Variation in numbers, territoriality and flock size of a Goldcrest Regulus regulus population in winter

A Goldcrest Regulus regulus population living in a spruce‐dominated forest near Oslo (c.60°N), in southern Norway, was studied during the winters (November‐March) from 1966–67 to 1971–72. A significant reduction in population size occurred between November and March, the recorded values varying from 76 to 96%, with a mean reduction for all six winters studied of 86%. This winter decline in numbers was significantly correlated with both precipitation (i.e., snowfall) and ambient temperature, but not with bird density. Spiders were the predominant prey items of the Goldcrests, and it is suggested that the birds experience food shortages that affect their survival rate. Although not statistically significant, a mean change in the ratio of males:females from 2:1 in October‐November to 10:1 in December—February was observed, suggesting that a greater mortality of females than males occurs during the winter.Partly based on data for colour‐banded birds, it is suggested that the Goldcrests form social flocks, which move around as individual units within territories of about 9 ha (7.5 12 ha, n= 5) in extent. The mean flock size decreased significantly from 4.57 (n= 157) birds in November‐December to 2.82 (n = 74) birds in January‐February, probably because of the reductions in the size of the local population during the winter. The mean flock size in November‐December varied inversely with the ambient temperature (r= ‐0.92).