At the time of going to press, photographs emerged (e.g. plate 125) of the ‘American Black Tern’ Chlidonias niger surinamensis in Northumberland apparently engaged in a copulation attempt with an Arctic Tern Sterna paradisaea. The behaviour and plumage of the American Black Tern previously suggested that it was a male (see here) but the set of images showing the apparent copulation attempt seem, at first glance, to bring that into doubt. However, Ian Nisbet offered the following comment: ‘This sequence of photographs indicate that the Arctic Tern was a male, but it does not necessarily show that the American Black Tern was a female. The actions of the Arctic Tern are mostly aggressive: I don’t know how to interpret the postures of Black Terns, but this bird seems to be mostly submissive (appeasing). Unmated male terns often react to other terns in submissive postures by trying to mount them. I have often seen young male Common Terns [S. hirundo] that are exploring the nesting area in mid to late June encountering large chicks that are unattended by their parents; the chicks know from experience that adult terns other than their parents are likely to peck them, so they crouch submissively. The inexperienced young males mistake the defensive crouching of the chicks for submissive crouching of females, and then mount the chicks and try to copulate with them. If [the Northumberland] American Black Tern brought in fish and displayed with them to other terns, it is more likely to have been a male, but I do not know enough about the behaviour of Black Terns to express a strong opinion about this. If it lays an egg this year, that will settle the matter!’
Furthermore, the American Black Tern was briefly observed briefly attempting to incubate an unattended Arctic Tern egg in June 2022 (Paul Hackett pers. comm.; plate 126. See also https://britishbirds.co.uk/file/22323). As both sexes of Black Tern undertake incubation (BWP, Cramp 1983), this again does not detract from earlier suggestions that the bird is a male – though it may demonstrate the bird’s frustration and desperation to breed after several fruitless years visiting the Long Nanny tern colony.
Ross Ahmed, Northumberland; e-mail [email protected]