The effect of manipulation (toggle recoil technique) for headaches with upper cervical joint dysfunction: a pilot study. Whittingham,W,Ellis WB,and Molyneux TP,
J Manipulative Physiol Ther. 1994 (Jul-Aug); 17 (6): 369-375

Twenty-six patients (16 males, 10 females) all had chronic headaches with upper cervical joint dysfunction. Significant diminishing of the severity and frequency of headaches was reported in a large majority of the subjects (24 out of 26).

A series of migraine changes following a manipulative therapy trial. TuchinPJ. 
Australasian Chiropractic & Osteopathy 1997 (Nov); 6 (3): 85-91

Four cases of migraine responded dramatically to spinal care. Many self reported symptoms were either eliminated or substantially reduced. Average frequency of episodes was reduced by 90% with the length of each headache reduced by 38%. Medication use dropped 94%. Other symptoms associated with migraine were reduced including nausea, vomiting, photophobia and phonophobia.

1 Braaf mm., Rosner SJ. Trauma of the cerviacal spine as cause of chronic headache. Trauma. 1975;15:441-446.

2 Margulies S. The postconcussion syndrome after mild head trauma, part II; is migraine underdiagnosed? Journal of Clinical Neuroscience. 2000;7(6):495-499.