Seasonality and Sociality in Tree Sparrows Passer Montanus

Abstract Despite the fact that the tree sparrow is often the object of behavioural studies and is a well-investigated species, no specific investigations were performed on the seasonal and daily pattern of the changes in flock size. Several observations suggest clear seasonal and daily periodicity of social organisation, and in connection with these changes, a similar periodicity in flock sizes. In this paper we present the results of a three-year observational study on the flock sizes of a suburban tree sparrow population. Our investigation resulted in statistical evidence for seasonal and daily periodic changes in the social organisation of the species. In the breeding season and in the mornings tree sparrows form smaller flocks than outside the breeding season and in the afternoons. Our results are consistent with the previous, non-systematic observations.