The White-throated Sparrow winters in southeastern US, breeding across Canada and the northern-most United States. During early spring, it stores large amounts of fat as an energy source during flight. Reproductive development occurs during spring. Following the June breeding, gonads regress and remain small until next spring. These events (fattening and gonad size increase) and migratory restlessness and orientation towards north or south are ALL AFFECTED BY PROLACTIN FROM PITUITARY and CORTICOSTEROIDS from the Adrenal cortex.
A. H. Meier injected corticosteroids into White-throated Sparrows at time zero. Prolactin given at 12 hours results in: rise of fat stores, rise in gonad size, rise in migratory restlessness and a rise in locomotor activity towards north (= spring migrant). Prolactin given at 4 hours results in arise in fat stores, rise in migratory restlessness, NO gonad increase, and an orientation to south (= fall migrant). Prolactin at 8 hours decreases fat stores, decreases locomotor activity, and decreases gonad weight (= winter bird).
This does not explain how the bird knows which way is north or South.