Highly gregarious, western jackdaws are generally seen in flocks of varying sizes, though males and females pair-bond for life and pairs stay together within flocks. Flocks increase in size in autumn and birds congregate at dusk for communal roosting, with up to several thousand individuals gathering at one site. At Uppsala, Sweden, 40,000 birds have been recorded at a single winter roost with mated pairs often settling together for the night. Western jackdaws frequently congregate with hooded crows or rooks, the latter particularly when migrating or roosting. They have been recorded foraging with the common starling ("Sturnus vulgaris"), Northern lapwing ("Vanellus vanellus"), and common gull ("Larus canus") in northwestern England. Flocks are targets of coordinated hunting by pairs of lanner falcons ("Falco biarmicus"), although larger groups are more able to elude the predators. Western jackdaws sometimes mob and drive off larger birds such as European magpies, common ravens, or Egyptian vultures ("Neophron percnopterus"); one gives an alarm call which alerts its conspecifics to gather and attack as a group. Occasionally, a sick or injured western jackdaw is mobbed until it is killed.