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Length 33ā39 cm, mass 76-116 g.
Breeding adult: Black forehead, nape and crown and white cheeks; the mantle, back and upper wings are grey and the collar, rump and underwing are white. The deeply forked tail is white with grey outer webs. The long bill, legs and feet are red. Non-breeding adult and immature: Forehead white, crown streaked black and white, nape black. The bill is black and the legs and feet dark red to blackish.
Common tern is on the left, Arctic on the right Photo by Steve G
Common and Arctic Terns are a species pair easily confused.
On the ground the short legs of Arctic terns give them a somewhat huddled look whilst the tail feathers in summer birds extends well beyond the folded wings. Common stand taller and the tail feathers don't extend past the primaries.
On flying birds the wing of Arctic when seen against the light is really quite translucent whereas in Common only the inner primaries are translucent. Common Terns have a black wedge or notch effect on the upper outer 5/6 primaries which is absent in Arctic and the trailing black edge of the primaries is narrower and neater in Arctic.
I find the easiest way to separate them in Summer is by the fact that Arctics have a very buoyant bouncy flight and are more compact in the head and neck coupled with the long tail giving the impression that the wings are well-forward on the body whereas Common Terns seem to have their wings more to the centre of the bird. The real clincher however is the beak - Common Tern has a fairly long somewhat decurved orange-red beak with a clear black tip whilst Arctic terns have a shorter solid blood-red beak (and are more likely to draw blood when you invade the nesting colony!!)
Written by Steve G