White-winged Black Tern at Elton Reservoir by Jill Islam

 

Pretty much all that can be said about the separation of Black Tern and White-winged Black Tern in juvenile plumage has been, so don't necessarily expect to find something new in this article, instead it's intention (as the title implies) is to consolidate their identification features utilising the excellent photographs of this 'marsh tern' pair taken within the county, by county birders, although some the photos clearly illustrate some features rarely seen, let alone photographed before, so please read on.

 

  Above: Black Tern (left) at Elton Reservoir, 07.08.08 (Adrian Dancy) and White-winged Back Tern (right) at Heaton Park Reservoir, 26.09.08 (Adrian Dancy)

Comparing the above two images clearly reveals some of the important features in the separation of these two species, they are as follows:

1/. RUMP: The rump of the Black Tern is evidently grey, shading nicely from the deeper grey of the back to the paler grey of the tail but still patently grey, whereas the rump of the White-winged Black Tern is plainly white, contrasting well with the much darker brown back and also the greyer upper tail coverts as can be seen in this image. This feature alone was apparent from great distance with the Heaton Park Reservoir bird, although as with any features care should be taken during assessment in extreme lighting conditions and observers should consider that Black Tern can show a paler rump, although never one quite so white and obvious as White-winged Black Tern.

2/. SADDLE (mantle, back, scapulars and tertials): The saddle of the Black Tern in the above image (which is in fresh juvenile plumage) is very dark on the mantle and paler grey on the back, the scapulars have conspicuous broad, pale tips and this lends itself to the overall paler, less solidly dark saddle. The White-winged Black Tern is solidly dark across the saddle, the scapulars appear similarly dark and are without evident pale edges although it should be noted that this particular individual has already begun to replace some of it's juvenile feathers on the mantle and scapulars for solidly grey adult types which generates it's slightly 'patchy' look.

3/. CARPAL BAR: The blackish carpal bar on the Black Tern is broad and conspicuous, whereas on the White-winged Black Tern is not immediately obvious.

4/. WING CONTRAST: On Black Tern there is little contrast between the saddle and inner wing coverts as both are somewhat concolourous but on the White-winged Black Tern the more solidly darker saddle contrasts significantly against the paler inner wing coverts. The upper wings of White-winged Black Tern are infact moderately paler than those of Black Tern, appearing whiter, less grey in the field, but it should be borne in mind that some Black Terns can occasionally show a stronger contrast.

5/. WING TIP: Note how the wingtip on the White-winged Black Tern appears perceptibly more rounded than that of the Black Tern which is clearly very pointed. As a supporting feature this has always worked well for the White-winged Black Terns I've seen and certainly held true for this Heaton Park Reservoir individual when viewed in the field.

 

  Above: Black Tern (left) at Elton Reservoir, 30.08.08 (Adrian Dancy) and White-winged Back Tern (right) at Heaton Park Reservoir, 26.09.08 (Adrian Dancy)

The above images again reinforce those features mentioned in former two images. The rump of the White-winged Black Tern is admirably displayed and the paler saddle of the Black Tern complete with it's broad pale edges to the rear scapulars is prominent but be aware that the sides of the rump on Black Tern may appear white and a vigilant check of the entire rump should be made. Also note:

6/. TAIL FORK: On these images (particularly because both tails are closed) it is feasible to see the slight difference in the depth of the fork in the tail; more obviously deep in Black Tern compared to that of White-winged Black Tern.

7/. HEAD PATTERN: An important attribute, the following features are apparent:

  • the forehead of White-winged Black is more extensively white than Black Tern, although be aware that the moult of the crown feathers in the latter species begins as early as August in some, lending to a more White-winged Black Tern appearance

  • White-winged Black exhibits more white over the eye, as a short but clear supercilium, broader than that of  Black Tern

  • White-winged Black Tern averages more white behind the ear coverts, reaching higher up the so the black on the nape is especially narrow and 'pinched in'.

These features lend themselves to depicting an overall appearance of less black and more white on the head of White-winged Black Tern, the ear coverts in particular appearing more isolated from the crown.

 

   Above: Black Tern (left) at Pennington Flash, 15.08.07 (Annette Cutts) and White-winged Back Tern (right) at Elton Reservoir, 25.09.08 (Jill Islam)

This image above of the White-winged Black Tern shows the wing contrast slightly more accurately as the lighting for this shot was taken in a 'flatter' light (all Heaton Park Reservoir shots were taken in bright sunlight). Note again the extensive white forehead and supercilium of the White-winged Black and how the ear coverts appear appreciably more isolated. However, these images display one of the most important features in the separation of the two:

8/. UNDERPARTS: The Black Tern shows the characteristic and plainly visible extensive dark patch on the breast sides, the White-winged Black Tern does not and appears typically cleanly white, although all is not always as it seems (see below).

