-Lesser Sheathbills (Chionis minor) were studied at Marion Island in the sub-Antarctic. Activity-time budgets of parents rearing chicks were converted into energy budgets and added to estimates of the food delivered to the chicks at the nest, in order to estimate the total energy costs of rearing chicks. Most of the food was obtained by stealing it from breeding penguins and it is improbable that Lesser Sheathbills could rear their chicks in the present manner without access to penguins, or possibly other colonial seabirds. Kleptoparasitism probably had little effect on the breeding success of the host Rockhopper Penguins (Eudyptes chrysocome), since a pair of sheathbills removed less than 1% of the food that the penguins brought into their territory. Investments of time, energy and risk of injury while rearing chicks were very similar for both sheathbill parents. The need to brood young chicks, owing to the harsh climate, restricted food delivery and caused chicks sometimes to starve. Kleptoparasitism, the stealing of food by one individual from another, occurs amongst many bird species but is seldom a basis for specialization (Brockmann and Barnard 1979). Sheathbills (Chionis spp.), which live in the Antarctic and sub-Antarctic, obtain a significant portion of their diets by stealing food from penguins, and occasionally from cormorants and albatrosses (Jones 1963, Burger 1979, in press a). Pairs of breeding sheathbills maintain foraging and nesting territories centered on colonies of breeding seabirds, usually penguins, and they obtain virtually all their food from these colonies. Reproduction in birds usually requires considerable investment of time and energy above the costs of normal maintenance (King 1973, Ricklefs 1974). Lesser Sheathbills (Chionis minor) apparently need to have access to a colony of penguins, or perhaps cormorants or albatrosses, in order to meet the costs of breeding (Burger 1979). In this paper I report time and energy demands of adult Lesser Sheathbills while they are rearing chicks, discuss whether breeding is theoretically possible if the birds have no access to penguins, and estimate the effects of kleptoparasitism on the breeding penguins. The division of labor between the sexes of breeding pairs is also examined. The chick-rearing period was selected as being the most demanding phase of breeding, as it is in most nidicolous bird species (Ricklefs 1974). Lesser Sheathbills’ nests are merely heaps of debris requiring little effort to make, their eggs are not large in relation to the size of the adult female and the clutch is small, averaging two or three eggs (Burger 1979). Both sexes incubate and the cost of incubation is likely to be far less than the cost of feeding chicks (King 1973, Ricklefs 1974, Drent 1975). MATERIALS AND METHODS Lesser Sheathbills are resident on four island groups in the southern Indian Ocean (Watson 1975). I studied the birds at Marion Island (46”54’ S, 37”45’ E) in the Prince Edward Islands. Observations in the austral summer of 1976/1977 concentrated on three pairs (A, B and C) which bred in adjacent colonies of Rockhopper Penguins (Eudyptes chrysocome). All six parents had been sexed (Burger 1980a) and color-banded two years before observations commenced. Pairs A and C and the female of pair B had bred successfully in the same territories for at least three seasons; the male from pair B was a three-year old bird breeding for the first time. Pairs A and B fed one chick each from hatching to fledging (about 60 days) and pair C fed three chicks for 39 days and two to fledging. Diurnal time budgets of these three pairs were determined at roughly weekly intervals from the time the chicks hatched (mid-January) until they left the nests to follow their parents (mid-March), making observations impracticable. I watched the birds from a blind from which the three nests and most of the three territories could be seen. The activities of each adult were recorded at five-minute intervals, and one of eight activities (see below) was assigned to part or the whole of each interval. The weekly observations were made on successive days to cover the periods dawn (t05:OO) to noon and noon to dark (t20:20). Adults roosted throughout the night within their territories, and the dawn-dark observations were thus sufficient to construct 24-h activity-time budgets. Bad weather severely restricted observations in the last week and where necessary in this case, the data from 385 min of observations were extrapolated to cover the 870 min of daylight. I estimated the mass of meals fed to chicks by placing ‘ chokers’ around the chicks’ necks to prevent swal-
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Apr 1, 1990
Gerald L Kooyman + 3
Open Access