IUCN threat status:

Vulnerable (VU)

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Biology

Cape gannets first return to breeding colonies after two to three years at sea (8). Males establish a nest territory while females wander on the outskirts of the colony ready to respond to inviting males, who entice females with much calling, head shaking and bowing. Once a mate is found, the pair bond is consolidated with mutual bill fencing and bowing. Partners then cooperate in building a nest and guarding their shared territory. Eggs are mainly laid from mid-October to mid-December, although some birds may lay as early as mid-June (4). The clutch typically consists of a single bluish egg, rarely two, which is then incubated for 42 to 46 days by both parents using the warm webs of their feet, which receive a rich blood supply (4) (5). The hatchling is naked and blind, but by eight weeks it outweighs the adults and continues to do so until it becomes a fledgling at 95 to 105 days of age. Both parents tend to the needs of the fast-growing, ravenous chick, primarily feeding it regurgitated anchovy (Engraulis capensis) and sardine (Sardinops ocellatus). Before fully fledging, juveniles wander off to the fringes of the colony where they practice hop-flying, but return to their nests to be fed, and most prefer to walk to the shore and swim rather than follow the adults into flight. This stage is fraught with dangers, such as falling victim to predation by seals. These birds fledge with a store of fat that enables them to survive without food for up to ten days, during which time they must learn the essential skills of capturing sufficient food for survival. The mortality rate for the Cape gannet is at its highest during this precarious stage of life (4). The Cape gannet hunts for fish with spectacular power, grace and precision, plunge-diving for prey from heights of 10 to 30 m with flexed wings, pointed tail and feet, and total focus on its quarry. Just before entering the water, the bird stretches and swings its wings backwards to form a streamlined arrowhead-like body as it pierces the water (4). Surprised fish are caught in the bird's dagger-like bill and may be swallowed before even leaving the water (5) (7). Shoaling fish in surface waters are preferred, including anchovy (Engraulis capensis), sardine (Sardinops sagax) and saury (Scomberesox saurus), and offal discarded by fishing boats may also be taken (2) (5).

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Source: ARKive

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