Abnormal Plumage in Adult Common Terns Sterna hirundo during the Breeding Season

Some bird species change their plumage colour on a seasonal basis, mostly between cryptic non-breeding plumage and conspicuous breeding plumage. Adult Common Terns Sterna hirundo moult their body feathers twice each year, resulting in two types of plumages, those typical to breeding and non-breeding seasons, which exhibit minor differences in the colours of some feathers, as well as the bare parts. Here, we report evidence of an abnormal plumage in adult Common Terns (sometimes referred to as ‘portlandica’), with two individuals exhibiting non-breeding like plumage during the breeding season. These terns were documented in a breeding colony in Northern Israel. This report could lead to future research into the causes and consequences of plumage maturation avoidance in adult birds.