X is for Xmas bird

ROBIN (ERITHACUS RUBECULA)

There is no British bird beginning with ‘X’ and I dislike the abbreviation of Christmas, but the show must go on. The timing is impeccable for a Christmas special.

No other British bird is such a yuletide symbol as the robin Erithacus rubecula. As I type, I’m surrounded by 10 manifestations of this little thrush (5 ornaments and 5 cards). Yet the robin is resident here all year.  Why aren’t our houses adorned with winter migrants like the gorgeous waxwing  Bombycilla garrulus, the redwing Turdus iliacus or appropriately arctic snow bunting Plectrophenax nivalis?

The reason robin’s stole the Christmas spotlight is twofold.  Firstly, they are one of few birds that sing this time of year*, adding winter cheer to an otherwise frosty atmosphere. Both sexes sing year-round, a complex tune of cheerful notes which reminds me of a blackbird Turdus merula on helium. Perhaps their splash of colour also brightened up the dreary winter woods.

The second reason is from the tradition of mailing Christmas cards.  At the time cards went commercial in the 1840’s , postmen wore red jackets, so were nicknamed ‘Robins’ for the similarity of their dress. The robin’s associations with postmen and winter time melded to inspire picture cards with caricatures of robins carrying letters through the snow and has made them the traditional Christmas favourite.

Our fondness for the robin in Britain extends beyond the Christmas season. They have been our unofficial national bird since the 1960’s¹, and they are seen as a friend above all else. British tradition dictates that it is taboo to capture or harm a robin^1,2. To kill one is a scandal, causing a lifetime of bad luck ^1,2 and egg-collecting children of past times left robin eggs be for fear of their fingers turning crooked if they stole one². The famous excerpt from William Blake “A robin redbreast in a cage puts all heaven in a rage” illustrates that though robins were sometimes caged as songbirds, it was generally frowned upon. Christians also adopted the reverence of robins, claiming the bird’s breast was coloured when it plucked the thorns from Jesus’crown  and bloodied itself¹.

The robin did not enjoy the same respect in mainland Europe, where it was more regularly trapped for food and cagebirds. Perhaps because of these different histories, the British robin is much tamer than its mainland relatives^1,2,3. We have our very own race of robin, E. rubicella melophilus (melophilus meaning lover’s song)^2,4. Gardeners and growers enjoy a  cupboard love with the bird, keen to exploit the insects and worms exposed by turned soil and confiding to the point of taking food from the hand (Something I’ve had the privilege of experiencing thanks to a friend).  A robin sat on the handle of a fork or a spade is another iconic image of this bird. Though this isn’t a special relationship since robins will follow other earth movers from moles to wild boar.

We often characterise the robin as red breasted, known as the ‘redbreast’ in England until recently, the Broindergh or ‘red belly’ in Gaelic and Yr hobel goc or the ‘red bird’ in Welsh ¹ and we often illustrate it in scarlet or crimson dress. However, look at any real robin and you’ll see it’s breast is not in fact red, but a rusty orange. The alternative name “Ruddock” referring to a warm, ‘ruddy’ tone is more descriptive. ‘Robin’ itself is short for ‘Robert’, following the practice for giving characterful birds affectionate human names.

The robin uses its colourful breast in display. Whilst humans see the robin as friendly, they view each other as anything but. Robin’s are fiercely territorial. Even the different sexes will clash in winter. Research into the true nature of robins by the devoted expert David Lack revealed that male robins respond to red feathers like a bull to a flag, even if it’s just a ball of fluff. After posturing, calling and chasing they may battle ferociously, sometimes to the death.

Within their territory, robins build a nest of leaves and moss in a nook or cranny. Aswell as bushes and tree holes, they are famous for using human objects including plantpots and other garden items and even furniture inside houses! When the young fledge, they are speckled brown and tan, often being mistaken for a different species.

So to all readers of EcoGeek, I hope you’ve enjoyed this celebration of this festive bird and enjoy your own festive celebrations if you haven’t done so already. And if Christmas dinner gets too much, to paraphrase one of my classmates, feed the birds, let them know it’s Christmas time!
Thank you and a Merry Christmas from Ecogeek!

