When the breeding season begins, there is a great deal of aerial chasing near the breeding site, with stunt flying like a lapwing, and sometimes foot-touching and rolling displays. The nuptial flights are spectacular, performed at high speed, and when coming into the nest tree the birds come in low, very fast, and sweep up on to a perch.
The nest is often built in the same tree that the birds rest in through the day. Both sexes build, but the female does more than the male. Twigs are broken off dead branches in flight, and many are dropped. It is usually high up in a tree, built on a large lateral limb, but sometimes in the middle of the tree. It is a fair-sized structure about three feet across by one foot deep, with a broad shallow cup, sometimes unlined, unlike other birds of prey.
Both in the East and in Africa it sometimes nests in busy towns. Only the female incubates, and she sits very close during the day, taking only an occasional flight. At dusk she leaves the nest and flies around, but does not always hunt for herself. Often the male feeds her on the nest or near it.
The incubation period is estimated at about a month. The young fledge in about 35-40 days, and are fed by both parents. The nestling only fed during the last fifteen to twenty minutes before dark. During this time the feeding rate is rapid, once every three or four minutes, and the parents often deposit the food on the nest edge and fly off, returning shortly with more.
The prey brought to the young often includes insects. During the early part of the fledging period the female remains on the nest with the young, but in the last week or ten days of the period the parents both sit on branches of the nest tree or adjacent trees.
The young only remain in the vicinity of the nest for a short time after fledging. Theses birds are regular breeders, rearing one young per pair in most years.
The Bat Hawk is mostly silent, but at dusk and in display it makes varied high-pitched, broken whistles, 'ki-kik-kik-kik-keee' or 'kwiek-kwiek'. These calls sound weak for the size of the bird. In flight, they appear as a dark, long-winged, slender bird about the size of a small buzzard, but with the flight silhouette of a large falcon. At rest in a tree its almost black body, white throat with median streak, and large bright yellow eye are good pointers.
The most important thing in life for the Bat Hawk is open space with loads of bats or swallows or swiftlets, or even large insects, which it can take on the wing at dusk. It can be seen in the evening hunting over pools, large rivers and creeks, beaches, railway platforms, or even large lawns near buildings. In the Far East it frequents the mouths of limestone and other caves inhabited by Birds-nest Swiftlets.
Due to its specialised habits it is a rare bird through most of its range, but is quite common where it is found, and frequents towns as well as wilder country. Provided its basic requirements for space and food are met, it can turn up almost anywhere in its range.
When hunting the Bat Hawk either quarters or circles over its chosen area, usually at great speed, looking above, below, and on both sides of it (unlike most birds of prey, which look downwards). It catches bats, swallows and swiftlets in flight, some other small birds and perhaps insects as well, and swallows them whole in the air.
The prey is caught in the feet and swallowed whole. The bird only hunts for about half an hour each day in Africa, but longer in the East, where the prey is swiftlets rather than bats. To enable it to catch enough to survive in the short time available the prey must be plentiful and small, as it is swallowed whole on the wing.
The Bat Hawk does not take large fruit bats, which are often very numerous in its range, but confines itself to small bats and birds. During the day the Bat Hawk sits motionless in a large tree, rarely, if ever, leaving it even for a short flight, though it may sometimes be seen flying about in daylight, especially in the breeding season. A little before dusk it takes wing, and hunts until the light has all but gone. It also hunts in the early morning.