Illumination of a chick's eye allows light to pass through to the retina of the contralateral eye. Electroretinographic (ERG) recording employing the scalp or comb as a reference results in shorter implicit time, higher amplitude and lower sensitivity during the day than during the night in a light:dark (LD) cycle and in constant darkness (DD). ERG recordings employing the contralateral eye as reference abolishes rhythmicity or reverses the phase angle (higher amplitudes at night). This is probably due to light transmission through the eyes to elicit visual responses in the reference. The contralateral eye is a poor choice for reference in birds and obscures physiological analyses of clock control of vision.