 

Above: Black Tern (left) at Elton Reservoir, 30.08.08 (Adrian Dancy) and White-winged Back Tern (right) at Heaton Park Reservoir, 26.09.08 (Adrian Dancy)

Whilst the dark breast patch of the Black Tern is detectable from a good distance, the Heaton Park Reservoir White-winged Black Tern infact exhibited a very subtle short pale brownish breast patch as can be seen in the image. Practically undetectable in the field (only when it passed several times within 8 feet of me did I notice it) and in the majority of the images, one or two of them however, such as this above, showed it to be tiny and entirely inconspicuous. Apparently it is a rare feature in White-winged Black Tern (Olsen and Larsson, 1995) and it was the first time out of many hundreds of White-winged Black Terns seen throughout Europe and Asia that I had experienced it myself. Another feature is also visible here:

9/. BILL SIZE: Black Tern averages a longer bill than White-winged Black Tern and it is fairly apparent on the above images, in the field I usually find it particularly noticeable.

10/. HEAD SHAPE: Whilst always a subjective feature and more useable through experience, the head shape of White-winged Black Tern appears more rounded than that of Black Tern, with a peak in the crown more clearly above the eye and although a cautious approach should be taken in assessing and utilising this feature it is apparent in the above images and has generally held true for me in the field.

 

  Above: Black Tern (left) at Elton Reservoir, 07.08.08 (Adrian Dancy) and White-winged Back Tern (right) at Heaton Park Reservoir, 26.09.08 (Adrian Dancy)

Note the prominence of the dark breast sides on the Black Tern and the length of it's bill. The mark on the sides of the breast on the White-winged Black Tern here is reduced to almost a very pale 'stain' and is practically unperceivable. Although not evident on any of these images, two further features useful for separation of the pair are:

11/. OVERALL SIZE: White-winged Black Tern is noticeably smaller than Black Tern and although this would generally require a side-by-side comparison, experience of either species lends itself to an overall 'feel' of a smaller bird with shorter and blunter wings when faced with a White-winged Black Tern.

12/. LEG LENGTH: Obviously only useful on a perched bird but the legs of White-winged Black Tern are longer than those of Black Tern.

 

    Above: Black Tern (left) at Elton Reservoir, 04.08.08 (Paul Wilson) and White-winged Back Tern (right) at Heaton Park Reservoir, 26.09.08 (Adrian Dancy)

Most of the aforementioned features are obvious here but note the head pattern on this Black Tern has much reduced black due to moult and is all the more White-winged Black like and it's true wing colour and it's contrast with the saddle and carpal bar are hindered by shadow. A final feature, not perceivable in any photographs is useful in the separation of these two marsh terns:

13/. FLIGHT ACTION: Yet again, proving the necessity to carefully study when they occur, not only rarely encountered birds such as White-winged Black Tern but those more regular like Black Tern, as the flight action of the two are perceivably different with experience. White-winged Black Tern has a steadier flight than Black Tern, lacking much of the latter's 'bounce' with every wing beat and seems to have slightly stiffer wing beats, all of which was apparent on the Heaton Park Reservoir bird in particular.

 

Combine all the features for White-winged Black Tern mentioned above and it gives rise to a comparatively characteristic bird which under good conditions and conscientious observation should not provide too much difficulty to separate from the corresponding plumage of Black Tern. However, it's not always that clear cut and examples of possible hybrids between the two species have occurred, apparently displaying a mixture of features, so observers of a potential White-winged Black Tern should remain vigilant.

 

Acknowledgements

Once again I would like to thank the county photographers for so splendidly documenting the occurrences of these terns and for allowing me to utilise them in this article, so once again, a big thanks to Annette Cutts, Adrian Dancy, Jill Islam and Paul Wilson.

 

 References

Terns of Europe and North America, Olsen and Larsson, 1995, Helm Identification Guides

A tern showing mixed characteristics of Black and White-winged Black Terns, Davis, 1982, British Birds 75: 579-580

Tern showing mixed characteristics of Black Tern and White-winged Black Tern, Vinicombe, 1980, British Birds 78: 223-225

 

Ian McKerchar, October 2008

 

 

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