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Regions > African Savanna > Cape shellduck

Cape shellduck

Location at the Zoo: African Savanna



Region:

Africa

Class:

Aves

Order:

Anseriformes

Family:

Anatidae

Genus:

Tadorna

Scientific Name:

Tadorna cana

Description

: In the Anatidae family, shelducks are intermediate between geese and ducks. The male Cape shelduck has an ashy-grey head and neck, with a thin chestnut collar. The breast and upper mantle are tawny yellow. The lower mantle and the underparts are rusty brown. Rump, uppertail coverts and tail are black. In flight, a chestnut back and tertiaries are displayed, flanked by white wing coverts. The metallic green of the secondaries forms the speculum. Primaries are black. The underwing has white coverts and black flight feathers. Chestnut undertail coverts finish the picture. Bill and legs are black.

The female has a white face and chin that extend in variable patterns around and behind the eyes. The remaining head and neck are brownish grey. Breast and upper mantle are the same rusty brown as her body. Most shelducks exhibit sexual dimorphism. Chicks have dark-brown upperparts with white patches. Underparts are white. Juveniles are grey-brown. During their first winter, they moult into adult plumage.

Distribution

: As its alternative name, South African shelduck, suggests, this species is based in the Cape Province of South Africa. Also found in the Orange Free State, Transvaal, Namibia and southeast Botswana.

Habitat

: Tadorna cana resides in semiarid areas and grasslands. In breeding season, it chooses shallow waters, either fresh or brackish, of ponds, dams, marshes, and mudflat pools surrounded by grass or scrubby vegetation. The rest of the year, it’s found around deep freshwater lakes, reservoirs, salt pans, and sewage plants. When they are flightless during the moulting period, dense marsh and islands are used.

Food

: In breeding season, it eats only vegetable matter, mainly seeds of maize, wheat, oats, barley, and sorghum plus algae, peanuts, rice, roots, and leaves of grasses. In nonbreeding season, it is omnivorous dining on insects, larvae, pupae, crustaceans, brachiopods, as well as grains.

Reproduction and Development

: Females are the aggressors, selecting a mate based on his response to the inciting display. While calling gaaa repeatedly, she lowers her head and points at a shelduck that is disturbing her. If (another) male wants to win her, he should be honking at and threatening the offending shelduck. To attract the male, the female has showy-white head feathering. This is an example of sexual selection, a process usually found in males.

The Cape shelduck uses a cavity to nest. This provides protection from predators and heat. The nest, often located in an old aardvark or porcupine burrow, is composed of plants covered by down. Approximately 7 - 11 eggs are incubated by the female for 30 days. A few days after hatching, chicks are led to ‘nursery water’, which could be up to a kilometer away. Nursery groups number from 20 to 40 under the care of one or more nonbreeding shelducks. After fledging at 10 weeks, they stay with their parents for another three weeks. They can reproduce between two and three years of age.

Adaptations

: Shelducks are semi terrestrial, grazing on land, scything on mudflats and dabbling in water. To filter food out of water, they have small ridges on the bill’s edges called lamellae. Long necks are useful for feeding underwater. They forage nocturnally and diurnally, spending much of the day sleeping and preening. Though they don’t dive for food, they dive if frightened. In breeding season, pairs are solitary, wary and quite aggressive towards intruders, signaling an attack with chin lifting and sometimes lateral head shaking.

To find water and moult, Tadorna cana is partially migratory, moving short distances north in November as the dry season begins. For millennia, flocks of 400 to 5000 used the same few deepwater lakes. Now dams provide suitable habitat for their 28 days of flightlessness. About 70% of Cape shelducks moult in just 23 locations, 21 of which are reservoirs behind large dams.

Birds such as the Cape shelduck that migrate in groups have wing markings that are highly developed and distinct. This helps to bring cohesion to the group in flight.

Threats to Survival

: As aardvark numbers diminish, fewer burrows are available for the Cape shelduck to nest in. Also declining are black-backed jackals, causing a disruption to the food chain. Jackals’ prey (smaller mammals that feed on the Cape shelduck’s eggs) are proliferating. In nonbreeding season, the large flocks of grain-loving Tadorna cana are pests to farmers who hunt them down. This species is susceptible to avian botulism.

Increasing numbers of dams have enabled this species to expand its range further into Namibia and Botswana. On the reservoirs and lakes, they are increasingly disturbed by boats and other craft.

Status

: IUCN: Least Concern; CITES: Not Listed.

Zoo Diet

: Chopped spinach, romaine lettuce, waterfowl pellets, crickets, mealworms and finch-seed mix.