Some particulars, for the most part fragmentary, have been recorded on the nesting of sixteen out of the twenty-six African species of hornbills.
The scanty data for Bucorvus cafer make it practically certain that the Ground Hornbills fail to exhibit any of the peculiarities in breeding biology that distinguish the Bucerotidæ as a family from all other birds, including the analogous Toucans. The features common to all the other African species we know anything about may be summarized as follows:—
They choose holes with narrow entrances which they constrict with plaster so as to leave a vertical slit barely the width of their own bills. They build mainly with earth when the entrance is comparatively wide, with food-remains and fibrous matter when it is small. Dung has probably been reported too often as a constituent of their plaster; it cannot be identified without chemical analysis. As a rule no lining is brought for the nest. The males in no way coerce the females, who will themselves in and remain inactive from two to four months continuously, moulting meanwhile. During this period their mates are solely responsible for the food-supply of the family. Irregularity in egg-laying is general. A first egg may not be laid for nearly a fortnight after a female's entrance, and a second may follow five days after the first. This is reflected in unevenness in size within a brood and in their irregular emergence. The primary provision for the sanitation of the nest is that the occupants defecate forcibly towards the entrance.
Within this general framework of habit the African species of hornbills show much variation in detail:—
- (a) The plastering may be perfunctory in some of the West African hornbills, but it is most careful in Lophoceros spp. and in some Bycanistes spp. In Ceratogymna the male brings the material and both he and his mate fix it. In Bycanistes there is division of labour between the male, who brings the whole of the material, and the female, who does all the actual building. In B. cristatus the plastering of a hole can occupy a pair for months, the progress of the work depending on the male and probably on his salivation. In Lophoceros spp., on the other hand, the plastering process takes only a few days, or no more than a few hours, and the male's part in it is relatively unimportant.
- (b) Bycanistes females do not come out until their offspring are ready to fly. A B. cristatus was in her hole for 108 days, the egg probably hatching about the fiftieth day. In Lophocoros the females remain inside for 50–70 days and emerge 14–28 days before their young, the incubation period of which averages 30 days and their fledging period 45.
- (c) In Bycanistes the male is responsible for the entire food-supply of the family until they all fly. In Lophoceros the female helps to feed the young as soon as she emerges. The fact that the Lophoceros bring the food to the nest in single morsels, not in batches like Bycanistes, and, moreover, have much bigger families to cater for, would probably make it impossible for the Lophoceros male to bring up his family without the relief afforded to him by the early emergence of his mate.
- (d) The fledglings of Lophoceros, only 25 days out of the egg when the mother leaves them, at once elaborate plaster, re-seal the hole, and take over the sanitation of the nest. There is no evidence of precocity in Bycanistes fledglings.
- (e) Sanitation appears to receive more attention from Lophocsros spp. than from Bycanistes and Ceratogymna, in the nests of which the insect fauna should be examined for scavengers.
- (f) In the genus Bycanistes some species moult so suddenly as to lose the power of flight, and some do not. In Lophoceros spp. and Tropicranus the females drop all their wing and tail-feathers very rapidly, in Lophoceros by about the time their clutch is complete. They are probably flightless for at least six weeks, because their new feathers have made but little progress by the time their young hatch. There may be specific variation in the extent to which the contour feathers participate in the sudden moult.
This habit of sudden moult raises several problems. It cannot be explained teleologically, and may be a physiological consequence of the brooding bird's reduced ration of light.