The Common Cuckoo (Cuculus canorus) is one of the most recognisable voices of early summer, its two-note "cuck-oo" call carrying across the countryside in May and June. It measures 32–36 cm with a wingspan of 54–60 cm and weighs 95–155 g. The male has slate-grey upperparts, a white underside barred with dark bands reminiscent of a Eurasian Sparrowhawk, a long graduated tail spotted white along the edges, a bill that is yellow at the base with a dark tip, and yellow eyes with a yellow eye-ring; in flight the pointed wings and flat tail add to the sparrowhawk-like silhouette. The female shares the male's grey pattern but with a rusty-brown wash across the breast and flanks, and a rarer reddish-brown "hepatic" morph also occurs. Juveniles are dark, mottled brown with a distinctive white patch on the nape.
The male's call is the familiar two-note "cuck-oo", the second note lower, heard most intensely in the early morning through May and June before falling silent after midsummer; the female instead gives a bubbling, gurgling "kwikkwikkwikkwik", quite different and often overlooked. The Common Cuckoo is Europe's most thoroughly studied obligate brood parasite: each female specialises on a particular host species and lays eggs mimicking that host's size and pattern, watching a chosen nest for days before laying her own egg in roughly ten seconds once the host has laid some of its own. Common hosts include Meadow Pipit, Whinchat, Sedge Warbler, European Robin and White Wagtail. The cuckoo egg has a shorter incubation period (11–12 days) than the host eggs, so it hatches first, and the chick instinctively ejects all other eggs and nestlings from the nest within its first days, after which the host continues feeding it even once it dwarfs them. The cuckoo builds no nest of its own and never incubates, and a female can lay 10–20 eggs in a season, each in a different host nest. Its diet specialises on hairy caterpillars that most birds avoid, including tiger moth larvae; the stomach lining, which becomes clogged with the caterpillars' bristly hairs, is periodically shed and renewed. The species breeds across the whole region, arriving in late April, with adults departing again as early as July and juveniles following alone in September without ever having seen a parent, wintering in sub-Saharan Africa. The species is classified as Least Concern (LC), though numbers are declining, likely linked to intensified farming reducing host populations, insect decline, and habitat overgrowth.
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