McMillan, J.P., Keatts, H.C. & Menaker, M. J. Comp. Physiol. (1975) 102: 251. doi:10.1007/BF01464359
The eyes and extraretinal brain photoreceptor(s) of the House Sparrow (Passer domesticus) both contribute to entrainment of the locomotor rhythm. During exposure to a light cycle of low intensity 25 sparrows were entrained and eight free-ran (Table 1). Birds free-running under these conditions became entrained after the intensity of light reaching the brain was increased by plucking their head feathers (Fig. 1). Following carbon black injection beneath the skin over the skull of 23 of these entrained birds, 13 remained entrained and 10 free-ran (Figs. 1 and 2). The injected-entrained birds resynchronized to a 6 hour phase delay in the light cycle. Six of these birds were blinded (bilateral orbital enucleation) and subsequently free-ran. When the carbon black was then removed they re-entrained (Fig. 3).