Among vertebrates, the sense of smell is mediated by olfactory receptors (ORs) expressed in sensoryneurons within the olfactory epithelium. Comparative genomic studies suggest that the olfactory acuity ofmammalian species correlates positively with both the total number and the proportion of functional ORgenes encoded in their genomes. In contrast to mammals, avian olfaction is poorly understood, with birdswidely regarded as relying primarily on visual and auditory inputs. Here, we show that in nine bird speciesfrom seven orders (blue tit, Cyanistes caeruleus; black coucal, Centropus grillii; brown kiwi, Apteryx australis;canary, Serinus canaria; galah, Eolophus roseicapillus; red jungle fowl, Gallus gallus; kakapo, Strigopshabroptilus; mallard, Anas platyrhynchos; snow petrel, Pagodroma nivea), the majority of amplified ORsequences are predicted to be from potentially functional genes. This finding is somewhat surprising as oneprevious report suggested that the majority of OR genes in an avian (red jungle fowl) genomic sequence arenon-functional pseudogenes. We also show that it is not the estimated proportion of potentially functionalOR genes, but rather the estimated total number of OR genes that correlates positively with relativeolfactory bulb size, an anatomical correlate of olfactory capability. We further demonstrate that all the ninebird genomes examined encode OR genes belonging to a large gene Glade, termed y-c, the expansion ofwhich appears to be a shared characteristic of class Ayes. In summary, our findings suggest that olfaction inbirds may be a more important sense than generally believed.
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