*Apart from the wren Troglodytes troglodytes, lending it the common name ‘winter wren’.

References

1- Mark Cocker and Richard Mabey. (2005) Birds Britannica. London, Chatto and Windus.

2- Fransesca Greenoak. (1979) All the Birds of the Air. A. Brown and Sons.

3- Peter Holden and Tim Cleeves. (2010) RSPB Handbook of British Birds (Third Edition). London, A&C Black.

4- R.A. Robinson. (2005) BTO BirdFacts: profiles of birds occurring in Britain & Ireland: Robin Erithacus europaea   [Linnaeus, 1758]. (BTO Research Report 407). BTO, Thetford. Available from: http://blx1.bto.org/birdfacts/results/bob10990.htm

W is for Woodpigeon

WOODPIGEON (COLUMBA PALUMBUS)

There is no such thing as ‘just a pigeon’. It amazes me that people don’t realise the difference between the feral, city pigeon (domesticated descendants of rock doves* Columba livia originally kept for food), and the fully wild wood pigeon Columba palumbus.  ‘Woodies’ can be discerned by their salmon-pink breast  and white neck clasp, and wing patches. Like feral pigeons, they have an iridescent flash on their their neck, glinting metallic green and purple. The rest of their body is a pastel blue- grey.

Woodpigeons are also larger than feral pigeons (around 50% heavier¹), one of our biggest garden birds. They’re often thought of as fat and can look quite comical strutting with their tiny legs. One of my friends likens them to the portly figure of Winston Churchill. In truth though, their breast is mainly powerful flight muscle, not flab. 

But they are certainly big eaters. Wood pigeons are mainly herbivorous, grazing on plants and shoots and gulping down seeds, berries, buds and leaves along with the occasional insect, worm or snail^2,3.  A wide gape allows them to swallow acorns whole and despite their size, they are capable of quite acrobatic feats to feed on fruiting branches.
Plant foods are difficult to digest, so they eat a lot. This makes them a serious pest as winter flocks descend in their hundreds on fields of cereals and vegetables, and are understandably shot to control their numbers. One woodpigeon was found with its crop and stomach full of 1020 grains of corn, whilst another had swallowed 198 beans⁴.  They’re also begrudged in gardens for `taking food meant for smaller birds. This can be averted by caging the feed or using smaller seed mixes⁵. I personally welcome woodies in the garden over the more piggish feral pigeons.

The woodpigeon was proclaimed Britain’s commonest garden bird in 2005⁶. Once limited to rural woods and fields, their numbers have soared due to changing practices on the farms they pilfer. Switching from spring to autumn sowing of cereals and increased growing of oilseed rape provide extensive winter food sources which help the pigeons survive the leaner times. It is from these farms that many have spilled into suburban and urban areas⁵. They even outnumber their feral cousins in London gardens⁷.

But winter survival is only half the story. Woodpigeons can also breed all year round provided there is enough food available, thanks to a trick up their sleeve, or rather down their throat.

Most birds feed their young on insects to provide the protein they need to grow, restricting their breeding season to the warmer spring and summer months. Pigeons have curbed this by producing a protein-rich milk. Yes, milk!  Of course this ‘pigeon milk’ does not come from teats but from a cell lining of the crop. More winter food means woodpigeons can raise more broods later.

Woodpigeons mate for life, and courting males go to great effort to earn their mates. Their haughty cooing has a drowsy quality that accentuates the humid late summer. They also perform display flights, flapping vertically into the air before gliding down in a wide arc. Any interested female is greeted with a display of tilting,tail-fanning and cooing . Though she will often spurn his advances, he follows her persistently. If they suit, they become hopeless romantics, cuddling up together, preening and even ‘kissing’ by locking beaks and exchanging food. 

Woodpigeon courting (Video by vriesap) and song (Recording by Ashley Fisher). 

