IDENTIFYING ADULT TERNS IN FLIGHT
By Tom Ennis
16 June 2012
A recent visit to Lady's Island Lake, Co. Wexford, gave me the opportunity to recap on the flight identification of summer plumage Common, Arctic and Roseate Terns. There are breeding coloniesof these terns as well as Sandwich Terns at this location and I spent a pleasant few hours there at the end of May, photographing adult terns coming back from fishing at sea, as they returned over the sand dunes with bills full of fish, on their way back to the breeding colonies.
Have a look at the photographs and you'll see I've focused on images of Common, Arctic and Roseate Terns. These three are, I believe, the confusion species. Flight identification of the bigger, bulkier Sandwich Tern, usually doesn't present a problem. Its long, black, yellow-tipped bill, short forked tail and strident, harsh calls mark it out from the others. See below.
So let's have a look at the “confusing three”. I was fortunate enough to get pictures of both the upperside and underside of each species and these show the field points required to clinch flight identification.
There are of course, the standard points of difference in the three species, some of which even the most elementary bird guide contains. For example bill colour; Common has a pillar box red bill with a noticeable dark tip, Arctic has an all crimson bill and Roseate has a black bill with a very small patch of red at the base (however as the summer progresses this red increases in area and some individuals show a noticeable degree of red in the bill). Other points listed include the very short legs of Arctic Tern, the rosy flush on the breeding Roseate, and various differences in body structure and calls. There is no question that the “chew-wick” of the Roseate is very distinctive but
birds don't always give voice and the items just mentioned may not be a great deal of use when we are watching Terns in flight at sea.
To my pictures. Wing pattern gives a very good guide to identification. Common Tern's upperwing shows dark smudgily-edged primaries with a dark, “protruding” wedge round about the middle of the primaries. As the summer progresses this mark becomes darker and easier to see, due to wear in the feathers.
Arctic Tern's upper wing is plain grey above with a neat dark edge to the primaries. There is no indication of the Common's wedge.
Roseate shows paler wings and often well contrasting darker, outer primaries without a darker edge.
The undersides of the wings also help in identification. Again the Common Tern shows an ill-defined dark trailing edge to the primaries,
whereas the Arctic has a neat, thin dark edge.
Roseate Tern lacks any contrasting trailing edge to the undersides of its darker primaries.
Books do often emphasize the long tail streamers of the Roseate but many Arctics can match or even surpass them. Roseates do tend to be shorter winged, which gives their flight a jerkier quality. This does tend to increase the impression of a very long tail. The rosy flush to the breeding Roseate's plumage is not always easy to see in fly pasts but they do tend to look very white in the field. Arctic Terns, on the other hand, can often appear very grey underneath and some particularly well marked individuals can have remarkably deep grey underparts contrasting with very white cheeks reminiscent of a Whiskered Tern. I was once regaled by a tale of a Whiskered Tern sitting on a buoy, spotted from a ferry in Belfast Lough. I recall thinking “why wasn't it an Arctic Tern?” but as it was a birding “one that got away” story, I let it go past me.
So now is the time to see if you find some (or any) of these points useful. An hour or two at Groomsport or the the Belfast Lough Harbour Reserve will give you the opportunity to test at least some of it out. Such practice may stand you in good stead when you are sea watching at Ramore Head, Co. Antrim or St. John's Point, Co. Down or anywhere else on the coast. It may also aid your finances as the time spent on this exercise is time away from your local hostelry where friends will assist you in speeding you towards your credit limit.