THE BREEDING BIOLOGY OF THE GREAT WHITE PELICAN PELECANUS ONOCROTALUS ROSEUS AT LAKE SHALA, ETHIOPIA

Abstract

Summary. Pelecanus onocrotalus roseus is known to breed more or less regularly in Africa at Lake Shala, Ethiopia; Lake Rukwa, Tanzania; St. Lucia Bay, Natal; Abou Tougour, Chad; Kapsikis, Northern Cameroons; and Wase Rock, Nigeria; and has been observed or reported breeding at Mweru Marsh, Zambia; Lake Ngami, Botswana; Seal and Dyers Islands, South Africa; and Lake Natron, Tanzania. The Shala colony, after Lake Rukwa's, is the largest known breeding colony in Africa and is probably of crucial importance to the species in Africa.For successful breeding regular colonies of P. onocrotalus must have an assured supply offish and an inaccessible breeding site. These conditions are met at the Lake Shala colony.Assuming that a Great White Pelican consumes about 10% of body weight or about 900‐1,200 g per day, the Lake Shala breeding colony with 7,500‐12,000 pairs would consume about 3,140‐5,040 tonnes in the breeding seasons.The partially unsuccessful breeding of perhaps 10,000 pairs at Lake Natron in 1962 is described. Failure was attributed to sudden failure of the food supply.Breeding of the Great White Pelican on Lake Shala takes place throughout the year although there is a peak of numbers breeding from December to the end of March, which is in the dry season. Observations recorded in this paper cover two full breeding seasons, 1965‐6 and 1966‐7.Although during the peak period of breeding the flock at Shala is made up of 2,500‐5,000 pairs, this mass is composed of smaller units, each made up of 300‐1,200 pairs, laying more or less together with little overlap from one group to the next.Three changes occur in the plumage of the Great White Pelican towards the onset of the breeding season: (a) the development of a knob or swelling on the forehead at the base of the beak with associated expanses of brightly‐coloured bare skin, pinkish yellow in males and bright orange in females; (b) the development of a crest; and (c) the development of a yellowish band across the chest or larger areas of darker brown in the plumage.Four distinct colour‐types of breeding plumage were observed: the dark‐brown type, the brown‐breasted type, the yellow‐banded type, and the type with scarcely any suggestion of a breast patch. No correlation was found between plumage type and sex, size, length of bill or any other obvious physical feature.Group display and individual displays of the adults are described. There appears to be no special pre‐copulation display.Elaborate nests are not prepared; the male collects the nesting material, and both sexes build the nest. Nests are small, 35–60 cm in diameter (averaging 46‐4 cm), and close together (364 nests averaged 1–55/m2).Average clutch‐size is 1–88. Incubation begins with the first egg; both sexes incubate; the incubation period is probably about 38 days.The fledging period is 65–70 days, of which about the first 28–30 days are spent in the nesting area. After that the young form into groups or “pods”.The development of the young is described. When it is very small it is fed by either parent several times each day. When the young is 30–35 days or older, it is probably fed less than once per day.The bright red nail‐like tip of the adult's upper mandible serves to direct the begging movements of the young chick, and it probably holds the liquid food on which the chick feeds.It is suggested that pod formation of young both in the middle of the day and at night possibly helps to avoid extremes of temperature, either of heat or cold.Parents recognise their own chicks; and young pelicans, at a later stage, recognise their parents. Indiscriminate feeding of young does not occur in P. o. roseus.Adults often are quiescent but not asleep at night. Activity is stimulated by the earliest light, and most pelicans begin leaving the island by 09.30‐10.00 hrs. Most departures and arrivals take place from about 10.00 to 16.00 hrs.The distance from which food is brought to the colony is not known for certain, but most birds probably fly to and from Lake Abiata. It is not known where the young go once they leave the colony; most do not fly to the obvious place, Lake Abiata.Although difficult to determine, we suggest that the breeding success of the pelicans is on the average less than one young per nest. Egyptian Vultures were the most important predators at the colony, but all forms of predation together accounted for less than 10% of the nesting losses.The timing of breeding in relation to food supply, climatic factors and inaccessibility of the breeding site is discussed. Inaccessibility appears to override seasonal factors, but in permanently inaccessible sites the peak of breeding is in the dry season.It is suggested that the Great White Pelican nests in discrete breeding units within the main colony to minimise the effect of predation.