|Avian Demography Unit
Department of Statistical Sciences
University of Cape Town
Cape Gannets Morus capensis are easily identified by their large size and black and white plumage which is noticeably silky, giving them a graceful appearance. When seen in flight the snow-white body with the black tail, primaries and secondaries, and dark bill echo this grace. At closer range the distinctive golden crown and nape which gradually become white on the neck are noticeable. On the other hand, the dark brown juveniles look completely black when seen in flight.
There are three species of gannets in the world, the Cape, Australasian Morus serratus and North Atlantic M. bassanus Gannets, the latter two species are occasionally found in southern African waters. These species are very similar in their morphology and breeding habits, and are thus regarded as a allospecies by some authors. They are classified either in their own genus, Morus, or in the genus Sula together with the boobies. Both gannets and boobies belong to the fish-eating sulid family that catch their prey with the most spectacular plunge-dives, unrivaled by any other bird. The sulid is one of the six families that makes up the order Pelecaniformes, the latter including birds like the pelicans, cormorants, darters, tropicbirds, and frigatebirds.
Cape Gannets are restricted to the coast of Africa. They are found in waters off the Western Sahara, around Cape Agulhas to the Gulf of Zanzibar (Tanzania) and occasionally to Mombasa (Kenya) on the east coast of Africa. Amongst sulids, the Cape Gannet has the second most restricted breeding range surpassed only by the Abbott's booby which breeds exclusively on Christmas Island in the Indian Ocean. Cape Gannets have a slightly wider breeding range, comprizing of six offshore islands. Three of these (Mercury, Ichaboe and Possession) are islands off the Namibian coast, two (Bird Island, Lambert's Bay; Malgas Island, Saldanha Bay) off the west coast, and one (Bird Island, Port Elizabeth) off the east coast of South Africa. Within its normal range Cape Gannets are restricted to the continental shelf, at no more than 100 km from the coast. Occasionally they have been recorded on oceanic waters.
The hot, dry and windy climate of these islands has shaped many aspects of the Cape Gannet's breeding biology. As a rule, breeding takes place in densely packed colonies either on the flat ground of the low lying islands, or on flat ledges of the steeply sloping Mercury Island.
The breeding life of Cape Gannets begins when juveniles return to the breeding colony in their third year. The breeding cycle begins with the males establishing a nesting territory. Typically, such a territory is just big enough to accommodate the nest and a pair of birds. In the meantime prospecting females wander on the outskirts of the colony ready to respond to inviting males. The choosing of a partner is an intricate process. The male has to impress the female with much calling, head shaking and bowing. The female will then respond by approaching the male, usually with the head tilted upwards. Once the pair-bond has been consolidated, after much mutual bill fencing and bowing, the partners co-operate in nest construction and in the constant guarding of the territory. Cape Gannets are fussy nest builders using the most readily available material, guano, to which feathers, bones and other debris are added for good measure.
Cape Gannets begin breeding in August or September. Whilst eggs are mainly laid from mid-October to mid-December, some birds may lay as early as mid-June. Typically the clutch is a single bluish egg, which soon becomes soiled. Very rarely two eggs have been found in a nest, which the parents have incubated, and successfully reared the chicks to fledglings. However, it is controversial whether or not a female can lay a two-egg clutch. The egg is relatively small, weighing about 98 grams (about 3.5 per cent of the adult's body weight). Both parents are actively involved in the incubation process which lasts for 42 to 46 days until hatching. Gannets use their foot webs to incubate the egg. The foot webs, which are richly irrigated with blood vesels are wrapped around the egg. The structure of the foot web enables efficient heat transference for both the incubating egg and thermal regulation of the adult bird.
The chick becomes vocal before it even breaks free from the shell. This signals the impending hatching process and precipitates a shift in parental behaviour. Parents switch from incubation (when the egg is under the foot webs) to brooding (when the pipping egg is placed on top of the feet webs). The hatchling is black, naked and blind. The hatchling weighs only about 70 grams, but within three weeks its body mass is one third of that of an adult. At eight weeks the chick outweighs the adult, and this remains so until it becomes a fledging at 95-105 days of age.
Once the chick has hatched, both parents continue to attend to the needs of the fast-growing, ravenous chick. During the first weeks of the chick's life it is fed primarily achovy Engraulis capensis. One of the parents constantly broods the chick at the nest. Larger chicks are usually fed whole fish, mainly pilchard Sardinops ocellatus, and it is not uncommon for the chick to be left unattended for several hours while both parents are out foraging. Feeding takes place in a mouth-to-mouth fashion: the chick inserts its beak deep into the adult's gape where the fish is regurgitated directly into the chick's throat. Within a few seconds the chick has downed the fish and is begging for more. It usually takes two or three feeding bouts to transfer the food that a parent brings back to the nest into the chick's hungry bowels. In some cases one of the parents is so overwhelmed by the needs of an insatiable chick that it is driven to one of the bachelor's clubs on the fringes of the colony to roost in peace.
Once the chicks have their full complement of brown feathers, they are ready to fledge. Now they often wander off to the fringes of the colony where they practise hop-flying but return to their nests to be fed. This phase is a vulnerable one for the fledgling and the first steps towards independence are fraught with dangers. At this stage most fledglings prefer to walk to the shore and swim than follow the adults into flight from one of the runways. But it is also at this time when they can become tasty morsels for seals. For example, at Malgas Island a single old male seal was spotted taking dozens of fledgings per day. Predation is only one of these dangers.
