ID immature Chlidonia’s terns

The Whiskered Tern (Chlidonias hybrida) occurs widely across the world in Africa, Europe, Asia and Australia. It is rare in Germany and only breeds in Mecklenburg-Western Pomerania and Saxony-Anhalt. Evidence of breeding was also recently reported in Brandenburg. In other parts of the world, however, this Chlidonia tern is the most common of the genus.

Even in the breeding plumage, Chlidonia terns are not always easy to distinguish from one another or from some of the smaller Sterna terns. This is all the more true if the breeding plumage has not yet developed or the species has moulted into non-breeding plumage. I find the Chlidonia terns to be particularly tricky in non-breeding plumage, especially in the first calendar year, when they have lost their conspicuous young plumage, the plumage is a mixture of molten (light) and old (dark) feathers and the contrasting drawing of the magnificent dress is far from being developed.

I have therefore tried to get as much information as possible for the best possible identification of the species of young Chlidonia terns.

Many identification guidesbooks emphasize the differences in physique. The best book is definitely still the (English) Helm Identification Guides “Terns” by Klaus Malling Olsen from December 1994. It is now sold as an antiquarian bookshop at a significantly higher price than it was then in the store and has unfortunately not been reissued. The drawings are still very helpful, even if the photos could use a refresher.

To start with the most likely Chlidonia tern in Western Europe. This is the Black Tern (Chlidonias niger). This species is an extremely elegant species with a slender body and bill, with short legs with no knee visible when it perches.

Compared to the Black Tern, the White-winged Tern (Chlidonias leucopterus) is slightly smaller and stockier, the bill is shorter and stouter, the legs are longer with a more visible ‘knee’. The Whiskered Tern is the larger and much stockier species in the genus. Compared to the Black Tern, it is larger and stronger in body length. The wings are longer and wider. Obvious is the particularly stout, deep-based bill with a strongly curved culmen and a general resembling of a Sterna tern. The legs are longer than those of the Black Tern, with a “knee” visible when standing. In flight it looks less elegant and more robust than the other two species.

Unfortunately, I found that I was rarely able to tell flying terns apart by body impression. This may be possible when different species fly over a lake at the same time and can be seen in good light, but most of the time you see several individuals of the same species together and then you have to keep in mind individual variance in body dimensions.

Statements about the color distribution in the plumage are perhaps more helpful. The Black Tern appears white on the head and underside in the first calendar year, with only a dark spot remaining on the crown, which extends over the ear covers in a “headphone” -like pattern. Visible is a dark ‘shoulder mark’ on the breast sides which is characteristic. The spot is actually on the sides of the chest.

White-winged Terns in the first winter (but also the adult birds in winter) are largely white-headed and as light-white on the underside as the Black Tern. However, there there is no sign of any dark ‘shoulder mark’ and the ‘headphones’ are a fairly weak feature, particularly across the crown which is paler than the ear coverts.

The light-colored rump and the white sides of the tail are striking. Juvenile specimens have a brownish-dark upper surface with a grid pattern like the Black Tern. However, the feathers on the mantle and scapulars are clearly darker than the closed wing which creates a clear, dark “saddle” that is clearly visible both at rest and in flight. Young (whether juvenile or in the first winter) White-winged Tern show this clear “saddle” contrast, which excludes the Black Tern, usually very well. Other important features of the White-winged Tern are the shorter, stouter bill, the reduced dark color distribution on the head, the “headphones”, the lack of a dark “shoulder mark”, contrasting white rump and upper tail coverts as well as narrow white tail edges.

The Whiskered Tern in its first winter plumage but also the adults in winter are white-headed and white on the underside like the black tern, with an extensive, dark-striped “cap” and a fairly straight lower edge. This reaches the back of the crown and therefore looks more like a sterna tern. The upperwings are relatively plain, with no strikingly dark primaries, and the rump, uppertail coverts and uppertail – the latter with narrow white tail-sides – are grey, concolorous with the mantle. Sometimes there is a hint of shoulder spots.

The stout bill is easy to see and the extent of the dark color distribution on the crown and neck are also easy to see. If a bird is photographed in flight, another useful difference to the juvenile White-winged Tern can be seen: the uniformly light grey rump.

Bills and legs of Chlidonia terns are dark in the non-breeding plumage and therefore no help to distinguish them. In summary, it can be said that the best way to distinguish the 3 Chlidonia terns in their young, non-breeding plumage is based on the size and shape of the dark head spots (“headphones”) and the contrast between the upper sides, tail and rump. Here it is necessary to see the characteristics in combination in order to secure oneself in the identification of the species.

Amazingly good, informative drawings and comprehensible descriptions can also be found in the rather old, but still excellent, book “Birds of Kenya and Northern Tanzania” from 1996 by Dale A. Zimmerman, Donald A. Turner and David J. Pearson.

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