BREEDING OF SACRED IBIS THRESKIORNIS AETHIOPICA AT LAKE SHALA, ETHIOPIA

Abstract

SummaryData are based on more than 200 h of observation at Ethiopia's Lake Shala from 1966 to 1972.Except for differences in size of bill, there are no useful field characters separating male and female Sacred Ibis. The breeding plumage is described; vivid blood‐red colour underneath the wings and the ornamental plumes are especially obvious when nesting commences.Physical and biological features of Lake Shala, Ethiopia, and its nesting islands are described; the species of birds nesting on the Shala islands are given.Ibises nest at Shah from March to August; no nesting has been recorded from September to February during the last months of the ‘big’ rains through the main dry season. Nesting normally begins in the ‘small’ rains (between 14 March‐24 April), although instances were recorded as early as 1 March and as late as 20 August. The ibises normally nest once per year, although it is possible that occasionally a second nesting may occur after an unsuccessful first attempt.The ibises at Shala nest in discrete groups; several nesting groups may form on any or all of the islands; the number of groups attempting to nest varied from year to year. Nesting activity begins when males arrive and establish pairing territories, usually in a small tree but sometimes on the ground. When females and other males arrive at the pairing territories, pair formation ensues. At this time males perform forward threat, modified forward threat, pursuit flight, supplanting attack and modified snap displays, while both sexes perform stretch and bow displays. Once established, the pair abandons the pairing territory and moves to the nesting area, usually near but always distinct from the pairing territory, and establishes a nest‐site territory. Most members of the nesting group move to the nesting area on the same day. Copulation then takes place, and is followed by collection of nest material, usually by the male. Nests are built close together. The average area of 10 nests measured was 0.09 m2. Nests are usually less than 20 cm thick and are made of many small branches and sticks.The average clutch in 34 nests was 2.24 eggs; the average size of 34 eggs was 63.4×43.5 mm. Incubation probably begins when the clutch is complete. Both sexes incubate, and the incubation period probably lasts 28–29 days.The development of the young is described. The young leave the nest‐site territory when 14–21 days old. Although they are capable of some flight when 35–40 days old, the young do not leave the colony until they are 44–48 days old. In the colony, both parents care for the young. Usually only one parent at a time is with the young. The parents recognize their own young and are usually recognized by them. The behavioural interactions between young and parents are described.Fledging success in 1968 was 1.06 young per pair. The number of pairs successfully rearing young varied annually from none to 81%, on average over six years (1966–70, 1972) 35%.Predation at the breeding colonies is minimal. The food of one one‐month old chick consisted of beetle larvae, lepidopteran larvae and beetles. Feeding areas, although undetermined, must be widespread.Inter‐specific competition between Sacred Ibis and other nesting birds at Shala is discussed.Among possible factors stimulating nesting at Shala one, fairly heavy rainfall, seems to be especially important. It is also suggested that especially heavy rain‐storms cause ibises to abandon the colonies, and result in poor breeding success.