Common chiffchaff one of the first avian signs that spring has returned. Its call is a "hweet", less disyllabic than the "hooeet" of the willow warbler or "hu-it" of the western Bonelli's warbler. The song differs from that of the Iberian chiffchaff, which has a shorter . However, mixed singers occur in the hybridisation zone and elsewhere, and can be difficult to allocate to species. When not singing, the common chiffchaff can be difficult to distinguish from other leaf warblers with greenish upperparts and whitish underparts, particularly the willow warbler. However, that species has a longer primary projection, a sleeker, brighter appearance
Genomic evolution of birds or two cone opsin genes. Birds also seem to have lost the functionality of their right ovary. There are two ovary development-related genes, MMP19 and AKR1C3, that have disappeared in birds. The fact that a high number of genes related to spermatogenesis are evolving fast (which does not happen in those related to ovogenensis) suggest that males undergo a stronger selective pressure. The abundance of genomic data available nowadays has enabled research to elucidate the early evolution and divergence of bird groups, and has produced a much more detailed phylogenetic tree. Genomic research has shown that in order to make