Migratory Birds Are Likely Responding to What Environmental Cue to Initiate

Multiple Choice

Migratory birds are likely responding to what environmental cue to initiate the behavior leading to migration from the northern breeding grounds to their more southern overwintering areas?

A) changes in temperature as seasons change
B) changes in food supply as seasons change
C) changes in predator activity as seasons change
D) changes in day length as seasons change