Common Hawk-Cuckoo is an Indian bird that bears a scientific name "Hierococcyx varius", popularly known as the Brain Fever bird scattered in the Indian states like West Bengal, Jharkhand, Bihar and Assam.

History of Common Hawk-Cuckoo
The locality of Common hawk-cuckoo is Tranquebar in the Indian state of Tamil Nadu, once a Danish settlement and from where a specimen reached Martin Hendriksen Vahl who described the species in 1797.

Category of Common Hawk-Cuckoo
Common Hawk-Cuckoo is placed under the genus Hierococcyx, which includes other Hawk-Cuckoos, but is sometimes included in the genus Cuculus. There are two subspecies, the nominate from India and ciceliae of the hill regions of Sri Lanka. The Indian population has paler plumage than ciceliae.

Behaviour of Common Hawk-Cuckoo
Common Hawk-Cuckoo is a medium-sized cuckoo. It bears a close resemblance to the Shikra, even in its style of flying and landing on a perch. The resemblance to hawks gives this group the generic name of Hawk-Cuckoo and like many other cuckoos these are brood parasites, laying their eggs in nests of babblers.

Breeding of Common Hawk-Cuckoo
During the breeding season of Common Hawk-Cuckoo in summer the male Common Hawk-Cuckoos produce loud, repetitive three note calls that are well-rendered as brain-fever, the second note being longer and higher pitched. These notes rise to a crescendo before ending abruptly and repeat after a few minutes; the calling may go on through the day, well after dusk and before dawn.

Structure of Common Hawk-Cuckoo
Common Hawk-Cuckoo is a medium- to large-sized cuckoo, about the size of a pigeon (ca. 34 cm). The plumage is ashy grey above; whitish below, cross-barred with brown. The tail is broadly barred. The sexes are alike. They have a distinctive yellow eye ring. Subadults have the breast streaked, similar to the immature shikra, and there are large brown chevron marks on the belly. At first glance they can be mistaken for a hawk. When flying they use a flap and glide style that resembles that of sparrow hawks (especially the shikra) and flying upwards and landing on a perch they shake their tails from side to side. Many small birds and squirrels raise the alarm just as they would in the presence of a hawk. The sexes are alike but males tend to be larger. Common Hawk-Cuckoo can be confused with the large Hawk-Cuckoo, which, however, has dark streaks on the throat and breast.

Young Common Hawk-Cuckoo
Young Common Hawk-Cuckoo has a pale chin but young large hawk-cuckoos have a black chin. The immature Common Hawk-Cuckoo is with orange bill and indistinct eye-ring which are specially seen in the streets of Kolkata, West Bengal.

Calls of Common Hawk-Cuckoo
During the summer months, Common Hawk-Cuckoos before the monsoons, the male Common Hawk-Cuckoo are easily detected by their repeated calls but can be difficult to spot. The call is a loud screaming three-note call, repeated 5 or 6 times, rising in crescendo and ending abruptly. It is heard throughout the day and frequently during moonlit nights. The calls of females are a series of grating notes.

Feeding of Common Hawk-Cuckoo
Common Hawk-Cuckoos feed mainly on insects and are specialised feeders that can handle hairy caterpillars. Caterpillar guts often contain toxins and like many cuckoos they remove the guts by pressing the caterpillar and rubbing it on a branch before swallowing it. The hairs are swallowed with the caterpillar and are separated in the stomach and regurgitated as a pellet.

Concentration of Common Hawk-Cuckoo
Common Hawk-Cuckoo occurs in most of the Indian subcontinent, from Pakistan in the west, across the Himalayas foothills, east to Nepal, Bhutan and Bangladesh and south into Sri Lanka. Some birds of the Indian population winter in Sri Lanka. In the hills of central Sri Lanka, ciceliae is a resident. It is generally resident but where occurring at high altitudes and in arid areas is locally migratory. It is found in the lower elevations (mostly below 1000m) of the Himalayan Mountain Range but in the higher areas, the large hawk-cuckoo tends to be more common. Common Hawk-Cuckoo is arboreal and rarely descends to the ground. Its habitat includes garden land, groves of tree, deciduous and semi-evergreen forests.