Horsfield's Cuckoo or Oriental Cuckoo (Cuculus optatus) is a bird belonging to the genus Cuculus in the cuckoo family Cuculidae. It was formerly classified as a subspecies of Cuculus saturatus but differences in voice and size suggest that it should be treated as a separate species. Some authors use the names Horsfield's Cuckoo for optatus and Oriental Cuckoo for saturatus while others use Oriental Cuckoo for optatus and Himalayan Cuckoo for saturatus.
The Common Cuckoo (C. canorus) is very similar in appearance but is slightly bulkier with longer wings and tail and a slightly smaller head and bill. It is slightly paler grey and the barring on the underparts is a little narrower. The vent is usually white with dark bars but is more similar to Horsfield's Cuckoo in a few individuals. Birds of the rufous morph have a plain rump with no dark bars unlike Horsfield's Cuckoo.
The Himalayan Cuckoo (C. saturatus) is extremely similar to Horsfield's Cuckoo but is slightly smaller and shorter-winged.
The call of the male Horsfield's Cuckoo is a series of low paired notes, "poo-poo", with both notes stressed equally. It is somewhat similar to the call of the Hoopoe. It may be introduced with a four to eight note phrase or sometimes with grating notes. The female's call is a deep bubbling trill. Outside the breeding season, the birds are usually silent. The call of the male Himalayan Cuckoo is a series of three or four note phrases with a short, high-pitched introductory note.
The exact extent of its wintering range is uncertain due to its secretive habits and the difficulty of separating it from the Himalayan Cuckoo and other similar species. It is believed to include the Malay Peninsula, Indonesia, the Philippines, New Guinea, western Micronesia, the Solomon Islands and northern and eastern Australia with occasional birds reaching New Zealand. It has occurred as a vagrant in Ukraine, Israel and Alaska.
It is a brood parasite, laying its eggs in the nests of other birds. The nests of Phylloscopus warblers such as the Arctic Warbler, Eastern Crowned Warbler, Willow Warbler and Chiffchaff are commonly used. Other hosts include the Olive-backed Pipit and Asian Stubtail. The eggs are smooth, slightly glossy and vary in colour, sometimes mimicking those of the host species. They are incubated for about 12 days. The young cuckoo is born naked and has an orange gape with black patches. Within a few days it pushes the eggs or young of the host out of the nest. Older nestlings have blackish feathers with white fringes; the belly is dark brown with white bands. The young birds fledge after around 17-19 days.