If differentiated forms are to persist alongside each other as separate species two condition must be met First they must avoid interbreeding In bird this is usually taken care of by differences in appearance and in the song It is no accident that bird watchers find male bird so easy to recognize correct identification is even more important for the female bird Dawin’s finches recognize each other chiefly by the beak We have often seen a bird start to chase another from behind and quickly lose interest when a front view show that the beak is that of a species other tham its own