Head orientation is aligned with take-off trajectory as chicks jump.

Chicks of up to one week of age were video recorded while jumping across gaps of different widths and depths. The orientation of the head in the sagittal plane was measured at the point where jumps began. As the gap became wider, or the far side of it was raised, chicks' heads rotated upwards, but this change in head orientation did not keep the far edge of the gap fixated at a particular elevation in the visual field. Except with narrow gaps, which chicks often stepped over, there was a consistent relationship between head orientation and the initial trajectory of a jump. Each 1 degree change in trajectory was accompanied by an approximately 1 degree change in head orientation. It is proposed that the visual control of head orientation and position in relation to the dimensions of a gap contributes to adjusting a chick's posture and, therefore, its trajectory at take-off.