Biometrics and colour forms of chicks of common terns and arctic terns

Abstract

Methods of distinguishing the chicks of Common and Arctic Terns based on measurement of tarsus length and on the colours of the dorsal down and the belly down are described. The variation of tarsus length with age was investigated by relating it to bill length, to mid‐toe length, and to body weight. In all of these comparisons, length of tarsus provided a reliable method of distinguishing the species in the larger chicks, but there was some overlap in the ranges of tarsus length in the smaller chicks. Length of tibia was of little value in separating the species. Grey chicks and brown chicks were found in Arctic Terns in a ratio of about 1:1, but only brown chicks were found in Common Terns. The colour of the belly down was pure white in all Common Terns examined and white with a dark cast in all Arctic Terns examined. Criteria based on dorsal and belly down colour in smaller chicks and tarsus length in larger chicks thus allow the species to be distinguished for all chick ages from hatching to fledging. ...