AP Biology Challenge Question. Wagtails?

European birds called wagtails eat insects. During winter, when food is scarce, a wagtail may defend a territory where it captures an average of 20 insects per hour, or it may join a flock that ranges widely over the countryside. A bird in a flock averages 25 insects per hour. Discuss the two strategies in terms of optimal foraging. Why do you think the wagtails do not always travel in flocks?