This year i got to view a group for a prolonged period fly around the pits, and I was struck immediately by their pristine appearance and swallow-like outline. their wings lacked the slightly dingy smudge present in the outer wing of the common tern, instead being a beautiful clean grey, with a crisp dark trailing edge to the outer primaries that appeared blacker on the underside, and which contrasted to the pale edged secondaries. The common tern can show a reduced smudge in the wing in certain lights, only the dark trailing edge being apparent, and at this time confusion with arctic is more understandable- but look for the slight seam between the inner and outer primaries in the form of a slightly darker wedge.
Arctic terns are always illustrated in the books as being darker than commons underneath, but i have found this to be an unreliable feature in the field, as lighting conditions,and the background tones in front of which the bird is seen, can drastically alter the perception of this underside shading. I have seen common terns on a dull day that look much darker than the arctics I saw the other day, whose undersides appeared washed out as light from the water bounced up at them.
Small details can be used as part of a holistic appraisal of the bird in front of you, but they should be considered carefully - bill shape and colour being another obvious feature that can sometimes muddy the water. Common terns can show little or no black on the bill tip, and the black can seem to disappear if the bird flies past a dark background. the bill should be shorter and straighter on an arctic, and the head should show a steeper forehead. I found that the profile of the black caps on birds I watched the other could offer a possible way of categorising the two species- steep angled forehead on the arctic, steeper upturn at the back of the head on common. The difference is subtle i admit, and leads to the common showing a flatter crown when compared with the arctic's more domed appearance.
To me though, the easiest method of telling the two species apart in flight is the position and shape of the wings. the arctic seems to have less of a neck, making the wings look as though they are placed in a more forward position than on common. the tail, as a result appears longer as well on arctic.the commons' longer head and bill should look sleek and extended. The arctics' wings are subtly narrower, and added with their generally paler trailing edge, their "hand" looks relatively longer in comparison to that of the common. This feature also adds to the "front heaviness" of the arctic. I think a lot of literature on the subject makes too big a deal over the comparative elegance of the two species, which in turn leads to more confusion, as our own definition of elegance is purely subjective, and it is actually pretty hard to think that there is anything more elegant than a common tern, in my book. Its slender features seem to fit the description pretty well perfectly, so its not surprising that on a sunny evening, when one skims past looking more lovely and serene than the usual busy and noisy fish-diver of a blustery late April out on the main pits, we can be deceived into thinking that this must be a different species of higher worth. Of course there's always the roseate tern.