The strangest aspect of hamerkop behaviour is the huge nest, sometimes more than 1.5 metres (4 ft 11 in) across, comprising perhaps 10,000 sticks and strong enough to support a man's weight. The birds decorate the outside with any bright-coloured objects they can find. When possible, they build the nest in the fork of a tree, often over water, but if necessary they build on a bank, a cliff, a human-built wall or dam, or on the ground. A pair starts by making a platform of sticks held together with mud, then builds walls and a domed roof. A mud-plastered entrance 13–18 centimetres (5.1–7.1 in) wide in the bottom leads through a tunnel up to 60 centimetres (24 in) long to a nesting chamber big enough for the parents and young.[4]
These birds are compulsive nest builders, constructing three to five nests per year whether they are breeding or not. Barn owls and eagle owls may force them out and take over the nests, but when the owls leave, the hamerkops may reuse the nests.[2] Snakes, small mammals such as genets, and various birds live in abandoned nests, and weaver birds, starlings, and pigeons may attach their nests to the outside.[4]
At the finished nest, a pair gives displays similar to those of the group ceremonies and mates, often on top of the nest. The clutch consists of three to seven eggs which start white but soon become stained. Both sexes incubate for 28 to 30 days. Both feed the young, often leaving them alone for long times; this unusual habit for wading birds may be made possible by the thick nest walls. The young hatch covered with grey down. By 17 days after hatching, their head and crest plumage is developed, and in a month, their body plumage. They leave the nest at 44 to 50 days but roost in it at night until about two months after hatching.[4]
The strangest aspect of hamerkop behaviour is the huge nest, sometimes more than 1.5 metres (4 ft 11 in) across, comprising perhaps 10,000 sticks and strong enough to support a man's weight. The birds decorate the outside with any bright-coloured objects they can find. When possible, they build the nest in the fork of a tree, often over water, but if necessary they build on a bank, a cliff, a human-built wall or dam, or on the ground. A pair starts by making a platform of sticks held together with mud, then builds walls and a domed roof. A mud-plastered entrance 13–18 centimetres (5.1–7.1 in) wide in the bottom leads through a tunnel up to 60 centimetres (24 in) long to a nesting chamber big enough for the parents and young.[4]These birds are compulsive nest builders, constructing three to five nests per year whether they are breeding or not. Barn owls and eagle owls may force them out and take over the nests, but when the owls leave, the hamerkops may reuse the nests.[2] Snakes, small mammals such as genets, and various birds live in abandoned nests, and weaver birds, starlings, and pigeons may attach their nests to the outside.[4]At the finished nest, a pair gives displays similar to those of the group ceremonies and mates, often on top of the nest. The clutch consists of three to seven eggs which start white but soon become stained. Both sexes incubate for 28 to 30 days. Both feed the young, often leaving them alone for long times; this unusual habit for wading birds may be made possible by the thick nest walls. The young hatch covered with grey down. By 17 days after hatching, their head and crest plumage is developed, and in a month, their body plumage. They leave the nest at 44 to 50 days but roost in it at night until about two months after hatching.[4]
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