An ID article on American Black Tern

Last October myself and Pete headed up to Longham Lakes in Dorset to twitch the long-staying American Black Tern. The close views inspired me to do some research on this nearctic sub-species and learn about the finer I.D features of Surinamensis and the potential pitfalls when ruling out nominate European Black Tern. This article will hopefully show how to age a Black Tern in the autumn months and then it will move onto separating juvenile American Black Tern from European Black Tern. Although an adult summer plumage bird has yet to turn up in Britain I have included a brief summary of their identification. I'm sure more of these vagrant Tern's will be found in the UK as observer awareness increases and I also wonder why it isn't yet a full species seeing that it is readily identified at all ages.  

Status:

American Black Tern breeds in wetlands in the northern provinces of U.S.A and Canada and has a breeding population of up to 500,000 birds. They migrate through North America on a broad front with some birds heading overland to their wintering grounds in Central and South America whilst other populations head along the East Coast of North America making them obvious candidates for vagrancy to Western Europe. By 2019 eleven birds had been found in Britain and Ireland with the first record being at Weston-super-Mare on 3^rd-11^th October 1999. All occurences have been in the autumn from July-November with the peak month of occurrence being September (four records).  

Ageing:

Learning how to age Black Tern in Autumn will be invaluable if the chance of finding an American Black Tern ever comes along. Ageing should be straightforward with care:

·        Pay attention to the overall coloration of the bird in flight and when  perched. If it has crisp, brown feathers with neat fringing on the upperparts (especially the mantle when at rest and in flight the majority of the upperwing) these are 1^st generation feathers and you can be sure it is a juvenile or 1^st winter bird. An adult will have plain dark grey upperparts with no fringing.

·        Juvenile birds show tinges of brown in the cap whilst adults have a unifom dark blackish cap.

·        The distinctive dark breast sides are more obvious in juvenile birds and on average feinter (yet always present) on an adult.

Note the brown tinged mantle and notice the neat fringing to the upperparts feathers, it is a juvenile (photograph by Gary Thoburn).

The upperparts look warn and if you look closely there is no neat fringing to any feathers on the upperparts, it is an adult (photograph by Gary Thoburn).        

Identification:

All records of Surinamensis in Britain so far have been juvenile or 1st summer birds, these have several features which can help separate them from their more common European cousins at the same age:

• Dark washed flanks 
• An all dark tail with little contrast to the rump and mantle
• Dark upperparts 
• Broad dark breast sides which extend onto dark grey flanks

                     

Note the brown tinged forehead and obvious brown breast sides which age this bird as a juvenile. The sooty grey flanks (which are white in juvenile European Black Tern) are a key feature of American Black Tern. The dark breast side also extends further on this subspecies, spreading onto the grey flanks.

The grey upperparts, black crown, pale rump and absence of prominent breast patch age this bird as an adult moulting out of summer plumage. Adults can show a varying amount of dark on the flanks in the autumn so are a potential pitfall (photograph by Gary Thoburn).

Juvenile European Black Tern has paler, more contrasty upperparts with a more prominent dark leading edge to the wing. The rump and tail are still rather dark but contrast more with the mantle. This photo shows the paler outer web of the outer tail feather, a characteristic of European Black Tern (photograph by Steve Rogers).

Juvenile American Black Tern has dark upperparts showing little contrast. The rump and entire tail are dark and don’t contrast much with the mantle.

For completeness juvenile White-winged Black Tern has a paler rump and upper-wing with a contrasting dark mantle. It also lacks the dark breast patch of the Black Tern (photograph by Gary Thoburn).

In summer plumage adult Black Tern, American Black Tern and White-winged Black Tern have key differences which allow identification highlighted by the photographs below:

Adult summer plumage American Black Tern hasn’t yet occurred in Britain but it is distinctive in its own right. It has the jet black body of a White-winged Black Tern and the darker upper-wing and underwing coverts of Black Tern.

Adult summer plumage White-winged Black Tern also has a jet black body but has a much paler upper-wing and underwing.

Adult summer plumage European Black Tern has a paler toned black body. It's upperparts and underparts are identical to American Black Tern being far more subdued than White-winged Black Tern.

I hope this article will help inspire you to look closely at any Marsh Tern you see in the future as a rare vagrant could occur just about anywhere even at your local patch. For instance I hope one turns up in Cornwall soon (surely it is overdue in my home county?). These stunning birds are always a treat to watch and yet can be equally frustrating to observe well so a bit of patience, good note taking and getting good photographs will always help with their identification.