Ginny tells me that the Black-billed cuckoo has quite a widespread range E. of the Rockies. It may sing at night, unlike most birds.
The Yellow-billed cuckoo has localized ranges in river valleys in the Southwestern US; spotty in CA.
Both species winter in S. America, as far south as Argentina, and both species breed in the USA. They come back from S. America March or April through September. They have the same song summer and winter, but they'd first be heard in the USA around March or April. So she's not sure where they'd be first heard in early July, but their summer range extends all the way from Mexico to Southern Canada, so maybe that's where the song is set?
The USA only has two of the 132 world species of cuckoo!

Mark