Sex- and age-dependent breeding plumage acquisition in monomorphic species, the Black-headed Gull Chroicocephalus ridibundus

Abstract

In monomorphic species, like in the Black-headed Gull, both sexes look alike in breeding plumage. With large sets of data on captured and photographed birds and using the Underhill–Zucchini moult model, we provided a detailed pattern of breeding plumage development in this species by age and sex. This study, similar to other studies, documented first adults with the initial stage of head moult at the beginning of January, yet the mean start date of nuptial moult in adults was the end of February. Half of adults acquired full breeding plumage about mid-March and almost all of them before mid-April. The start date of nuptial moult was more variable in immatures. The mean start of head moult in immatures was 19th April, which is 52 days later than in adults, and the majority, i.e., 70%, did not complete moult until the end of May. We showed for the first-time sex-dependent breeding plumage acquisition in monomorphic species. According to the Underhill–Zucchini moult model, males started to moult on average 7 days earlier than females and their moult lasted 7 days longer. Hence, the final date of completed head moult was the same in both sexes. A fully developed hood is an important part of the status signalling during pairing; therefore, completing the moult before mating is important for both sexes.