Seabirds

Southern Africa hosts a distinctive and isolated community of resident seabirds.  These are usually considered to consist of fourteen species.  They are: African penguin (Spheniscus demersus), Cape gannet (Morus capensis), Cape cormorant (Phalacrocorax capensis), bank cormorant (Phalacrocorax coronatus), whitebreasted cormorant (Phalacrocorax carbo), white pelican (Pelecanus onocrotalus), swift tern (Sterna bergii), Caspian tern (Hydroprogne caspia), Damara tern (Sterna balaenarum), roseate tern (Sterna dougalii), Kelp gull (Larus dominicanus vetula), Harlaub`s gull (Larus hartlaubii) and greyheaded gull (Larus cirrocephalus).  Of these 14 species, 13 breed in the Benguela system.  The roseate tern is the only southern African seabird that breeds solely in Agulhas Current waters.  Seven of these species and two subspecies are endemic to our coasts and most of these are entirely or largely confined to the Benguela system.  As we have seen in a previous post the Benguela current supports a prolific population of fish of the size and kind which seabirds find particularly to their liking.  Sardines, herrings, mullet, anchovies and mackerel abound in these waters.

The availability of breeding sites is also a very important determinant of seabird distribution and abundance.  All the seabirds listed are colonial breeders, although Damara terns breed in loose aggregations and several species may occasionally breed singly.  Seabird colonies would provide easy pickings for terrestrial predators and most seabirds breed entirely on islands or at inaccessible mainland sites.  Offshore islands and guano platforms provide the safest and most important breeding sites.  The Islands of the South Western Cape between Dyer Island and Lambert's bay are therefore very important for the seabird breeding success.

Fish is the main food of most seabirds in Southern Africa.  Exceptions are the gulls, which eat large quantities of crustaceans, molluscs and refuse.  The bank cormorant may feed extensively on crustaceans and other invertebrates.  Most seabirds eat small surface-shoaling fish, the three most abundant, the Cape gannet, Cape cormorant and African penguin, feeding mainly on pilchard and anchovy.  Most seabirds obtain their food near the surface.  Even the birds which can dive, the cormorants and penguins, are thought to catch their food within a few metres of the surface.  The cormorants and penguins are pursuit divers, diving from the surface in pursuit of their prey.  The cormorants use their feet for propulsion and the penguin its flippers.  Gannets and terns are surface-plungers, diving from the air into the water.  The gulls, particularly the kelp gull, are very adaptable in feeding technique.  They feed by seizing prey at the sea surface, scavenging food at sea and on land, piracy and by dropping mussels onto rocks to break them open.  All of the species discussed here feed close to shore, although gannets, and to a lesser extent kelp gulls, range offshore to scavenge from demersal or bottom trawlers, particularly in the area between Cape Columbine and the Cape Peninsula.

Moving further south, we can see an increase in seabird numbers. Apart from the obvious gathering of birds in estuaries, seabird life was rather sparse between the Orange river mouth and the Olifant's river.  Here and there a few cormorants flying low over the sea, sea gulls on the beaches and a few spottings of our project bird, the African Black Oystercatcher scavenging for food among the rocks.  At the different estuaries these were supplemented by terns, white pelicans and a whole lot of different waders and freshwater species.

South of the Olifant's river, things changed. During our reconnaissance trip in July, we noticed that getting closer to St Helena Bay, the numbers of seabirds increased. And believe me - it can be mighty overwhelming to sit on the quay of Laaiplek with a portion of hot fish and chips, while the Cape Cormorants swarm in in their thousands through the mist, on their way to their roosting sites in the Berg river estuary.

Cormorants are often seen drying their feathers by perching on rocks, holding their wings open to the sunshine.  They are agile swimmers, although their feathers are not as water-repellent as those of other seabirds.  Cormorant feathers are not so closely knit and tend to become sodden.  This forces the cormorant to dry itself at intervals and indulge in much preening.  When a big shoal of fish is discovered, cormorants gather from considerable distance, forming up into their huge “V”-shaped flocks.  The leader of the flock is periodically relieved by another pace setter.  As they settle on the water above the shoal, any semblance of the orderly control apparent during their flight, disappears.  It is each-bird-for-itself, diving, darting underwater and gorging to the point of discomfort.

The Cape Cormorant is the most abundant seabird in the region. 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. The Southern African population of Bank cormorants has also decreased alarmingly in recent years.  From a population of 8700 breeding pairs recorded in 1980, numbers have dropped to a current level of 4900 pairs.  Environmental conditions that are thought to have caused a scarcity of food, plus competition with seals are both cited as reasons for this dramatic drop in numbers.  When breeding, Bank Cormorants are also vulnerable to disturbance by humans.  Kelp gulls are quick to prey on eggs and chicks in abandoned nests.

In the early part of the twentieth century, egg and guano collecting had a dire effect on seabirds, reducing their populations to a fraction of their original size.  Today oil spills, food shortages, climate change and marine pollution all take their toll on seabird populations.  Cormorants are no exception.  For instance, changes in fish abundance and distribution, which may be brought on by Global warming, or by overfishing, might cause cormorants to forage further and further away, abandoning their nests in the process.

On the flip side of the coin, gulls have actually adapted so well to human beings that many gull populations are found hundreds of kilometers from the ocean.  Instead of a diet of marine organisms, gulls have become accustomed to feeding on offal from fishing boats, human refuse, terrestrial insects, snails and dead animals washed up on our beaches.  Earlier in the twentieth century large numbers of Kelp gulls were shot and their eggs destroyed because they were known to prey on the eggs and chicks of guano producing birds, such as penguins and cormorants.  The highest concentrations of Hartlaub`s gull are found inland, but there are still approximately 12 000 breeding pairs confined to the productive Benguela system and the Western Agulhas bank.  It`s natural diet made up of the invertebrates that are associated with stranded kelp (Ecklonia maxima).  Greyheaded Gulls favour human environments, such as sewage farms, rubbish dumps or tall buildings for nesting.

Probably because they have adapted well to the human environment, the populations of all three Southern African gulls are stable.  Gulls and humans do not always live in harmony, however.  In 1987 a colony of 4000 Hartlaub`s Gulls established itself at Ysterplaat, the air force base near Cape Town.  Birds and aircraft proved to be a dangerous mix and the colony had to be dispersed by breaking eggs, collecting chicks and shooting about 300 birds.