Looking at field guides like „ Collins Bird Guide“ from Peter Grant, Dan Zetterstrom, Lars Svenson and Killian Mullarney the ID look quite simple. But even in the breeding plumage Chlidonias – or Marsh – Terns can cause some headache identifying in the field. Then even the Whiskered Tern (Chlidonias hybridus) can be confused with the Black Tern (Chlidonias niger) especially if seen in poor light facing the sun when both terns look remarkably dark.
A remarkable fact is, that the scientific name arises from Whiskered Tern´s similarities in appearance to the Black Tern but also to the (more whitish) Sterna – Terns.
To distinguish Black Tern (Chlidonias niger) and the 3rd member of the genus, the White-winged Tern (Chlidonias leucopterus) is even more sophisticated. The wings do not always look decidedly white – as the name suggests. Often the upper parts of the wings do not look so much brighter than in the Black Tern. On the other hand a good deal of black is shown in the underwing-coverts. Hence the black & white contrast of the underwings might be the best criterion to distinguish flying White-winged Tern from Black Tern in the field.
In flight, all these terns appear slim – less so the Whiskered Tern. The wing-beats are full and dynamic, and flight is often erratic as they descend to the surface for food. Chlidonias – or Marsh – Terns do not dive for fish, but forage on the wing picking up items at or near the water’s surface or catching insects in flight. They mainly eat insects and fish as well as amphibians. The feeding habit is quite unlike Continue reading Identifcation of flying Chlidonias Terns in breeding plumage