cuckoo

Black-billed and Yellow-billed Cuckoos

Black-billed and Yellow-billed Cuckoos

Cuckoos are secretive, so spotting them and making an identification only by their bill color is difficult. Both can be found in the same habitats which adds some complication to ID, but there are a couple other good ways to tell them apart. 

Photo by Andy Reago and Chrissy McClarren