Graceful in flight but skulky and retiring in habit, the Black-billed Cuckoo is among North America's most elusive birds. It is frequently confused with the more common Yellow-billed Cuckoo (Coccyzus americanus), with which it shares similarities in plumage, behavior, and many vocalizations. Although both species occur sympatrically through much of their ranges, the Black-billed Cuckoo has the more northerly distribution. In addition, Black-billeds prefer more densely wooded areas and can be found more frequently within coniferous vegetation.
The Black-billed Cuckoo is rarely seen during migration and on wintering grounds in South America due to its silent and secretive manner. As a result, its nonbreeding distribution remains controversial. In North America, it is among the later migrants to return each spring; arrival on breeding grounds is announced by its staccato, repetitive call— cu-cu-cu cu-cu-cu —uttered as individuals fly overhead on late spring evenings. Vocal night flights increase as breeding commences. These flights, in concert with its quiet, sluggish behavior during the day, has led some ornithologists to suggest that the Black-billed Cuckoo is nocturnal in summer.
Like other cuckoos, the Black-billed exhibits unusual breeding behavior. The onset of nesting is apparently correlated with insect outbreaks, particularly those of caterpillars and cicadas. Furthermore, localized food abundance has been linked to increased clutch size and nesting success and to the frequency of brood parasitism. Young Black-billed Cuckoos are robust among altricial birds. Shiny, black nestlings hatch following a brief 11-day incubation period. Within 3 hours of hatching, they can raise themselves onto twigs, using their feet and bills. They mature rapidly, and at 6 days of age resemble porcupines, with their long, pointed feather sheaths. Just prior to the young leaving the nest on the following day, the sheaths burst and the chick becomes fully feathered, a process once likened to the commotion in a corn popper. The agile, young cuckoos are capable of hopping and climbing rapidly through the vegetation. When threatened, they assume a bizarre defensive posture—necks outstretched, bills pointed straight up, eyes wide open—that resembles the erect pose employed by American Bittern (Botaurus lentiginosus) chicks.
The Black-billed Cuckoo is a notorious consumer of caterpillars, with a demonstrated preference for noxious species, including the eastern tent caterpillar (Malacosoma americanum), fall webworm (Hyphantria cunea), and larvae of the gypsy moth (Lymantria dispar). Observations of cuckoos consuming 10–15 caterpillars per minute are testimony to the great service this species provides in forests, farms, and orchards. Stomach contents of individual cuckoos may contain more than 100 large caterpillars or several hundred of the smaller species. The bristly spines of hairy caterpillars pierce the cuckoo's stomach lining giving it a furry coating. When the mass obstructs digestion, the entire stomach lining is sloughed off and is regurgitated as a pellet.
The Black-billed Cuckoo was formerly much more common in North America. Population densities have declined across its range throughout the twentieth century, with particularly severe decreases in the 1980s and 1990s. Naturalists' accounts from the late 1800s speak of flocks of cuckoos descending on caterpillar-laden trees and not departing until every insect was consumed. Caterpillar irruptions still occur, but since they have been controlled by pesticide use, cuckoos are rarely seen more than singly. It is likely that pesticides, and the concomitant reduction of prey availability, have caused Black-billed Cuckoo mortality and reduced breeding success, but these detrimental effects have never been quantified.
Few aspects of Black-billed Cuckoo life history have been adequately studied. Typical breeding and nestling behavior has been described by Spencer ( Spencer 1943 ) and Sealy ( Sealy 1985c ). In addition, some anomalous breeding behaviors, such as brood parasitism and resource-dependent clutch sizes, have been examined by Nolan and Thompson ( Nolan and Thompson 1975 ), Sealy ( Sealy 1978b ), and Hughes ( Hughes 1997 ). Many workers have observed an association between local cuckoo abundance and insect irruptions (e.g., Jauvin and Bombardier 1996 ), and detailed examinations of stomach contents have been made ( King 1883 ; Beal Beal 1897 , Beal 1898b ). However, no comprehensive studies of the effects of resource availability on Black-billed Cuckoo abundance, distribution, or fecundity have been performed.