The couple builds a messy platform of sticks in the branches of a tree. Their plain white eggs can sometimes be seen from below. The young pigeons, called squabs have faces only a mother could love with bulging eyes, a roman nose and straggly down, growing into juveniles which lack the white markings of the adults.

Male woodpigeons are defensive of their breeding territories and mates, getting into fights by smacking each other with their wings. These tussles look silly, but the blows are powerful. Woodpigeons also clap their wings together over their backs when fleeing a predator, sounding a clattering alarm signal. Buzzards Buteo buteo, female sparrowhawks  Accipiter nisus, foxes Vulpes vulpes and humans all enjoy a meaty woodpigeon.

All too often we hear of species declining due to human land use. The woodpigeon provides a brilliant example of the opposite, even if it is at our expense. Whether viewed as a pest or handsome garden visitor, even the humble woodpigeon has a remarkable life. It is far from “just a pigeon”.

* There's no real difference between 'pigeons' and 'doves'. The sacred white dove symbol of christianity is simply a colour form of domestic pigeon. 

References

1- R.A. Robinson. (2005) BTO BirdFacts: profiles of birds occurring in Britain & Ireland: Woodpigeon Columba palumbus   [Linnaeus, 1758]. (BTO Research Report 407). BTO, Thetford. Available from:  http://blx1.bto.org/birdfacts/results/bob6700.htm

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- Mark Cocker and Richard Mabey. (2005) Birds Britannica. London, Chatto and Windus.

5- BTO (No date) A BTO Garden BirdWatch factsheet: Woodpigeon. Available from: http://www.bto.org/sites/default/files/u23/downloads/pdfs/factsheet_woodp.pdf

6- BBC News. (2004) Wood pigeon 'most common UK bird'. Available from: http://news.bbc.co.uk/1/hi/uk/4646685.stm

7- BBC News. (2008) Wood pigeons 'flocking to towns'. Available from: http://news.bbc.co.uk/1/hi/7166536.stm

V is for Vanellus

LAPWING (VANELLUS VANELLUS)

Lapwings Vanellus vanellus  are beautiful birds, with a stately figure and sweet rounded face topped off with a whisp of a headcrest. They appear pied, but to call them black and white would be an understatement, since their wings iridesce with green, purple and bronze. A flock of lapwings flashing dark and light as they twist and turn is a sight to behold, and one of my favourite childhood memories was watching these flocks descend on fields in my hometown.

The lapwing’s flight, with sweeping strokes of its distinctive broad wings earned it both its scientific name, meaning a ‘a waving fan’¹ and the common name, from the Old English ‘Hleapewince’ to leap with a waver². Their aerial prowess peaks in spring when males show off in tumbling display flights whilst giving their unusual  call, giving it it’s other common name the ‘Peewit’ (Though I’d personally translate it as ‘peao-wee’).

Lapwings are a familiar site in the British countryside, commonly encountered in open areas from moors and marshes to sheep pasture and ploughed fields.  Here they indulge their habit of plucking invertebrates off the ground, feeding in a teetering motion.

 It seems that lapwings are faring well in Britain, but unfortunately this is not the case. The numbers of adult birds, which live for over ten years, are masking a poor breeding success.

Breeding lapwings need open areas with short vegetation where they can dig a nest scrape and look out for predators, alongside insect-rich areas nearby for the chicks to feed. Naturally, lapwings nest in marshy habitats, but with the take-over of agriculture spring-sown cereal fields became choice habitat, especially in mixed farms with food-rich pastureland nearby. 

In the past, the lapwing had made a success in Britain’s agricultural landscape, but as goes the story for many farmland birds, increased agricultural efficiency and changes in land use have not been in their favour. From around the 1960’s, drainage of wet fields, decreases in mixed farming, ‘improvements’ of pastureland reducing their insect life, and switching from spring to autumn sowing have contributed to a shocking 50% decline in breeding lapwings since 1985³. It is now an RSPB red status species.

