Saturday, 14 May 2011

Bristol Waterfowl 2: Wood Duck

A.sponsa male
  One of the commonest non-domestic ducks to be seen in any waterfowl collection is the American Wood Duck or Carolina Duck, Aix sponsa. With its small size, ease of care, and free reproduction in captivity it is popular with both beginner and experienced amateur keepers as well. The only rival it has is with the very closely related east Asian Mandarin, Aix galericulata.

A.sponsa female
Both of these species have rather unusual habitats and breeding behaviour for ducks. They are specialists in deciduous woodland pools and slow-moving rivers, especially those with dense cover close to the waters edge. Their diet is mainly seeds of waterside plants, but they are also fond of acorns and beech mast in the autumn, and will also feed on aquatic invertebrates, especially females in lay and growing ducklings, which need more protein.
A.sponsa duckling
The Wood Duck and Mandarin are both classed in a separate tribe to the mallard and similar dabbling ducks, the Cairini or perching ducks, This is a reference to their fondness for trees, both for perching when out of the water (they have sharp claws on their toes and can climb well), and also as a nest site for their eggs. Hollow trees, woodpecker holes, and similar sites are all used, often at a considerable distance from the ground – up to 90m have been recorded – although 10-15m is more usual. Whatever the height, the ducklings climb to the nest entrance in response to the female’s call and then jump to the ground or water if the nest hole overhangs the pool. Unusually for ducks, especially where there is a great difference in colour between the sexes, the male stays with the female and helps guard the brood, although he does not help with incubation.

In the past, deforestation and overhunting brought the Wood Duck to the verge of extinction, but protection measures, nest box schemes (they take readily to nest boxes), and a reduction in deforestation have improved their prospects considerably.

As one might expect from such a widely kept bird, Wood Ducks have escaped and bred in the wild in the UK, but the population is apparently not self sustaining. This is in contrast with the Mandarin, which has an estimated UK population of perhaps 7,000, mostly in southern England. The reason for the difference is not clear, but may be because the Mandarin fledges at around 8 weeks, whereas Wood Duck take nearer to 10. This makes them more vulnerable to predators and the vagaries of the British climate than their Asian relatives.

Although there are now no European species of Aix, there is a report of Mandarin, or possibly a related and now extinct species, from around 500,000 bp in one site in the UK at West Runton. If this is not a later contaminant, it would suggest that Aix species were once resident in Europe but later became extinct during one of the intervening glacial periods. As a specialist in deciduous woodland they would be very vulnerable to loss of habitat, for example during the height of the last glacial maximum the nearest extensive oak woodland to western Europe was apparently along the southern shores of the Black Sea, with deciduous woods even in Spain and Italy reduced to isolated small copses in sheltered and damper areas. This raises a point about the surviving fauna of modern Western Europe – it is extremely impoverished in species compared to comparable forests in America and Asia. This is because the main geographic barriers in Europe, mainly the Alps and the Mediterranean, run east-west, and block southward dispersal of birds, reptiles, and mammals and their return north in response to climate change. The high percentage of invasive or naturalised species in Europe, as well as the spread more recently of eastern birds such as the Collared Dive, may be a reflection of the lack of competition in Europe compared to other regions.

Next week – the Pochard

The History of British Birds DW Yalden & U.Albarella
The Naturalised Animals of the British Isles C.Lever
(Images from wikipedia)

A.galericulata male

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