Cape Gannets fledge with just enough fat reserves for them to be able to survive without food for up to ten days. It is during this short time that they have to learn the necessary skills of capturing sufficient food to ensure their survival. In fact, it is during this period that the mortality rate of the Cape Gannet is at its highest. The many carcasses of brown gannets washed ashore on the beaches near breeding colonies attest to this. Those young gannets that survive the fledging stage migrate northwards to rich fishing grounds on tropical latitudes, such as those in the Goulf of Guinea.
Cape Gannets are powerful fliers. They use mainly a flap-gliding technique, which is more energy consuming than the dynamic-soaring favoured by albatrosses. They are highly skilled, generally master almost any kind of weather conditions and only very rarely are they blown off course. One factor that does work against their excellent navigational skills however, is heavy mist. In such conditions the birds cannot do anything else but sit on the water and wait for improved visibility.
It is when hunting, where power and control of speed and weight combine with spectacular precision, that gannets truly display their mastery of the elements. When actively hunting, gannets fly at a height of 10-30 m depending on the conditions of the sea. With heads tilted downwards, they scan the waters below. When a fish is spotted, they rise or even make a half-turn, come to a halt above the intended prey, poised and ready for action. With flexed wings, pointed tail, feet and total focus on the prey, the headlong plunge-dive is executed with unerring precision. A split second before entering the water, they stretch, swing their wings backwards and dive in like a streamlined arrowhead. Gannets can seize their prey either from above on the way down or from below on the way back to the surface, but in either case the fish usually has been swallowed before the gannets re-surface.
The height from which gannets plunge-dive into the water depends to a certain extent on the depth and availability of the fish. If there is an abundance of fish near the surface, gannets may dive at a low angle from a height of one or two metres. It can be a spectacular sight to observe flocks of gannets rain down in a shower of splashing descents upon large shoals of fish. Although most dives are of this more shallow nature, more powerful plunge-dives from a greater height of about 30 metres can precipitate a dive-depth of up to 10 metres.
The total gannet population has been estimated at about 80 000 to 100 000 breeding pairs, about 54% of the population estimated in 1956. The colonies of Possession and Ichaboe along the Namibian coast are in sharp decline whereas the colony at Mercury Is has remained stable during the past years. At present all three colonies in South Africa are increasing in numbers. The Cape Gannet, one of the seabirds endemic to southern Africa, has not been formally considered a candidate for conservation management in South Africa, as for example are the African penguin Spheniscus demersus, the Bank cormorant Phalacrocorax neglectus and the Crowned cormorant P. coronatus. Many authors have expressed concern about its vulnerability due to oiling, disturbances at breeding sites, and commercial fisheries. Even though only a small number of "oiled" gannets are received at the rehabilitation centre of the South African National Foundation for the Conservation of Coastal Birds (SANCCOB) in Cape Town, their population is as vulnerable to the effect of oil spills as all other seabirds. Although information of other pollutants affecting Cape Gannets has been scarce, traces of DDE, DDT, Dieldrin and PBC's have been found on eggs.
Cape Gannets are extremely tolerant towards close human proximity and do not desert their colony as most birds would. They however, are susceptible to disturbances during the breeding season. Especially visitors may disturb prospecting birds on the fringes of the colony and may cause chicks to abandon their nests. The colony at Bird Island, Lambert's Bay, is of particular concern as it is connected to the mainland by a causeway. Since being fenced off to the public in 1974, its guano yield has increased. Until recently, guano, used as a fertilizer had been extracted annually from gannetries. This practise has been discontinued on all the islands for the time being with the last guano scraping having taken place on Bird Is (Lambert's Bay) in 1991. Although the extraction done in April or May, during the non-breeding season did not interfere directly with the breeding process of the Cape Gannets, the removal of guano certainly affected other aspects of the colony. As has been previously intonated, guano is an integral component of their nests and thus a precious commodity to gannets. Removal may delay the onset of breeding by as much as a month and moreover insufficient availability of guano may increase the risk of eggs rolling out of too shallow nests.
All breeding colonies of the Cape Gannet are under some form of protection. Malgas Is. has been part of the West Coast National Park since 1986. The other two colonies in South Africa fall under the administration of the Cape Nature Conservation and the colonies along the Namibian coast under that of the Ministry of Fisheries and Marine Resources. Public access to these areas is restricted, with the exceptions of Bird Island (Lambert's Bay) where visitors are able to view the fenced off colony from state-of-the-art observation facilities and Malgas Island, to which guided tours organised by the West Coast National Park are available.
Fisheries have always regarded the Cape Gannet and other seabirds as competitors. The fishing industry claims that thousands of tons of commercially important fish are lost to seabirds each year. In the past, this misguided fear had been blown out of all proportion on several occasions, to the extent that in 1953 a certain senator S.M. Petersen was pushing for legislation to be passed to end the protection of all seabirds. In fact he even recommended that all eggs of seabirds be collected to ensure the subsequent destruction of these birds. These ideas must have seemed strange even in those days because all of this prompted the Government to finance research into the diet and populations of seabirds and seals.
It must be stated that gannets and other seabirds have been the victims rather than the villians of the fishing industry. The sharp decline in population numbers along the Namibian coast was directly precipitated by the collapse of the pilchard stock due to overfishing during the 1960s. Recent studies have shown that gannets remove about 1% of the adult anchovy biomass annually. These results confirm the assumption that in relation to other predators in the system, the consumption by seabirds is marginal and thus of little threat to the fishing industry. Moreover, the constant monitoring of seabirds and particularly the monitoring of their sampling of the marine environment provides valuable data to assist the management of fisheries.