Lapwing conservation presents special problems since they don’t always benefit from measures for other farmland birds. For example, hedgerows which provide food and shelter for songbirds also act as highways for lapwing nest predators such as foxes and stoats.  Special ‘lapwing plots’ in cropped fields also benefit  smaller birds⁴.

Lapwings begin breeding in March. The male makes several scrapes lined with grass or leaves and the female chooses where to lay her eggs.  The eggs are a mottled brown, concealing them against the ground and the downy, bobble-headed chicks are equally well camouflaged. Both parents care for the eggs and chicks and are very defensive of the nest. They will give aerial chase to any corvid or raptor that gets too close, and will distract ground predators by loudly calling, fluttering and even feigning injury away from the nest. This has earned the lapwing a deceitful reputation, famously observed by Chaucer who refers to the “false lapwynge, ful of treacherye” in his poem ‘The Assembly of Fowls’, and is established in their collective noun, a deceit of lapwing.

Whilst the adults go on the defensive, the chicks respond to threat by lying low and making like a pebble. If they are discovered, they can run away just minutes after hatching, with the charming side effect of scampering off still stuck with eggshell ². Shakespeare draws on this behaviour in Hamlet, when Horatio refers to young Osric getting ideas above his station “The lapwing runs with shell on his head”. These curiously astute observations of rare behaviour stem from the past practice of harvesting lapwing eggs².

After breeding, lapwings may move about depending on the weather. Small flocks arrive from Northern Europe in June swelling numbers through to winter, whilst British breeders may fly as far south as Spain in harsh winters ^3,5.

History has shown that the fate of the lapwing is interlinked with that of the British countryside. It would be a great shame to lose this culturally and visually enigmatic bird from our landscape. Hopes lie with our farmers and agrienvironmentalists to keep the peewits calling evermore.

References

1- R.A. Robinson. (2005) BTO BirdFacts: profiles of birds occurring in Britain & Ireland: Lapwing Vanellus vanellus   [Linnaeus, 1758]. (BTO Research Report 407). BTO, Thetford. Available from: http://blx1.bto.org/birdfacts/results/bob4930.htm

2- Fransesca Greenoak. (1979) All the Birds of the Air. A. Brown and Sons.

3- Peter Holden and Tim Cleeves. (2010) RSPB Handbook of British Birds (Third Edition). London, A&C Black.

4- D. Chamberlain, S. Gough, G. Anderson, M. Macdonald, P. Grice and J.Vickery.(2009)Bird use of cultivated fallow 'Lapwing plots' within English agrienvironment schemes. Bird Study 56, 289-297.Summary available from: http://www.bto.org/sites/default/files/u31/downloads/details/lapwingsinplots.pdf

5- Peter Holden and J.T.R. Sharrock. (1988) The RSPB Book of British Birds. London, Papermac. 

U is for Upupa

HOOPOE (UPUPA EPOPS)

From the moment anyone sets their eyes on a hoopoe* Upupa epops  in the UK, they know, they are  looking at something exotic. With their peachy body, zebra stripe wings, long down-curved beak  and huge headcrest, the hoopoe is unlike any other British bird.

Hoopoes are indeed from sunnier climes. Their usual range is the Meditteranean, Africa and the Middle East. Yet every year, around 100 hoopoes visit Britain. Unlike the swallow or the cuckoo they do not stay and revel in the delights of the British summer. So what are they doing here?

In spring, some hoopoes from Sub-Saharan Africa migrate to the northern Mediterranean.  Those which end up in Britain have overshot across the channel. Of course, most make land in southern England, but hoopoes have been found as far north as the Scottish Highlands and Orkney¹.  The first recorded hoopoe in the UK was in Greater London in 1666² (When the city was a lot greener). They are surprisingly hardy, with one recorded overwintering in Britain, feeding in leaf litter³. Hoopoes are known for enduring the cold, since they migrate across the Alps and even the Himalayas³.

Whilst most hoopoes arriving in Britain realise their mistake and return south, some do stay around for a while. There are around 30 records of attempted breeding in the past two centuries, though unfortunately none have been successful. Though some nests have fledged, the fledglings have been easy targets for predators³.

