Retinal asymmetry in birds

Vertebrate sensory systems are generally based on bilaterally symmetrical s ense organs. It is evident, nevertheless, that birds preferentially use eit her their left or right eye for viewing novel or familiar stimuli [1], and perform visual discrimination tasks under monocular viewing conditions bett er with one eye than with the other [2,3], Because of the nearly complete c ontralateral decussation of the optic nerves in birds [4], it has been assu med that this division of labour is due solely to cerebral hemispheric spec ialisation, generated as a result of uneven photostimulation of the eyes of the developing embryo during the last three or four days before hatching [ 5,6], Here, however, we present evidence that in the European starling, Stu rnus vulgaris, even the retinae are morphologically asymmetrical in terms o f photoreceptor distribution. This is the first evidence for such asymmetry in any bird and suggests that retinal photoreceptor composition should be assessed during studies involving the lateralisation of visually mediated b ehaviours.