SHELDUCK (TADORNA TADORNA)
One of our largest of ducks ~, the shelduck Tadorna tadorna casts a handsome and commanding figure with its pure white body, chestnut chest band, black wingbar, bottle green head and a bright vermillion beak. The shelduck is named after it’s parti-coloured, or “Sheld” plumage (black and white with some colour)1. Males have a knob atop their beak, whilst females have a smudge of white feathers at the base. They also sound different. Males have a soft whistle, whilst the female has a deeper honk (Both can be heard below).
Somewhere between a duck and a goose in
appearance, the shelduck is our only native member of a waterfowl group known
as tadornids *. Some of its relatives have become established in the UK from
captive birds, including the ruddy shelduck Tadorna
ferruginea and Egyptian goose Alopochen aegyptiacus.
Shelducks are easily spotted on sandy or muddy coasts,
estuaries and marshes. Unlike most
ducks, they are more of a wader than a swimmer, foraging in a characteristic bill-sweeping manner in
shallow water or wet silt. The shelduck uses tooth-like lamellae on the side of
its bill to filter out its food. Their diet
is mainly tiny Hydrobius water snails
along with other aquatic invertebrates, fish fry and some plant matter2. In deeper water shelduck will upend to reach
the bottom, with just their chestnut rump above the water. In flight, the shelduck casts a particularly
large and impressive figure.
The shelduck’s characteristic feeding behaviour. Video by J R Martinez.
The shelduck’s breeding behaviour is also unusual. A clue is in one of their old names, “burrow duck”1. They like to nest in a hole, often an abandoned rabbit burrow but also in buildings, under dense scrub cover, haystacks, spaces between straw bales, tree holes or any other secluded hollow2,3. This habit caused their persecution in the middle ages, when privately-owned rabbit warrens were a valuable asset on sand dunes and shelducks were killed for apparently competing with the residents4.
Males are fiercely territorial during the breeding season. Disputes
involve a lot of posturing, such as neck stretching, head-bobbing and flapping
whilst uttering a deep chuckling warning call. In an attack, the male viciously
bites his opponent and deals blows with his powerful wings. Their heightened aggression even extends to
other ducks, gulls and waders
5.
Chaotic aggression between shelduck pairs during the breeding season. Video by Frank Vlamings.
Females deal with limited nest holes by laying eggs in
another’s nest if they cannot get their own1. The foster parent’s
brood may increase by ten or more, but surprisingly she will still hatch them
all successfully1. This probably has little effect on the foster
parent in practice since shelducks show minimal parental care.
Almost as soon as they hatch, the black and white, marbled
ducklings are lead out of the hole in a procession to their watery nursery. Sometimes the parents may nest miles inland,
so the family faces quite a trek to their new home1. Although the parents will
defend their brood fiercely along the way, once at the water they usually abandon
their ducklings1,2,3,6.
However, the babies are not totally helpless. They gather into crèches numbering from around twenty to over one hundred ducklings, which will be guarded by a few unrelated, non-breeding ‘aunties’1,2,6. Like other ducklings, baby shelducks are quite independent and can run and dive well to avoid predators 1,6. The juveniles remain black and white until they reach adulthood at 2 years old4.
For the adults, leaving their offspring isn’t simply
shirking their duties, it is a matter of urgency. Once breeding season is over
they must migrate to their moulting areas before they are unable to fly2,6.
Like most waterbirds they are grounded during their four week moult, when they
renew their wing feathers, so they must stay in an area with a rich food supply
to last them out. One such place is Bridgewater Bay off Somerset, where around
4000 shelduck gather6. Their aggression is lost and they gather in
flocks6.
Come October, shelducks migrate again for the winter6.
British shelduck return to their resident breeding grounds, but our populations
swell with flocks of migrants from mainland Europe2. Britain is a
very important place for shelducks, since it is home to over half the wintering
population of Western Europe3.
Shelducks are increasing in Britain, driving their expansion
to inland waters2. However, they face several recent threats,
including loss of feeding and nesting habitat to coastal development and tidal
barrages2. Commercial cockle dredging decreases the shelduck’s
invertebrate prey7, whilst invasive American mink (Neovison vison) are significant nest
predators8.
The shelduck is a handsome oddball among our waterfowl with
a strange life history. I for one will have a new appreciation of this Common
duck the next time I go birdwatching on an estuary.
~Our heaviest duck is the eider Somateria mollissima, beating the shelduck by a whole kilo, but the shelduck has a slightly greater body length and larger wingspan.
*Note that while all ducks are anatids (in the same family
as geese and swans) ‘ducks’ do not belong to a single related subfamily within
it, they are simply named by appearance.
References
1- Fransesca Greenoak. (1979) All the Birds of the Air. A.
Brown and Sons.
2- Peter Holden and Tim Cleeves. (2010) RSPB Handbook of
British Birds (Third Edition). London, A&C Black.
3-Peter Holden and J.T.R. Sharrock. (1988) The RSPB Book of
British Birds. London, Papermac.
4- R.A. Robinson. (2005) BTO
BirdFacts: profiles of birds occurring
in Britain & Ireland: Shelduck Tadorna tadorna [Linnaeus, 1758]. (BTO
Research Report 407). BTO, Thetford. Available from: http://blx1.bto.org/birdfacts/results/bob1730.htm
5- M. Makepeace and I. J. Patterson. (1980) Duckling
mortality in the Shelduck, in relation to density, aggressive interaction and
weather. Wildfowl, 31, 57-72.
Available from: wildfowl.wwt.org.uk/index.php/wildfowl/article/download/609/609
6- Michael.J.Seago. (no date) Birds of Britain: Shelduck
Tadorna tadorna. Available from: http://www.birdsofbritain.co.uk/bird-guide/shelduck.asp
7-BirdLife International (2010) Commercial shellfisheries can negatively impact migratory waterbirds.
State of the world's birds. Available from: http://www.birdlife.org/datazone/sowb/casestudy/23.
8- M. Nordström, J. Högmander, J.Nummelin, J.Laine, N.Laanetu
and E. Korpimäki.(2002) Variable responses of waterfowl breeding populations to
long-term removal of introduced American mink. Ecography , 25, 385-394. Available from: http://www.jstor.org/discover/10.2307/3683549?uid=3738032&uid=2&uid=4&sid=21103095401163
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