During the breeding season, the costs and benefits of social foraging of goldfinches (Carduelis carduelis L.) feeding in groups of different sizes was evaluated. With increasing flock size (up to six individuals) individuals became less alert: the individual head-jerk rate decreased but the whole group vigilance level increased. The handling time for the consumption of a Rumex seed was simultaneously reduced. Compared with individuals feeding alone, in a group of six birds the consumption of corns amounts to 20 % more within the same period of time. However, with increasing group size, the individuals have to fly greater distances to reach another profitable plant, which consumes more time. Another cost of flocking may be that larger flocks are more easily spotted by predators. Thus, flocking involves benefits in shortening handling time of the consumed seeds but also causes costs in having a larger travelling time to the next plant stem. Besides plant density calculated optimal foraging group size depends on the number of seeds provided by one seed head.