When are Atlantic puffins Fratercula arctica in the North Sea and around the Faroe Islands flightless?

Capsule: Atlantic Puffins in the North Sea can replace their primaries, and hence be flightless, any time between September and March but there are peaks in wing moult in October and March and a smaller proportion of birds moult between November and February. Aims: To determine when Atlantic Puffins wintering in the North Sea and around the Faroe Islands replace their primaries and are flightless. Methods: We examined 1431 Atlantic Puffins washed ashore on the coasts of the North Sea and 165 shot in the Faroe Islands. Birds were aged using bill characteristics and the state of wing moult and age of the primaries recorded. Results: Flightless adult Puffins were recorded in all months between September and March but the proportions of moulting birds were higher in October and March and lower in January and February. Most juveniles did not become flightless during their first winter and probably do not moult their primaries until about one-year old. On average, adults completed their moult slightly earlier than immatures, consistent with a typically earlier return to the breeding colony at the start of the breeding season. Conclusion: Despite the Atlantic Puffin being one of the most abundant birds in the North Sea we still know little about its moult. The available data indicate that the species exhibits a highly unusual pattern with the timing of the flightless period showing much greater variation than normal. The causes and consequences of this flexibility are currently unknown but results from ringed birds indicate that timing of moult can vary markedly within a breeding population