Lenses were attached to rigid plastic rings mounted on Velcro support rings. Mated Velcro rings were attached with collodion to the feathers around one eye of each bird. Astigmatic lenses with +5 D along one axis and −5 D along the orthogonal axis, 12 mm in diameter, were made from CR39 plastic (Tri-Supreme Optical, Farmingdale, NY). Spherical lenses (plano [no optical power], +3 D, +6 D, −3 D, and −6 D) were made from PMMA with a base curve of 7 mm and an optic zone diameter of 10 mm (Conforma Contact Lens Co., Norfolk, VA). To combine astigmatic lenses with spherical ones, the spherical lenses were placed on top of the astigmatic lenses, and both were fitted into a clear plastic tube. The resultant lenses had the following powers along their two orthogonal axes: +8/−2 D, +11/+1 D, +2/−8 D, and −1/−11 D, respectively.
Lenses were fitted over one eye at 6 days of age. To align the lenses in relation to the normal head angle, chicks were photographed from the side in their cages and an estimate made of the average angle between the horizontal and the line formed by the auditory meatus and the beak tip—an angle of 18° (beak down). Consequently, we marked the Velcro rings that the birds wore at that angle, and we fitted the astigmatic lenses either in the WTR or ATR orientation or at 45°. When combined with spherical lenses, the astigmatic lenses were always oriented at 45°. This orientation produced maximum image distortion, because it blurred the vertical and horizontal bars of the chicks’ cage. It also caused maximum image motion during horizontal or vertical head movements.