A fairly small cuckoo measuring about 21-23 centimetres in length, Cacomantis merulinus is a species of bird belonging to family Cuculidae. This species is commonly known as the plaintive cuckoo. It inhabits forest edges, open woodlands, scrubs, grasslands, farmlands, parks and gardens. The plaintiff cuckoo mostly feeds on insects such as beetles, bugs, termites and numerous soft-bodies species. It may take some fruits too.

The male has grey–brown upper plumage, orange below plumage, pale grey head, throat and upper breast, dark greyish brown mantle and wings, white-tipped tail feathers, orange-yellowish legs and feet, red eyes and yellowish bill below and black above. While the female mostly resembles the male; but often occurs in a "hepatic" morph with a reddish-brown upper plumage with dark bars. Its plumages below are paler with fainter barring. It also has a light stripe over its eyes and the tail is barred along the entire length. The juvenile looks like the hepatic female but is duller and has dark streaks instead of bars on the crown and throat.

As numerous cuckoos, the plaintive cuckoo is a brood parasite. It favours laying its eggs in the closed nests with a narrow entrance, built by small passerines such as ioras, prinias, cisticolas and tailorbirds. The eggs are similar to those of the host species but are larger.