A recent study in this journal revealed that avian influenza A viruses (AIVs) H5Nx (N1, N6 and N8) showed pathogenicity,1 and accordingly had different adaptation to the codon usage pattern of its hosts.2 AIVs classify into 16 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes.3 The abundance and distribution of different subtypes varies greatly.4 Subtype H5 is the most often isolated AIV, followed by H7 and H9.4 It remains unclear which factor(s) associate with this abundance in birds. Considered that codon usage pattern was suggested to influence the adaptation of H5Nx AIVs,2 in this study, we calculated the codon adaptation index (CAI) to assess how well AIV HA subtypes H1–H16 and NA subtypes N1–N9 correspond to host codon usage patterns.