The black-billed cuckoo is separated from the yellow-billed by its all-black bill, lack of rufous in the wings, and muted undertail pattern, which barely shows contrast between the gray and white spots. The common call is a long soft series of cu notes, all at the same speed and pitch.
![A black-billed cuckoo perches on a branch.](https://bwdmagazine.com/wp-content/uploads/2015/02/Black-billed_Cuckoo_13883974479.jpg)