A Population of Birds Is Found on a Remote Island

A population of birds is found on a remote island. Which of the following information is most important in deciding if the birds all belong to a single species?

A) That all the birds appear to eat the same range of food.
B) Whether they can breed with one another.
C) Whether the matings produce viable eggs or offspring of some matings are sterile.
D) That they share many physical characteristics.
E) That the males all sing very similar songs.