It has been suggested that positive selection, in particular selection favoring a change in the protein sequence, plays a role in the evolution of olfactory receptor (OR) gene repertoires in fish and mammals. ORs are 7-transmembrane domain (TM) proteins, members of the G-protein-coupled receptor superfamily in vertebrate genomes, and responsible for odorant binding and discrimination. OR gene repertoires in birds are surprisingly large and diverse, suggesting that birds have a keen olfactory sense. The aim of this study is to investigate signatures of positive selection in an expanded OR clade (group-gamma-c) that seems to be a characteristic of avian genomes. Using maximum-likelihood methods that estimate the d(N)/d(S) ratios and account for the effects of recombination, we show here that there is evidence for positive selection in group-gamma-c partial OR coding sequences of 9 bird species that are likely to have different olfactory abilities: the blue tit (Cyanistes caeruleus), the black coucal (Centropus grillii), the brown kiwi (Apteryx australis), the canary (Serinus canaria), the galah (Eolophus roseicapillus), the kakapo (Strigops habroptilus), the mallard (Anas platyrhynchos), the red jungle fowl (Gallus gallus), and the snow petrel (Pagodroma nivea). Positively selected codons were predominantly located in TMs, which in other vertebrates are involved in odorant binding. Our data suggest that 1) at least some avian OR genes have been subjected to adaptive evolution, 2) the extent of such adaptive evolution differs between bird species, and 3) positive selective pressures may have been stronger on the group-gamma-c OR genes of species that have well-developed olfactory abilities.