The hoopoe feeds mainly by probing the soil with its long beak searching for insect larvae. They prefer to settle in areas with plenty of bare ground or short turf where it is easier to feed. For this reason, they often stop off on manicured lawns such as golf courses, bowling greens and gardens. They particularly favour grazed areas, where there are also abundant beetle grubs and fly larvae associated with animal dung⁴.
They also snatch small animals from the surface, mainly grasshoppers and crickets as well as beetles, spiders, centipedes and occasionally small lizards and rodents. Hoopoes are one of few birds unphased by millipede toxins.

In flight, the hoopoes boldy- patterned, broad wings and unique fluttering technique have earned it the nickname ‘butterfly bird’.  This may give the impression that their flight is weak and haphazard but along with their migratory feats, hoopoes are highly agile in the air and excellent at evading avian predators such as falcons³.

But perhaps the hoopoes most distinctive feature is it’s crest. The long feathers are raised when excited or alert, spreading into a black-edged fan. Most of the time it is sheathed away as a spike jutting out of the back of its head, giving the bird a passing resemblance to the pterosaur pteranodon.

Given the hoopoes’ unique appearance, it seems odd that both its common and scientific names are onomatopoeic, referring to its call. The soft, but resonant “oop oop oop” is made by the hoopoe lowering its head and inflating its throat whilst calling, often from a perch.

Hoopoe calling from a perch. Video by Sanal Kumar

The hoopoe’s uniqueness is recognised in its taxonomy (Its relationship to other birds). It is the only species in the ‘Upupidae’ (though the Madagascan subspecies  U. epops marginata may be a separate species). Traditionally they are included in the Coraciiformes along with kingfishers, bee-eaters and hornbills, though some have argued for a separate order, the ‘Upupiformes’ which seperates the hoopoe along with wood-hoopoes and scimitar bills⁶.

Like other Coraciiformes, hoopoes are hole nesters, using trees, banks or buildings⁴. They cannot hammer their own hole, but the parents dig out loose matter and enlarge the cavity, as well as lining it with soft materials⁴. The single brood of around 7 young start off ugly and partly bald before fledging into grey, comically scruffy versions of the adults, with yellow lips or ‘gapes’  to direct their parents to their mouths.

With the warmer summers brought about by  future climate change, it is likely that Mediterranean birds, including the hoopoe will increasingly visit and breed in Britain⁷. Though changing weather patterns may cause problems for several species, I’m sure some birdwatchers will see the charismatic hoopoe as a little silver lining.

Finally, if you have seen a hoopoe, please report it to @Hoopoesuk on twitter https://twitter.com/Hoopoesuk.

*(Pronounced ‘hoo-poo’ or ‘ ho-po’)

References

1- R.A. Robinson and J.A.Clark. (2013) Summary of all Ringing Recoveries for Hoopoe (Upupa epops). Available from: http://blx1.bto.org/ring/countyrec/resultsall/rec8460all.htm

2- R.A. Robinson. (2005) BTO BirdFacts: profiles of birds occurring in Britain & Ireland: Hoopoe Upupa epops   [Linnaeus, 1758]. (BTO Research Report 407). BTO, Thetford. Available from: http://blx1.bto.org/birdfacts/results/bob8460.htm

3- Michael.J.Seago. (no date) Birds of Britain: Hoopoe. Available from: http://www.birdsofbritain.co.uk/bird-guide/hoopoe.asp

4- GRRLSCIENTIST. (2011) Mystery bird: Eurasian hoopoe, Upupa epops. Available from: http://www.theguardian.com/science/punctuated-equilibrium/2011/sep/27/2

5- RSPB and Birdlife International. (2007) Birds on the Move: Introducing A Climatic Atlas of European Breeding Birds.  Available from: http://www.rspb.org.uk/Images/birdsonthemove_tcm9-180310.pdf

T is for Tadorna

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 hollow^2,3. 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 ducklings^1,2,3,6. 

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 fly^2,6. 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