An Arctic Tern at Hollowell Reservoir provides the opportunity to study a plumage rarely seen in the UK in summer.
Found by Gary Pullan during the morning of 20th June, a first- (or possibly second) summer Arctic Tern was still present there yesterday, allowing Jon Cook to capture some clearly instructive images of a bird which would normally be expected to spend the northern summer south of the Equator. A tiny proportion is, however, known to accompany adults when they move north in spring, although they are likely to wander and do not usually appear in breeding colonies.
Any tern with a white forehead and a predominantly dark bill, seen in late spring/early summer, is likely to draw attention, as it stands out from the typical fully black-capped, red-billed Common and Arctic Terns normally encountered on spring passage or, in the case of Common Tern, breeding in the county. Identifying it to species is one thing and ageing it correctly is another.
Jon’s excellent series of photos nicely illustrate its identity, which is straightforward, given good views in the field. In the first image, against the light the translucent primaries are clearly visible, immediately putting the bird in the Arctic camp before any full assessment of the plumage detail.
A check of the upperside of the primaries also adds to this identification, as they are ‘clean’ and uniform, lacking the darker, unmoulted outer primaries of both first- and second-summer Common Tern.
There is a darker bar on the leading edge of the wing coverts, which looks more prominent in some photos than others. First-summer Common Tern would also show this but in combination with darker (not white) secondaries.
The overall ‘neat’ proportions also look right for Arctic Tern, although in some photos the bill looks rather long but this is probably accentuated by the white forehead.
Leg colour is said to be variable – see, for instance, Terns of Europe and North America (Larsson & Malling Olsen, 1995) and this bird has a definite redness associated with its legs.
Bill colour of second-summer is said to be red like that of an adult but with a dark tip to the upper mandible and a darker base. The images above appear to depict an all dark bill but the one below, from Adrian Borley, taken on 20th in different light conditions, appears to show some redness in the bill’s centre, as well as some brown tones to the dark crown. The underparts are a mixture of grey and white. The last two features are said by Larsson & Malling Olsen (1995) to be indicitave of second summer birds.
The Hollowell bird appears to show features of both first- and second-summer. Any further comments on its age would be welcomed.