関連ワード:  why do birds migrate in v shape    why do birds fly in v shape    why do birds migrate in av shape    why do birds fly in v shape class 4    why do birds fly in v shape short answer    why do birds fly in v shaped formations    why do birds fly in v shape joke    why do birds fly in v shape class 4 short answer    why do birds fly in av shape    why do birds fly in av shape formation