Articles - Juvenile Common And Arctic Terns
Arctic Tern by Steve Young
Keith Vinicombe - Posted on 01 Jan 2010
Key featured species
- Common Tern Sterna hirundo
- Arctic Tern Sterna paradisaea
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 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
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
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.
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
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
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
Adults in autumn
As with juveniles, the main passage of adult Arctic Terns in southern
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