Black-billed Cuckoo

The breeding season runs from May through September, with peak activity in June and July. Pairs form in mid to late May, sometimes not until June. Courtship involves the male landing near a female with a food item in his bill and calling; if receptive, the female moves closer and performs a distinctive tail-flicking display — flicking her tail up and down repeatedly for up to 15 minutes while giving a soft "mew" call. The male remains motionless throughout, then mounts the female (still holding the food item), after which he either eats it or feeds it to her. The pair is monogamous for the breeding season.

Both adults build the nest — a loose, flimsy platform or shallow cup of twigs, grasses, and plant stems, lined with dead or green leaves, pine needles, catkins, plant fibres, rootlets, mosses, and spiderwebs. The finished structure is approximately 15 cm across and less than 2.5 cm deep. Nests are typically placed in a shrub or low tree, 0.3–6 m above the ground, usually below 3 m, concealed among dense foliage or tangles of vines. Unusually, nest construction continues even after incubation has begun.

Clutch size is typically 2–3 eggs, sometimes 4–5; in years of caterpillar abundance, clutches may be larger and laying begins earlier. Eggs are elliptical, greenish-blue, and unmarked, measuring approximately 22.6–32.3 mm long and 18.3–23.5 mm wide; they may take on a marbled appearance after a few days of incubation. Both parents share incubation continuously; the incubation period is just 10–13 days — one of the shortest of any North American bird.

Chick development is equally rapid. Nestlings leave the nest at just 6–7 days old, still flightless, and clamber through branches using their feet and bill. Full flight is achieved at approximately three weeks. The total time from egg-laying to fledging is around 17 days — among the shortest of any bird species in the world. Both parents feed the young, crushing food before thrusting it into the chicks' mouths. In years of caterpillar abundance, the species occasionally engages in brood parasitism, laying eggs in the nests of other Black-billed Cuckoos, Yellow-billed Cuckoos, Chipping Sparrows, American Robins, Gray Catbirds, Wood Thrushes, and at least six other species.

The Black-billed Cuckoo is a relatively short-lived bird. The typical lifespan in the wild is 2–4 years, and the maximum recorded longevity for a wild individual is 5 years, based on banding data. Sexual maturity is reached at approximately one year of age, meaning most birds attempt to breed in their first full summer.

Survival rates are influenced by the hazards of long-distance migration — the species must complete a round trip of several thousand kilometres between eastern North America and northwestern South America each year, crossing the Gulf of Mexico and traversing Central America. Collisions with communication towers and buildings during nocturnal migration are a significant source of mortality, and the species appears disproportionately vulnerable to these strikes compared to many other migrants.

On the breeding grounds, adults face predation from birds of prey, and nests are vulnerable to mammalian predators including raccoons and squirrels. The rapid developmental timeline — from egg to fledgling in just 17 days — may be an adaptation to minimise the period of nest vulnerability in a species that builds a notoriously flimsy, exposed nest.

Compared to the Common Cuckoo, which can live up to 6–8 years in the wild, the Black-billed Cuckoo has a shorter recorded maximum lifespan, though both species face similar pressures as long-distance migrants. The oldest known Black-billed Cuckoo was recovered through banding records at 5 years of age.

The Black-billed Cuckoo is listed as Least Concern on the IUCN Red List, with a global population estimated at approximately 880,000 mature individuals. However, the population trend is firmly downward. The North American Breeding Bird Survey recorded a cumulative decline of approximately 67–68% between 1970 and 2019, at a rate of over 2% per year. Partners in Flight rates the species 13 out of 20 on the Continental Concern Score and has placed it on the Yellow Watch List-D for species with population declines and moderate-to-high threats. If current rates of decline continue, the species could lose half its remaining population by 2037.

In Canada, the Breeding Bird Survey suggests an overall moderate decrease since 1970, and the species is a candidate for assessment by COSEWIC (the Committee on the Status of Endangered Wildlife in Canada). At the state level, it is listed as Threatened in Illinois and as a Species of Special Concern in New Jersey.

The primary threats are interconnected. Pesticide use — particularly insecticides applied in orchards and agricultural areas — reduces caterpillar prey populations and may directly poison cuckoos. Habitat loss and fragmentation on both breeding and wintering grounds, including loss of shrubland through forest maturation and over-browsing by deer, removes suitable nesting habitat. The species also appears disproportionately susceptible to collisions with communication towers and buildings, particularly during nocturnal migration. Climate change is projected to alter the species' range and the timing of caterpillar outbreaks on which it depends.

The species is protected under the US Migratory Bird Treaty Act. Conservation priorities include reducing pesticide use in key foraging habitats, maintaining shrubby woodland edges, and addressing tower collision mortality through lighting and structural modifications.

Courtship in the Black-billed Cuckoo is understated but precise. The male initiates by landing near a female with a food item — typically a caterpillar or other large insect — held in his bill, and calling softly. The female's response, if she is receptive, is one of the more unusual displays in North American birds: she moves closer to the male and begins flicking her tail up and down in a slow, rhythmic motion, continuing this display for up to 15 minutes while giving a soft "mew" call. The male remains completely motionless throughout.

After mounting the female — still holding the food item in his bill — the male either eats it himself or passes it to her as a courtship gift. This food-offering behaviour is thought to signal male quality and provisioning ability, giving the female information about how well he is likely to feed their chicks. The pair is monogamous for the duration of the breeding season.

Nest building is a shared activity, with both adults contributing to the loose twig platform. Unusually, construction continues even after incubation has begun — the nest is never truly "finished" in the way that many songbird nests are. Both adults also share incubation duties continuously, with no clear division of labour between day and night shifts. This shared investment from the outset of the breeding cycle is consistent with the species' monogamous pair bond and contrasts sharply with the entirely parasitic strategy of the Common Cuckoo, which forms no pair bond and invests nothing in parental care.

Across much of its North American range, the Black-billed Cuckoo is known as the "rain crow" — a name rooted in the folk belief that it calls more frequently in the hours before rain. The name appears in rural American tradition from at least the 19th century and is shared informally with the Yellow-billed Cuckoo, which occupies much of the same range. Despite the persistence of the belief, no scientific study has confirmed a reliable correlation between cuckoo calling behaviour and incoming precipitation. The name likely reflects the species' habit of calling loudly and persistently in warm, humid weather — conditions that often precede summer storms — rather than any genuine meteorological sensitivity.

The "rain crow" name is part of a broader tradition of associating cuckoo calls with weather prediction that spans multiple continents. In Europe, the call of the Common Cuckoo has long been treated as a harbinger of spring, and various folk traditions hold that the number of calls heard on first encounter predicts the number of years until marriage, death, or good fortune. The Black-billed Cuckoo carries none of these European associations in North America, but the rain crow name gives it a distinct cultural identity in the rural communities of the eastern United States and Canada where it breeds.

The species' secretive habits and ventriloquial call — which seems to come from everywhere and nowhere at once — have contributed to its reputation as a mysterious, elusive bird. It is rarely depicted in popular culture but holds a quiet place in the natural history of the eastern woodlands, where its hollow, rhythmic call drifting from a thicket on a July evening is one of the characteristic sounds of the season.