Distribution, diet and kleptoparasitic behaviour of gulls (Aves: Laridae) in the southwestern Cape Province, South Africa

Doctoral Thesis


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University of Cape Town

The southwestern Cape coastline supports only two common, resident gull species, Hartlaub's Gull Larus hartlaubii and the larger Kelp Gull L. dominicanus. Hartlaub's Gull is endemic to southern Africa and the Kelp Gull that occurs in this region is sub-specifically distinct, L. d. vetula. Although locally abundant, both the Kelp and particularly Hartlaub's Gull have been relatively little studied. This study addressed three aspects of the two gull species' ecology related to foraging. The diet of both species was studied through direct observation and analysis of pellets, scats and stomach samples. Diet was quantified at a range of foraging habitats; (i) a sandy beach; (ii). a rocky shore; (iii) a fishing harbour; and, (iv) a refuse dump. In addition, the proportion of marine prey in the diet of Kelp Gulls at a refuse dump, a sandy beach and at archaeological sites was estimated using stable carbon isotope analysis of bone collagen. Both gulls are generalist foragers feeding on a wide and diverse range of prey species. The Kelp Gull is able to feed on a wider range of prey species than is Hartlaub's Gull due to its larger size. The preferred natural foraging habitat of the Kelp Gull is sandy beaches, where the sand mussel Donax serra is the most important prey. Present day Kelp Gull diet at an undisturbed• sandy beach includes a similar proportion of marine protein to prehistoric gull diet in the same area. Hartlaub's Gull most commonly forages on swarms of invertebrates associated with accumulations of stranded kelp e.g. Ecklonia maxima.

Bibliography: leaves 159-175.