Generally shy and elusive, the yellow-billed cuckoo can be easily overlooked. Its calls are usually loud and often provide the best evidence to the presence of the bird. It favors eating caterpillars and seems to respond well to outbreaks of tent caterpillars. Monotypic. Length 12".
It has a slender body, a long tail, and rounded wings. Adult: it has grayish brown upperparts with whitish underparts. The crown can be noticeably grayer than the rest of the upperparts on some individuals. The rounded wings have reddish primaries. The long tail is graduated with brown central rectrices tipped in black; the remainder are black and broadly tipped with white. It has a yellow orbital ring. The bill is curved with a black culmen extending over much of the upper mandible; the lower mandible is yellow with a black tip. Immature: It looks similar to the adult but has buffy undertail coverts. The undertail pattern is muted and the tips of the rectrices are buffy and not as prominent. The orbital ring is a dull yellow.
Presently considered monotypic; however, there have been 2 subspecies described: americanus in eastern North America and occidentalis in the Southwest. The differentiation of these taxa is weak and limited in the contact zone.
The yellow-billed cuckoo most closely resembles the black-billed cuckoo, but it is distinguished by the yellow orbital ring, rufous primaries, more prominently white-tipped tail, and the yellow lower mandible. Some calls, however, are quite similar.
Call: a rapid staccato kuk-kuk-kuk that usually slows and descends into a kakakowlp-kowlp ending; sounds hollow and wooden.
Status and Distribution
Common in eastern North America, becoming increasingly rare and local in much of the West. Breeding: open woodlands with dense undergrowth, riparian corridors, and parks. Southwestern populations increasingly limited to riparian corridors. Migration: trans-Gulf migrant as well as southeastward over Caribbean Islands. In spring, the Gulf Coast peak occurs ±1 May; southern Great Lakes ±10 May. In fall, it wanders up eastern seaboard as far north as Newfoundland. Southern Great Lakes peak ±20 August; Gulf Coast peak ±10 Sept. Rare in the United States after 1 November Southwestern populations arrive late May–early June. Winter: casual to accidental along Gulf Coast, these records could pertain to lingering fall migrants. The majority of population winters in northern South America, as far south as northern Argentina.
Populations in the western United States are declining fairly rapidly and are of considerable conservation concern. These populations depend on riparian corridors, which are under increasing pressure from exotic plants, water impoundment, and other factors. The validity of occidentalis may become be of greater importance because some conservation groups would like to have the population considered for federal protection under the Endangered Species Act. Populations elsewhere are declining at a less precipitous rate.
—From the National Geographic book Complete Birds of North America, 2006