IDENTIFICATION ARTICLES

North American and European Black Terns.

Black Terns (Chlidonias niger) are only occasional visitors to Hawai'i and the Pacific Ocean, however it is the most regular occurring of the "Marsh Terns" in Hawai'i. So far all birds which have been recorded have presumably been of the North American race surinamensis, but there is a very remote possibility that the European nominate race niger could conceivably occur as well in the Pacific region.

This short paper outlines the distinguishing features of both races and their distribution.

American Black Tern is fairly widespread in North America and breeds in most of the northern states but is an uncommon non-breeding migrant on the East coast. Its numbers on the West coast, in particular California have been reported as decreasing. It is a long distance migrant wintering as far south as northern South America. European Black Tern breeds across Europe and Western Asia from Sweden and Spain across to SW Siberia and the Caspian Sea and the border with China. Birds winter off West Africa as far South as South Africa.

Vagrants have occurred in Japan and Australia, although it is not known which race was recorded (Harrison 1983). American Black Tern has been recorded five times in Europe: three specimens from Iceland in 1956, 1957 and 1970; one record from Ireland in 1999 and a further recent record from England, also in 1999. Interestingly the three Icelandic records were from June, whereas the other two records were from September/October (Adriaens 1999, Andrews et al. 1999).

Adult Summer Plumage

Birds of both races are similar in adult plumage but American Black Tern usually shows much darker black on the body, whereas Eurasian shows a paler more sooty grey colouration. The race niger shows grey underwings which contrast with the body and bright white under-tail coverts. Males are darkest and usually exhibit a dark head, whereas females often show a paler throat and cheeks. The dark head merges into the upperparts which are mid-grey, although the rump is often slightly paler. The secondaries and outer primaries tend to be darker than the fresh inner primaries, although the contrast is usually slight. Adults moult from late May and the throat and eye areas are moulted first, which become progressively white, as well as a white eye-ring forming, according to Olsen and Larsson (1995) this is so that the chicks which are being reared at this time can more easily see where the bill is for feeding by the parents. The rest of the plumage is moulted after mid-summer, ending with the belly. Winter plumage has usually been acquired by late summer and autumn. North American surinamensis have shorter wings and longer legs than niger, although this is of little use on a vagrant tern in the field. Adults in summer have a dark black head and underbody (in darkness like White-winged Black Tern), which contrasts much more with the upperparts than the black on niger. The black of the head extends back onto the mantle (stops before mantle on niger), and the upperparts are generally slightly darker in the North American race. The leading edge of the wing can be almost white, which is rare in niger and can sometimes be useful in the field. As in the European form, males are darker than females, which also exhibit slightly paler throats, although not often as marked as in female niger. The tail is grey. The American race tends to moult later than the European race, usually not before mid-August and many are still in summer plumage in September.

Adult Winter Plumage

Birds of the race niger in this plumage have a uniform black cap, which is broader behind the eye than in front. The eye is surrounded by a white eye-ring, which is widest above the eye. The sides of the breast exhibit a broad patch of dark feathers at the wing base, and although this may be reduced in some individuals is always present to some extent. The upperparts are pale brownish-greyish with darker lesser coverts. The tail and rump are also pale brownish-grey. The underwing is pale grey. In surinamensis birds are distinguished by broader dark neck patches on the breast side which merge into grey flanks (clean on niger). The facial mask tends to be paler, this effect being highlighted by white streaking in the cap/forehead. The ear-coverts are black and there is a broad white eye-ring. The upperparts are uniform dark grey, with blackish-grey lesser coverts and upper mantle. The rump is dark grey and the tail is the same colour as the back. The rump on niger is paler and the face mask is more solid black (no white streaks).

Juvenile

Juvenile niger recall adult winter plumage but often have a brown-tinged cap and often exhibit larger and browner breast patches. The upperparts are also usually darker and browner than adult individuals, with the darkest areas being the mantle and lesser coverts, which can "form a continuous dark area from carpal to carpal that may continue down onto the back as a dark T shape. The contrast with the rest of the dark upperparts, however, is never striking" (Olsen & Larsson, 1995). The upperparts have feathers which have pale fringes. The rump is slightly paler than the back, especially at the sides but is never white. The tail is pale greyish with pale sides, and occasionally a faint dark terminal band.

