Saturday, 21 May 2011

Bristol Waterfowl 3: Diving Ducks

Common Pochard
 As part of the legacy of earlier days when Bristol’s collection of waterfowl was focused on ornamental ducks we still have a couple of species of diving ducks in the collection – Tufted Duck Aythya fuligula and Pochard A. farina. When these last individuals pass away they will probably be replaced with more endangered waterfowl species. On the whole, the various Aythya species are common ducks around the northern hemisphere, with a few species south of or near to the equator. One of these, the Critically endangered Madagascar Pochard, was believed extinct and is now the focus of an intensive conservation programme with the aid of DWCT and the WWT. Not in quite as bad a position, but still declining to a worrying extent, the East Asian Baer’s Pochard A. baeri is believed to number less than 20,000 adults, possibly under 10,000, and is classed as Endangered by the IUCN.

Madagascar Pochard
Diving ducks get their name from their habit of feeding in deep water – often diving several metres down to feed on water plants, freshwater mussels, and other aquatic life. Mallard and Wigeon by contrast are called dabbling ducks – they feed at the water surface or on land. The various ecological specialisations of waterfowl are what enable a single lake to be home to a multiplicity of species.

Diving ducks as part of their adaptation to a more aquatic lifestyle have their legs set further back on their bodies than dabbling ducks. This gives them more manoeuvrability on water but makes them ungainly on land, and as a result when nesting they often prefer to build their nests at the edge of the water, often in reeds or other dense vegetation. The usual clutch is 6-8 eggs. When they hatch, the ducklings immediately enter the water and will dive deep from a few hours old. This means if rearing them away from the parents they need a deep tank, otherwise they are liable to bang their heads on the bottom if startled!

A small duckling is a tasty meal for large predatory fish, and the introduction of tilapia to Lake Alaotra, combined with habitat loss, is probably the cause of the near-extinction of the Madagacar Pochard. Even other species experience high losses during the fledging period.

Tufted Duck
Outside the breeding season, most diving ducks are migrants, often over long distances. In the UK, the wintering population of Tufted Ducks and Pochard is increased by large numbers of migrants from Scandinavia and Siberia. Among these, some rarer species such as Scaup and Ferruginous Duck can be found, causing much excitement among birders. Unfortunately, diving ducks readily hybridise, even in the wild, and the resultant hybrids can resemble the vagrant species, so duck ID is one of the trickier aspects of birding.

In a mixed waterfowl collection diving ducks need to have their food provided either at the water edge or preferably in the water. They often have a higher protein requirement than dabbling ducks, but keeping other birds from getting their first can be a problem. I have seen at the Wildfowl and Wetland Trust at Slimbridge one way round this – an open-bottomed chicken-wire cage on the water which the diving ducks will enter from below, while keeping off the dabbling ducks on the same lake.

(images from Wikipedia)

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