1. In a field experiment with a winter population of house sparrows at a farm, flock size at a given feeding site appeared to be positively related to seed density. 2. The positive relationship between flock size and seed density resulted from individual birds spending longer in areas where seed density was high and from birds being recruited from elsewhere on the farm to experimental sites. 3. Mean flock size was positively related to and the variance of flock size negatively related to the time of day and both showed a tendency to be negatively correlated with the amount of human disturbance. Mean flock size was also positively related to ambient temperature and flock size variance negatively related to daylength. 4. The rate of fighting per bird increased with flock size and time of day but decreased with increasing ambient temperature. 5. The pattern of flock size distribution in house sparrows is compared to that in juncos and some differences between the two may be attributable to differences in the type of food supply and temperature range over winter.