The most obvious feature between the two races is the presence of gray flanks in surinamensis, whereas niger shows clean white flanks. The upperparts tend to be more uniform and dark and the head pattern shows more of a "headphone" effect, similar to that shown in White-winged Black Tern (Chlidonias leucopterus). The dark face mask tends to show white streaking, which contrasts with the dark ear-coverts. Frequently the mask reaches to the mantle.

First Winter/First Summer Plumage

This is much as adult winter in niger and is acquired after the new year and continues into the summer (2nd calendar year). It can be distinguished from adult winter by the grey-brown and worn retained juvenile remiges (4-6 outer primaries) and usually a dark bar on the lesser coverts. The upperparts are washed out and untidy which is distinctly different from both juvenile and adult winter plumage. Both races are very similar in this plumage to their respective adult winter plumages.

Table 1. Juvenile features of Black Terns.

		surinamensis		niger
Flanks		Gray.		White.
Breast Patches		Large and dark, merging with flanks.		Usually smaller but isolated on white background.
Rump		Dark gray.		Pale-mid gray.
Tail		Dark gray.		Pale-mid gray.
Head Pattern		Pale Crown, nape and forehead. Crown sometimes streaked.		Dark crown extending down the nape.
Upperwing Coverts		Very dark gray.		Pale gray with some barring/pale feather edges.
Underwing Coverts		Usually gray or off-white.		Usually completely clean white.
Mantle		Narrow pale edgings to mantle.		Obvious pale edgings to mantle.

Observers should be aware that adult niger molting from summer plumage could be a potential pitfall as individuals in this plumage may show large dark breast patches merging with darks flanks, although usually the underparts are patchy in coloration, the upperparts look more ragged and worn and there is a dark primary wedge on the outer primaries. Also the mantle does not show the pale fringes or sub-terminal markings of juvenile birds.

However, if a bird is found showing all or most of the above features for each respective race then it should be possible to identify the bird specifically. For an excellent text on the species as well as illustrations and photographs of both races (species?), the Helm identification guide "Terns of Europe and North America" by Klaus Malling Olsen and Hans Larrson is highly recommended.

Although at present both are only considered races of the same species there is a good case for splitting the two forms, based on plumage and geographical distribution. Obviously further work is needed on these two forms, but with Least and Little Terns now treated as separate species there may well be a case for a re-appraisal of this species status.

Note the typical marsh Tern appearance. The wings are quite heavily abraded indicating that the bird is a moulting juvenile, although the upperpart plumage appears to be in full winter plumage, being pale gray with none of the brownish markings expected of juvenile marsh Terns. The head shows an extensive white forehead, dark eye patches and a white collar. In favour of the bird being niger are the pale rump and the pale grey tail, as well as the extensive dark cap and pale grey upperparts.In favour of surinamensis is the amount of grey on the flanks/axillaries.

In favour of this bird being surinamensis are the grey underwing, the pale forehead with white streaks on the cap, the dark outer tail feathers and the wide white collar. European birds should show a clean white underwing and clean white flanks and although this bird does not exhibit the amount of flank colouring one would expect in surinamensis, the dullness of the underparts (wing and flanks) would point to the bird being of North American origin.

Note that in this photograph, compared to figure 1, how the flanks appear paler and whiter and may appear this way from the brightness of the light and reflection of light off the sand. Shades of grey and brownish-grey can be difficult to judge under harsh light conditions, and photographs may not capture truly the shades of plumage involved, fieldnotes are therefore extremely important. Age of the bird and stage of moult will also have an effect on the paleness or darkness of a particular individual, and of course birds of the same species differ from one another in exact colour shades as well. It should also be noted that this individual exhibited a slight brownsih tinge to the back and mantle indicating that the bird is a juvenile.

References

ADRIAENS, P. 1999. The American Black Tern in County Dublin. Birding World 12/9. Norfolk.

ANDREWS, R.; HIGGINS, R. and MARTIN, J. 1999. The American Black Tern in Avon. Birding World 12/10. Norfolk.

CRAMP, S. et al. 1985. Birds of the Western Palearctic. Volume 4. Oxford.

HARRISON, P. 1983. Seabirds - An identification guide. Helm. London.

OLSEN, K.M. and LARSSON, H. 1995. Terns of Europe and North America. London.

Christian Melgar. Worthing, West Sussex. UK. 2001-2002.

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