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Articles - Juvenile Common And Arctic Terns



Arctic Tern by Steve Young
Arctic Tern by Steve Young

Keith Vinicombe - Posted on 01 Jan 2010

Key featured species

  • Common Tern Sterna hirundo
  • Arctic Tern Sterna paradisaea

The problem

The separation of adult Common and Arctic Terns was covered in the May 2005 issue of Birdwatch (155: 28-29), while Roseate Tern was dealt with in the June 2006 issue (168: 26-27). To complete the trilogy, we now turn our attention to Common and Arctic Terns in their confusing juvenile and adult winter plumages. 

The solution

Timing

The appearance of the first juveniles depends on where you live. In areas where either species breeds, you will see them once they fledge, by the end of June or early July. Although I have seen migrant juveniles as early as 12 July, the passage of juvenile Common Terns in southern England usually begins from the end of July and may last until mid-October, occasionally into November. Being a more northerly breeder, juvenile Arctic Terns do not appear in southern England until the last 10 days of August. Like Commons, passage often lasts into October and I have even seen them as late as mid-November. In autumn, Common Terns regularly migrate overland. Small numbers of Arctics sometimes accompany them, but may also appear independently. Reflecting their more pelagic lifestyle, however, the largest numbers of Arctics are usually associated with westerly gales.

Shape and flight

As with spring adults, there are subtle yet distinctive differences in shape. Common is quite a big, sturdy tern in comparison with Arctic, which is a smaller, shorter-necked and daintier bird with proportionately shorter, narrower wings that produce a slightly quicker flight action. In addition, their wings often appear ‘pushed forward’, but with their pointed primaries swept back. A most distinctive feature is that juvenile Arctic is distinctly longer-tailed and, as with spring adults, this produces a proportionately ‘short-winged, long-tailed’ swallow-like shape. I have even seen people mistakenly assume that juvenile Arctics are adults, simply because of their tail length. Note that in normal flight, the tail is usually held closed, creating a tapering and pointed profile to the rear-end. At rest, Arctic looks small, short-necked, short-billed and short-legged.

Plumage

The important point to stress is that, given a reasonable view, juvenile Common and Arctic Terns are easily separated. By the time they head south, juvenile Common is essentially a greyish bird right across the upperparts, but with a prominent dark carpal bar along the leading edge of the wing coverts. The secondaries are a darker shade of grey, forming a slightly darker secondary bar, and the upper primaries are also grey, diffusely darker along the trailing edge. Arctic has a more diffuse and much less prominent carpal bar but, most importantly, its median and greater coverts and secondaries appear largely white. Its upper primaries are also paler, but with a clearer dark trailing edge.

The overall effect is that, from any distance, Common looks a relatively uniform greyish bird with a prominent dark carpal bar and a grey secondary bar, whereas on Arctic the whole of the rear of the upperwing looks very white, and there is a less prominent carpal bar. The impression of whiteness is further enhanced by the very white underwings which, like the adults, have a narrow, sharply defined black trailing edge to the under-primaries. This combination of very white-looking rear upperwings and very white underwings enables juvenile Arctic to be identified at considerable distance, especially when flying back-on. For some reason, juvenile Commons appear a slightly less pure white below and, like the adults, they have a thicker, more diffuse greyish trailing edge to the under-primaries. Juvenile Arctics always strike the observer as particularly attractive grey, black and white birds, whereas juvenile Commons somehow come across as less smart and, consequently, less eye-catching.

Young juveniles

To go back in time a month or so, shortly after fledging, Common is a gingery brown colour across the back and scapulars, but this fades to brownish-grey and then to grey, the rate at which it fades varying individually. Young juvenile Arctic is greyer but shows buff-brown submarginal feather fringes and some buff shading, which produces an overall buffy impression from a distance.

As with Common, they too soon fade to grey, although some retain limited brown scalloping into the autumn. Both species have pale orangey bills to begin with. Whereas Common retains the pale at the base, the bill of juvenile Arctic rapidly darkens to black, forming a smart contrast with its white face. However, the rate a which Arctics lose their pale bill base also varies individually and some may retain dull reddish to late August at least.

Adults in autumn

As with juveniles, the main passage of adult Arctic Terns in southern England starts later than that of Common, usually lasting from early August to the end of September. Although I have seen adult Arctics as early as mid-July, they are in fact rare to uncommon before mid-August.

There are important moult differences in autumn adults. Common Tern starts its post-breeding wing moult on its breeding grounds and this is suspended during its migration south. This is important as it means that any Common or Arctic Tern showing wing moult in autumn should prove to be a Common. Such birds show gaps in the inner primaries and they may also start to moult their inner tail feathers. They may also lose their long tail feather projections by the time they head south. In addition, they also start a variable body moult in late summer, soon acquiring a white forehead. Many also gain a dark carpal bar and also a dark secondary bar, superficially resembling juveniles.

Moulting adults may also show pale areas across the wing coverts, due to missing feathers, and they often show greyish rumps and dark in the old outer tail feathers. Their active moult and their darker, more worn primaries and tail ensure that such birds always look scruffy compared with their pristine offspring, which do not start to moult until they reach their winter quarters. Adult bill colours may also darken at the end of the summer so that some autumn migrants show completely black bills.

In contrast, adult Arctics do not commence their primary moult until they arrive in their winter quarters (although I did once see an adult on 30 August that had already dropped an inner primary on each wing). They also retain their summer body plumage as they head south, although some may acquire a white forehead. This means that, on autumn migration, adult Arctics continue to look neat and relatively smart, compared with the scruffy moulting Commons. As with Common Terns, their bills may also start to darken.

First-summers

First-summers of both species remain in their winter quarters but, occasionally, some individuals may head north. First-summers are, on the face of it, similar to winter adults, so ‘winter-plumaged’ Common or Arctic Terns seen in Britain in mid-summer are likely to be first-summers. First-summer Commons in particular may be quite heavily worn.

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