21 July 2012

Common Tern showing some characteristics of Sterna hirundo longipennis – Dhahran Hills

Whilst birding the local ‘patch’ on 9th July 2012 I saw an unusual tern flying around the percolation pond. Superficially it looked like a Gull-billed Tern as it had an all dark bill, but the bill did not appear to the right size or shape for Gull-billed Tern. On closer inspection it became apparent it was a Common Tern which is a scarce to uncommon migrant & winter visitor to the Eastern Province of Saudi Arabia with records from February to June and once in September on the coast. Inland migrants were noted in April, May & August at Abqaiq & Hofuf with records of single birds in February, June & July & three birds in September at Hofuf. Well inland at Haradh one was recorded in September and was thought to have been a trans-desert migrant (Bundy et al 1989). Common Terns of the subspecies hirundo are the birds that should normally occur in Arabia but these have a red coloured bill and legs, whereas this bird had a black coloured bill and legs resembling longipennis. The first photograph below was taken by Phil Roberts and is used with his permission.

Subspecies of Common Tern
Sterna hirundo hirundo (Linnaeus, 1758) - North America to northern South America, Atlantic islands, Europe, north Africa (Tunisia) and west Africa (Mauritania, Senegal, erratically Nigeria), through Middle East and Black and Caspian Seas to Yenisey Valley; winters south of Tropic of Cancer
Sterna hirundo minussensis (Sushkin, 1925) - Central Asia through Transbaikalia to northern Mongolia and South Tibet; winters mainly north Indian Ocean. This is oftern regarded as a hybrid between longipennis and hirundo
Sterna hirundo tibetana (Saunders, 1876) - West Mongolia south to Kashmir, Tibet and Sichuan, at high altitudes; winters mostly east Indian Ocean
Sterna hirundo longipennis (Nordmann, 1835) – North-east Siberia south to north-east China (central Heilongjiang to Inner Mongolia and Shanxi); winters South-east Asia to Australia

The following features have been mentioned as those that can help differentiate an Adult Eastern Common Tern longipennis from a Common Tern hirundo:
Slightly more svelte appearance; Smaller more domed head/crown; Slightly longer wings, outer tail feathers project slightly beyond wing-tips; Bill shorter & finer, sharper, less dagger-shaped (though some variability); Bill black, some with crimson-purple at base of lower, brightening in spring; Bill has less arched culmen; Black crown has more sharply defined edges, sharper contours behind head; Dark trailing edge to secondaries on underwing; Upperparts more ash grey; White cheek stripe, esp. in front of eye; Under-parts dusted with lavender grey, isolating a white cheek stripe; Legs dark reddish brown/brown/chestnut; Call less shrill

The problem with longipennis Common Tern, also known as Eastern Common Tern, is it breeds and winters a very long way away from Saudi Arabia and is therefore an unlikely visitor. There have also been records of hirundo Common Terns with all black bills. Adult breeding longipennis should show a black bill and legs but also dusky under-parts that this bird did not show in all conditions. The bird did, however, have a modest length tail, a Common Tern like under & upper-wing pattern, as well as flight action. The bill was very obvious though and did not fit a typical hirundo Common Tern at all in my eyes being a slightly odd shape, maybe an illusion caused by the black colour, and was entirely blackish along its length. The completely black cap and neat body plumage suggested it was in adult summer plumage. There are many records from Israel of longipensis type birds where it has been regarded as an uncommon but regular migrant, The Birds of Israel (Shirahai 1996). Although the bird I saw showed some characteristics of longipennis Common Tern, it’s a far from conclusive record and is more likely a black-billed Common Tern than a true longipennis.