was quite interesting:  She not only had a brood patch, but her belly was quite distended.  We think she was about to drop an egg!

We had a lot of birds at that net check, so I didn’t get a picture of the Vireos’ bellies.  But I did get one of a female Common Yellowthroat.  The brood patch is skin on her belly that she can put directly on the eggs when incubating.  The lack of insulation allows her body heat to transfer more effectively to the eggs:

Learn more:

• http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Common_Yellowthroat.html
• http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Red-eyed_Vireo.html