The separation of Common and Arctic Terns has traditionally been regarded as problematical, despite some landmark identification papers in the 1960s and 1970s.
|Common Tern (location unknown, 14 July 2007). Think of Common as slightly larger, sturdier and longer winged than Arctic, without the very long-tailed impression of the latter. Photo by Steve Young.|
|Arctic Tern (location unknown, 23 June 2009). Arctic is narrower in the wing and longer in the tail than Common, and the underwing is very white. Photo by Oliver Smart.|
One of the easiest ways to separate adults is by the pattern of the upper primaries, this being dependent on two facts: first, on Common Terns, as the grey primaries wear, they become darker; in fact they are quite blackish when heavily worn; and second, Common’s inner primaries are moulted twice a year, whereas the outer primaries are moulted only once. This means that, in late summer, most adult Common Terns show an obvious contrast between the old, dark, blackish outer primaries and the newer, fresh, pale grey inners.
In contrast, adult Arctic Terns moult all their primaries only once a year (on their winter quarters) so that, when in Britain, the upper surface of all their primaries is uniformly pale grey.
The problem is that this difference is much less obvious in spring and early summer, when Common Tern shows much more uniformly pale grey upperwings. However, even then, there is still a contrast between the darkest of the old outer primaries and the adjacent grey inners. This manifests itself as a small dark wedge in the centre of the primaries.
This may be obvious, facilitating easy separation, but it must be stressed that it can be hard to see, particularly at a distance or in bright light. Furthermore, the problem of seeing the wedge can be particularly acute on spring seawatches, particularly when distant migrating terns are whipping past at some speed low over the waves.
But it can also be surprisingly difficult to see on mid-summer Common Terns flitting around over inland lakes and reservoirs. The upshot of this is that the potential for misidentifying adult Common and Arctic Terns is far greater in spring and summer than it is in the autumn.
Size and structure
The best way around this is to really look closely at any close-range spring or summer ‘Commic’ Terns that you see, so that eventually you develop a good ‘feel’ for them. As you become familiar with them at close range, you will gradually become much more confident with them at long range. At Chew Valley Lake, we regularly separate feeding birds at ranges of half a mile to a mile. However, we do have the advantage that inland birds usually come closer, so that their identification can be confirmed.
Compared with Arctic, I always think of Common Terns as slightly larger, longer-necked, heavier, sturdier, longer-winged birds with an easy, languid flight action. Although long tailed, they do not have the very long-tailed impression of Arctic Tern. To put it simply, Arctic Tern – with its shorter neck, rounded head, shorter, narrower, more sharply pointed wings and long tail – really does look Swallow-like in shape, an analogy that is not nearly as appropriate for Common Tern.
Another useful difference is that the narrower-winged Arctic Tern really does look ‘Persil white’ on the underwings, with a thin, but clear-cut black trailing edge to the primaries. In addition, when viewed against the light, all the under primaries are translucent. The underwings of Common Tern are a less pure white, with something of a slight silvery effect. Most importantly, the dark trailing edge to the under primaries is obviously thick and diffuse, readily apparent at some distance. When viewed against the light, only the newer inner primaries appear translucent.
When feeding over fresh water, Common Tern has an easy flight action, whereas, to quote Rob Hume (British Birds 86: 217): “Arctic is more bouncy and at the mercy of the wind. Common has a fast, powerful downstroke, but a faintly lumbering look. Arctic is fluttery and butterfly-like, with a quick, snapped upstroke and a slower downbeat – it is easier to see the downstroke than the upstroke on an Arctic instead of the other way around.
For birders in the south, it must also be stressed that Arctic Terns are much less regular than Common and, as a consequence, it is a fundamental mistake to assume that you will be encountering the two species in equal numbers. Although in spring and autumn some Arctic Terns may head overland or tag on to flocks of migrating Commons, their more pelagic lifestyle means that they often turn up after strong westerly winds, particularly in September.