Took me ages to get the rotation constraints working. There were no transforms on bones or mesh before parenting but nothing worked.
When I began the build I pressed 1 on the num pad to go to front view and built the head facing that view. As it turns out this is facing along the negative y-axis. (You can see from the little, coloured widget in the bottom left of the 3D viewport.) Eventually, I rotated the head bone 180 deg around the z-axis in object mode and applied the transform. Then the numbers entered into the rotation constraint boxes for each eye bone began to make sense.
Great learning experience.