Text and photographs by Michelle du Toit
Animal Demography Unit
White-breasted Cormorants occur along the entire southern African coastline, and is common in the eastern and southern interior, but occurs only along major river systems and wetlands in the arid western interior. The coastal population breeds from Ilha dos Tigres in southern Angola, to Morgan Bay in South Africa. In 1977-81 there were 2 524 pairs breeding along the coast between northern Namibia and the Eastern Cape Province. Currently, the overall breeding population in southern Africa is estimated at 3 700 pairs, representing some 11 000 individuals, of which approximately half is entirely non-marine.
Along the coast, White-breasted Cormorants occur mainly within 10 km offshore, often near reefs. Inland populations frequent rivers, streams, dams and impoundments with adequate fish populations. A variety of nesting habitats are used, including offshore rocks and islands, cliffs, trees, reeds and man-made structures such as disused jetties, marine platforms, shipwrecks and pylons. Such artificial structures are used by over half the coastal population for breeding. The birds nest singly or colonially, occasionally utilising the same nests year after year. White-breasted Cormorants that forage in the marine environment feed on bottom-living, mid-water and surface-dwelling prey of little or no commercial importance, such as Sparids.
Changing water levels in the interior of southern Africa results in nomadic movements of White-breasted Cormorants, with distances of up to 1 054 km covered by young birds, as evidenced by ring recoveries. Inland (freshwater) and coastal (marine) populations interchange in both directions. Birds have been shown to move from Barberspan, South Africa, over 900 km away to Gazangula (Zambia), Bulawayo (Zimbabwe) and the Orange River estuary.
White-breasted Cormorants nesting
Human disturbance poses a serious threat at breeding sites. These cormorants are more susceptible to disturbance than the other marine cormorants, and leave their nests for extended periods if disturbed, exposing eggs and chicks to Kelp Gull predation. Black-backed Jackals and Cape Fur Seals occasionally take adult White-breasted Cormorants as prey. Other mortality factors include Avian Cholera, oil pollution, discarded fishing line and deliberate killing by humans inland.
A co-ordinated census of breeding White-breasted Cormorants at all southern African colonies, both coastal and inland, is necessary to obtain an accurate population estimate.