This week bird migration has had a distinctly trans-Atlantic theme, with the following very rare and rare North American species being recorded – a brown-headed cowbird on Fair Isle; a Franklin’s gull at Sullom: two black ducks at Hillwell, and then at Scatness; a laughing gull, a solitary sandpiper and a pectoral sandpiper on Foula; and a lesser scaup at Nesting.
The rarest vagrant was the brown-headed cowbird which arrived at Fair Isle on the 8th and remained there throughout the 9th. This was only the second record of this species for Britain.
Slightly larger than a starling, the cowbird is a brownish-black passerine with a stout finch-like bill. Brown-headed cowbirds were originally a species of the great American prairies, associated with feeding bison which stirred up their insect prey.
They are now widespread throughout America, having adapted to forage alongside domestic livestock. The first British record was from Islay in 1988 and the only other European record came from Norway in 1987.
The Franklin’s gull on the 11th was also an extremely rare vagrant with only nine Scottish records (The Birds of Scotland 2007). It was first recorded in Scotland in 1980 and in Shetland in 1990.
Superficially resembling a black-headed gull, the mantle is darker, the wing-tips have a bold black and white pattern, and there are white crescents above the eye, contrasting with the dark hood.
Franklin’s gull breeds colonially around inland lakes in the prairie regions of North America, wintering along the coasts from Central America south to Chile. It is thought that vagrants may reach Europe from the wintering areas in the Gulf of Mexico and the Caribbean.
All the Scottish birds have been found in or near coastal habitats. The laughing gull is also an American vagrant with 22 Scottish records.
Black ducks are very rare American vagrants with around 10 Scottish and 28 British records up to 2007. The males resemble a dusky female mallard and are found in similar habitats.
The first Shetland record of this species was from Spiggie in 1990, with a second occurrence at Hillwell in May 2002. Black ducks breed in eastern North America and most of the vagrants reaching this country tend to linger for several months.
The solitary sandpiper found on Foula was only the second Shetland, and fifth Scottish record for this very rare vagrant. Breeding in the coniferous forests of North America, the species winters in the Caribbean and in parts of Central and South America. It is one of the rarest American waders on the British list, with around 30 records, most of which have been in autumn.
The solitary sandpiper is the American equivalent of the green sandpiper but lacks the white rump of the latter, whilst the head, neck and breast are more coarsely streaked with white.
The pectoral sandpiper is a local rarity and is identified by a sharply demarcated gorget of streaks in the centre of the breast which is present in all plumages. It is the most regular of the North American migrants to Britain, usually occurring in the autumn. It is thought that spring records are of birds, which crossed the Atlantic Ocean the previous autumn, spending the winter in Africa and Europe, before moving north.
Among the commoner migrants this week, have been widespread records of carrion crows, a jackdaw, several Iceland gulls, wood pigeon, swallow, sand and house martins, and swift.
Another white-tailed eagle was observed circling over Mossy Hill and Fitful Head on the 11th, there was a marsh harrier at the Loch of Spiggie, and a kestrel at Wester Quarff. Five long-tailed skuas and 29 pomarine skuas were counted flying past Wats Ness in the West Mainland on the 10th.
There were records of a few passage migrant waders such as knot, sanderling, black-tailed godwit and a wood sandpiper. A small group of dotterel were also found on Ronas Hill. Small passerines have included hawfinch, willow warbler, goldfinch, common redpoll, siskin, blackcap and grey wagtail.