Yellow-billed Cuckoo is a widespread species and very common in the southeastern United States. It occurs more rarely in areas to the north, where it tends to be replaced by Black-billed Cuckoo. Both species have populations that are known to fluctuate significantly in response to local caterpillar populations, most notably with tent caterpillars.
Yellow-billed Cuckoo is widespread in a range of eastern forests, but also occurs in riparian areas in the desert southwest of New Mexico, Arizona, and southeastern California. One fascinating thing about Yellow-billed Cuckoo migration is the pronounced difference in timing between east and west. The species reaches the Gulf Coast in mid-April and can be reliably found in the mid-Atlantic (e.g., Maryland) in late April. Despite the lower latitude of southeast Arizona and southern California, however, those sites never get cuckoos before late May. The STEM map shows this timing accurately, with the faint blush in the desert southwest happening well after the species has established itself in the East.