Almost all birds incubate their eggs: keeping them warm while the embryo develops into a chick. In order to transfer heat better from their body to the eggs, many birds develop brood patches (a.k.a. incubation patches). The bird loses feathers from her belly, and the bare skin becomes wrinkly and swollen with fluid. In juncos only the female develops a brood patch, since she does all the incubating, but in species where males also incubate, males can develop brood patches too.
Brood patches are a good way to tell what breeding stage a bird is at, since usually the brood patch begins to develop during nest building, becomes very swollen with fluid (“edema”) during incubation, and then declines. I had planned to use brood patches to tell breeding stage with the juncos; however, I’ve been having a lot of trouble catching females, so it isn’t working as well as I’d hoped.
On our last trip, however, we caught a for-certain-female junco with a brood patch!
This looks like an early-stage brood patch to me—the skin is still pink and normal-looking, not swollen—so probably she is building her nest right now.