SummaryA study of the breeding biology, feeding ecology, and behaviour of the Little Auk was carried out on Horse Head Island (73° 38′ N, 57° 08′ W) in west Greenland between July and August 1974. Distinct subcolony groups were recognized within the colony. During incubation, attendance at the colony increased to a peak between 2100 and 2400 h, with large rafts building up on the sea before engaging in aerial flights and then settling on land. During the nestling period, this pattern changed with further peaks at 0600 and 1800 h, and a decline during the afternoon, particularly during the late nestling period.Little Auks fed exclusively on copepods and amphipods (94%Calanus finmarchicus) usually captured within 2.5 km of the colony. Feeding rhythms fitted closely the known diurnal vertical migrations ofCalanuswith peaks between 2200 and 0800 h. Some individuals within subcolony groups showed synchrony of feeds, mainly between 1800 and 2400 h. The mean number of feeds to a chick in 24 h was 5.25. Growth rates of young were comparable between all four experimental groups and controls, although the latter attained higher weights. Mean daily weight increases were constant until 18 days of age when they declined sharply. This was not reflected in a change in mean daily weight increases with date and so suggests that food availability had not declined. Instead there was a reduced feeding rate to chicks from age 21 days to fledging.Breeding success was 50 %, mainly the result of egg desertion and predation by Arctic Foxes, and this may have been reduced by our own activities. Behaviour postures and the responses they elicited are described together with the diurnal patterns of different activities. Aerial flights are also described and their possible function discussed.It is concluded that the shortness of the time when abundant food is available has encouraged close breeding synchrony and this is achieved by mass flights and rafting on the sea beside the colony, and subdivision of the colony into groups with communal behaviour close to the