|Avian Demography Unit
Department of Statistical Sciences
University of Cape Town
Text and photographs by Michelle du Toit
Though looking like nondescript black birds from a distance, at close range the black plumage of Cape cormorants can be seen to have a blueish sheen, which is iridescent in breeding adults. The eyes are turquoise in adults, with blue beads on the eyelids, and grey in juveniles. The gape at the base of the bill of adults is yellow-orange.
Cape Cormorants are endemic to southern Africa, where they are abundant on the west coast but less common on the east coast of southern Africa. They breed between Ilha dos Tigres, Angola, and Seal Island in Algoa Bay, South Africa. Non-breeding birds range as far north as Lobito, Angola, and Inhaca Island, Mozambique, and vagrants have been recorded from Gabon. On the east coast they follow shoals of sardine Sardinops sagax that migrate along the KwaZulu-Natal coast (South Africa), occasionally irrupting into southern Mozambique.
Cape Cormorants breed on offshore islands, cliffs, rocks and artificial structures such as jetties, platforms and even moored fishing vessels and yachts. Occasionally they will nest on breakwaters and ruins and in estuarine wetlands and coastal sewage works. They construct shallow nests using seaweed, sticks and guano, and nest in dense colonies at 'pecking distance' from one another. They generally feed within 10-15 km of the shoreline, preying on Pelagic Goby Sufflogobius bibarbatus, Cape Anchovy Engraulis capensis, Pilchard Sardinops occelatus and Cape Horse Mackerel Trachurus trachurus.
Large interannual fluctuations in breeding numbers due to breeding failure, nest desertion and mass mortality are related to the abundance of Anchovy, for which they compete with commercial fisheries. This makes it difficult to accurately determine population trends. A co-ordinated census of breeding Cape Cormorants at all colonies is necessary to obtain an accurate population estimate. In 1977-81, the breeding population was estimated at 277 032 pairs, but by 1996 there were only 72 000 pairs.
Major mortality results from disease, with tens of thousands of birds dying during outbreaks of avian cholera Pasteurella multocida. Cape Cormorants are also vulnerable to oiling, and are difficult to catch and clean. Discarded fishing gear and marine debris entangle and kill birds. Kelp Gulls (Cape Gulls) prey on eggs and chicks; this is exacerbated by human disturbance, especially during the early stages of breeding, and the increase in gull numbers. Rain and gales may result in the flooding of nests and mortality of chicks. Both adults and fledglings are taken by rogue Cape Fur Seals as prey, and Great White Pelicans prey on nestlings and eggs. Summer tourists, recreational fishers and scuba divers occasionally disturb birds breeding on small rocks. Cape Cormorants have benefited from the guano industry, through the construction of guano platforms in Namibia, on which they nest in large numbers.