関連ワード:  why do birds fly in flocks    why do birds fly in flocks in a circle    why do birds fly in flocks at sunset    why do birds fly in flocks short answer    why do birds migrate in flocks    why do parrots fly in flocks    why do birds fly in huge flocks    why do migratory birds fly in flocks    why do some birds fly in flocks    why do black birds fly in flocks