Going cuckoo – the ins and outs of atlassing cuckoos

Mike Burrell, Ontario Ministry of Natural Resources and Forestry

June 18, 2021

Figure 1: Photos of Yellow-billed Cuckoo (left) and Black-billed Cuckoo (right)


Ontario is lucky to have two species of cuckoos. Both are birds of wooded and semi-open habitats and share the same secretive behaviour. Black-billed Cuckoo (BBCU) has the larger Ontario range (Figure 2), breeding throughout southern Ontario north to Sault Ste Marie, Rainy River, and Thunder Bay and with scattered birds north to Cochrane and occasionally even southern James Bay. They can be found in virtually any wooded habitat, especially those with abundant shrubs. Yellow-billed Cuckoo (YBCU) is more southern in distribution, mostly breeding south of the southern edge of the Canadian Shield. YBCU tends to favour mature deciduous forest slightly more than BBCU.

Figure 2: Atlas-2 breeding evidence map for Yellow-billed Cuckoo (left) and Black-billed Cuckoo (right)

In Ontario, cuckoos are famous for their ferocious appetite for caterpillars, especially hairy/spiny ones, like tent caterpillars (Lepidoptera: Lasiocampidae, Malacosoma). While most other birds can’t eat the caterpillars because of the spines and hairs, cuckoos’ unique stomach lining collects the hairs and is periodically shed and re-grown. As a result of their ability to take advantage of periodically abundant food sources like caterpillar outbreaks, their numbers can fluctuate on a regional scale greatly from year to year.

While cuckoos, especially those from Europe are known for their parasitic nesting behavior, both Ontario species build their own nests, and, as Percy Taverner put it “In regard to their parental duties, our birds show considerable [sic] more realization of responsibility than the European.” Brood parasitism is regular in both of our cuckoo species though the frequency has not been quantified. The most common hosts are other cuckoos (usually the same species, but not infrequently the other) but a fairly wide range of other species have been documented. Nest parasitism may be more common when food supply is abundant, and cuckoos produce larger numbers of eggs.

Both cuckoos build fairly large, flimsy platform or shallow cup-style nests of twigs and grasses, usually placed in deciduous trees, shrubs, vines, or tangles about one to two metres above the ground. If you are lucky enough to find a nest, check the contents; YBCU eggs are noticeably larger than BBCU.


Identification is straight-forward when seen well but trickier for heard-only birds. Both species are fairly large, with a long tail, plain brown upperparts and pale underparts. Distinguishing characteristics include lower mandible colour (yellow on YBCU, black in BBCU), wing colour (reddish in YBCU, plain brown in BBCU), orbital ring colour of adults (yellow in YBCU, red in BBCU), and tail pattern (large, distinct white tips in YBCU, small, indistinct whitish tips in BBCU).

The difficulty of “heard-only” birds is amplified by the fact that there are a number of different song/call types, some of which are very similar between the two species. In fact, during Atlas-2, atlassers were instructed to record heard-only cuckoos to the genus level (e.g. Black-billed/Yellow-billed Cuckoo). This was partly based on the belief that each species could learn the song/calls of the other, but this has not been conclusively demonstrated, and in fact, cuckoos do not appear to “learn” songs the way many other birds do; the vocalizations are innate. In Atlas-3, we are relaxing this rule slightly, allowing entry of heard-only cuckoos to species but we urge caution; if you are going to do so, please ensure you understand the range of variability in vocalizations within and between the two Ontario species and preferably have heard more than a single call or sequence of calls from the individual.

One of the goals of Atlas-3 is to understand the distribution and relative abundance of each species as accurately as possible. If you aren’t sure, please report as “Yellow-billed/Black-billed Cuckoo” and/or consider using playback of one of the two species (sparingly) to see if you can get a visual to ensure accuracy.

Vocalization Types

Yellow-billed Cuckoo

Song is a repeated “kow, kow, kow” or “coo” “coo” “coo” several (5-11) times, spaced about a second apart.

Black-billed Cuckoo

Song is a rhythmic “po-po-po-po”, with the “po” notes repeated 2-6 times per sequence with each sequence lasting about 0.5 seconds, and each sequence spaced about one second apart.

Typical Long call a rattle followed by repeated “Kowlp” notes. Could be described as a rapid stuttering “Ka-Ka-Ka-Ka-Kowlp-Kowlp-Kowlp”. Rises then falls in volume, loudest in middle. Rattle can sometimes be omitted.

Typical Long call is a loud “chortle” followed by repeated “Kowlp” calls. Can be confused with YBCU song.

An interesting example, basically a typical long call fading into a typical song.

Other calls: rattle/knocker call is like the start of the long call.

Other calls: Kowlp calls can be mixed in with other calls in combination or repeated.

Other calls: chortle call is similar to the last few notes of the long call. Often given during nocturnal migration.

Other calls: chortle call similar to Yellow-billed Cuckoo; often given at night and during nocturnal migration.

Other good recordings: Song, then a rattle, then another song.

Other good recordings: Interesting song, with long intro.

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