Distribution: The taxonomy of yellow-billed cuckoo subspecies has been debated. Most authors have recognized both an eastern (Coccyzus americanus americanus) and western (C. a. occidentalis) subspecies (A.O.U. 1957). Only the western yellow-billed cuckoo occurs in Utah (Behle and Perry 1975). Its historic range included all states west of the Rocky Mountains and extended into southern British Columbia at the northern extent and into the northwestern states of Mexico at the southern limit. Estimates of the number of current breeding pairs range widely; however, it is apparent that the cuckoos' population and range have been largely diminished since Ridgway (1877) first described the subspecies. Currently, the range of the cuckoo is limited to disjunct fragments of riparian habitats from northern Utah, western Colorado, southwestern Wyoming, and southeastern Idaho southward into northwestern Mexico and westward into southern Nevada and California. Cuckoos are long-range migrants that winter in northern South America in tropical deciduous and evergreen forests (Ehrlich et al. 1988).
The western yellow-billed cuckoo is a Federally listed threatened species. Historically, cuckoos were probably common to uncommon summer residents in Utah and across the Great Basin (Ryser 1985, Hayward et al. 1976). The current distribution of western yellow-billed cuckoos in Utah is poorly understood, though they appear to be an extremely rare breeder in lowland riparian habitats statewide (Walters 1983, Behle et al. 1985, Benton 1987).
Ecology: Western yellow-billed cuckoos are one of the latest migrants to arrive and breed in Utah. They arrive in extremely late May or early June and breed in late June through July. Cuckoos typically start their southerly migration by late August or early September. Western yellow-billed cuckoos feed almost entirely on large insects that they glean from tree and shrub foliage. They feed primarily on caterpillars, including tent caterpillars. They also feed frequently on grasshoppers, cicadas, beetles, and katydids, occasionally on lizards, frogs, and eggs of other birds, and rarely on berries and fruits (Ehrlich et al. 1988, Kaufmann 1996).
Nesting habitat is classified as dense lowland riparian characterized by a dense sub-canopy or shrub layer (regenerating canopy trees, willows, or other riparian shrubs) within 100 m (333 ft) of water. Over story in these habitats may be either large, gallery-forming trees (10-27 m [33-90 ft]) or developing trees (3-10 m [10-27 ft]), usually cottonwoods. Nesting habitats are found at low to mid-elevations (750-1820 m [2500-6000 ft]) in Utah. Cuckoos may require large tracts (40-80 ha [100-200 ac]) of contiguous riparian nesting habitat; however, cuckoos are not strongly territorial and home ranges may overlap during the breeding season. Nests are usually 1.2-2.4 m (4-8 ft) above the ground on the horizontal limb of a deciduous tree or shrub, but nest heights may range from 1-6 m (3-20 ft) and higher. The nest is a loosely arranged platform of twigs lined with softer materials such as grass, rootlets, and dried leaves. Nests are built in 1-3 days. The female lays 1-8 (usually 3) eggs over a period of several days; laying often begins before the nest is complete. Both males and females incubate eggs for a period of 9-11 days, beginning when the first egg is laid. Nestlings are altricial and hatch asynchronously over several days. Young are brooded by both adults for 7-8 days before leaving the nest, an unusually rapid development for a bird this size. Young climb on branches for about 2 weeks after leaving the nest until they are capable of flight at about 3 weeks of age. Both adults tend the fledglings, and in some cases early fledglings are attended by the male and later fledglings are attended by the female. It is not know whether cuckoos have more than one brood per season in Utah, but multiple brooding has been recorded in California.
Western yellow-billed cuckoo nesting behavior may be closely tied to food abundance. In years of low food abundance, cuckoos may forego nesting; in years when the food supply is abundant, cuckoos may lay a large number of eggs and even parasitize the nests of other species (Nolan and Thompson 1975). Cuckoos are rarely hosts to brown-headed cowbirds.
Habitat Requirements: Western yellow-billed cuckoos are considered a riparian obligate and are usually found in large tracts of cottonwood/willow habitats with dense sub-canopies (below 10 m [33 ft]).