The hamerkop is mostly silent when alone, but is fairly vocal when in pairs or in groups. The only call it usually makes when alone is a flight-call, a shrill “nyip” or “kek”. In groups, vocalisations include a range of calls including cackles and nasal rattles. One highly social call is the "yip-purr" call. This call is only made in a social context, when at least three birds, but up to 20 are gathered in a flock. Birds start by giving a number of "yip" calls, eventually giving way to purring notes. This call is made with the neck extended and sometimes accompanied by wing flapping, and becomes more vigorous when larger numbers of birds are present.
Another common social behaviour is "false mounting", in which one bird stands on top of another and appears to mount it, but they do not copulate. This behaviour has been noted between both mated pairs and unmated birds, and even between members of the same sex and in reversed mountings, where females mount males. Because of this, the behaviour is thought to be social and not related to the pair bond. Dominant birds may signal to subordinates by opening their bills slightly and erecting their crests, but the species is not very aggressive in general towards others of its species. Birds in groups also engage in social allopreening when in groups. One bird presents its face of back of the head to the other to be preened.
This species normally feeds alone or in pairs, but also feeds in large flocks sometimes. It is a generalist, although amphibians and fish form the larger part of its diet. The diet also includes shrimp, insects, and rodents. The type of food they take seems to vary by location, with clawed frogs and tadpoles being important parts of the diet in East and Southern Africa and small fish being almost the only prey taken in Mali. Because it is willing to take a wide range of food items and also take very small prey, it is not resource-limited and only feeds for part of the day.
The usual method of hunting is to walk in shallow water looking for prey. Prey is located differently depending on circumstances; if the water is clear, it may hunt by sight, but if the water is very muddy, it probes its open bill into water or mud and shuts it. It may shuffle one foot at a time on the bottom or suddenly open its wings to flush prey out of hiding. Prey caught in mud is shaken before swallowing to clean it, or if available, taken to clearer water to do so. The species also feeds while in flight. A bird flies slowly low over the water with legs dangling and head looking down, then dipping feet down and hovering momentarily when prey is sighted. The prey is then snatched with the bill and swallowed in flight. This method of hunting can be very successful, with one birds catching prey on 27 of 33 attempts during one 45-minute session. It is also opportunistic, and feeds on swarming termites when they conduct their nuptial flights, snatching as many as 47 alates (flying termites) in five minutes.
This species has been recorded foraging for insects flushed by grazing cattle and buffalo, in a manner similar cattle egrets, and has been observed fishing off the backs of hippopotamuses.It has also been recorded feeding in association with banded mongooses; when a band of mongooses began hunting frogs in dried mud at the side of a pool of water a pair of hamerkops attended the feeding group, catching frogs that escaped the mongooses.
The strangest aspect of hamerkop behaviour is the huge nest, sometimes more than 1.5 m (4 ft 11 in) across, and strong enough to support a man's weight. When possible, it is built in the fork of a tree, often over water, but if necessary, it is built 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 cm (5.1–7.1 in) wide in the bottom leads through a tunnel up to 60 cm (24 in) long to a nesting chamber big enough for the parents and young. Nests have been recorded to take between 10 and 14 weeks to build, and one researcher estimated that they would require around 8,000 sticks or bunches of grass to complete. Nesting material may still be added by the pair after the nest has been completed and eggs have been laid. Much of the nesting material added after completion is not sticks, but an odd collection of random items including bones, hide, and human waste.
Pairs of hamerkop are compulsive nest builders, constructing three to five nests per year whether they are breeding or not.[25] Both members of the pair build the nest, and the building of nests may have a function in creating or maintaining the pair bond between them. Barn owls and eagle owls may force them out and take over the nests, but when the owls leave, the pair may reuse the nest. Owls may also use abandoned nests, as may snakes, small mammals such as genets, and various birds, and weaver birds, starlings, and pigeons may attach their nests to the outside. A few reports exist of hamerkops nesting close together, including in Uganda, where 639 nests were seen in an area of 8 km2 (3.1 sq mi); even if each pair had made seven nests, this would mean 80 pairs were nesting in that area. The species is not treated as colonial, as it does not habitually nest close together, but is not thought to be highly territorial, either. Even where pairs have home ranges that are more spread out those home ranges overlap and are the boundaries are poorly defined.
Breeding happens year-round in East Africa, and in the rest of its range, it peaks at different times, with a slight bias towards the dry season. Pairs engage in a breeding display, then copulate on the nest or on the ground nearby. The clutch consists of three to seven eggs which start chalky white, but soon become stained.[10] The eggs measure 44.5 mm × 33.9 mm (1.75 in × 1.33 in) on average, and weight around 27.8 g (0.98 oz), but considerable variation is seen. Egg size varies by season, by the overall size of the clutch, and from bird to bird. Both sexes incubate the eggs, but the female seems to do the most of the work. Incubation takes around 30 days from the first egg being laid to hatching, eggs are laid with intervals of one to three days, and they hatch asynchronously.
Both parents feed the young, often leaving them alone for long times. This habit, which is unusual for wading birds, may be made possible because of 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 first leave the nest around 44 to 50 days after hatching, but continue to use the nest for roosting at night until they are two months old.
