Age differences in primary moult of the Black-headed Gull Chroicocephalus ridibundus

Abstract

Flight feather moult is an energetically expensive stage of the annual cycle of birds. Its timing is adjusted to other important time- and energy-demanding activities, including migration. In the vast majority of migratory birds, primary moult occurs before or after migration or moulting is suspended during migration. The Black-headed Gull is an exception: it starts moulting flight feathers at the onset of the autumn migration. The course of primary moult in this species is still poorly understood due to the difficulty in obtaining sufficient data from the entire period of flight feather replacement. In this paper, we employed digital photos taken in the field to estimate the start, variation in the start date and duration of primary moult in adult and immature Black-headed Gulls, by using the Underhill–Zucchini likelihood moult model. On average, immatures started their moult on 7 June, 25 days earlier than adults, but their moult lasted 5 days longer. Consequently, there was a 20-day difference in the end of moult – that is, on average it occurred on 16 September and 6 October in immature and adult gulls, respectively. Adults showed greater variability in the mean primary moult start date than immatures, as the beginning of flight feather replacement may depend on breeding success (earlier after breeding failure) and on breeding phenology. The overlap of primary moult and migration produces a trade-off in resource allocation between those two processes, which leads to a low number of simultaneously growing primaries and a decrease in the growth rate of the two external, heaviest flight feathers. Existing reports of a later completion of primary moult in the first half of November in this species are based on the faulty assumption that the outermost primary is always the longest, which is not the case in 34% of individuals.

Keywords

• Gulls
• Underhill–Zucchini model
• flight feathers
• migration