Do nine-primaried passerines have nine or ten primary feathers? The evolution of a concept

The number of primary feathers in a bird's wing has been used as a systematic character since the first half of the nineteenth century. During the years, though, the definition of which feathers to count as a primary has changed and today the species historically denoted as having only nine primaries are instead said to have, for example, nine 'functional' primaries. In this study, I investigated the borderline between 'nine-primaried' and 'ten-primaried' birds to search for a proper definition of the term 'nine-primaried'. A total of 161 specimens of 104 bird species, mainly passerines, were examined. All species examined had ten primaries although the 'nine-primaried' species had primary ten more or less concealed under primary covert nine. The number of primary coverts has decreased over time, with ten primary coverts as the ancestral state within Passeriformes and nine primary coverts among most oscine species. In conclusion, a proper definition of 'nine-primaried' might be with primary ten concealed by primary covert nine''. This definition includes all taxa historically denoted 'nine-primaried', i.e. systematically it is a definition of a paraphyletic group. The term 'nine-primaried' is thus too inclusive to be of more than very limited systemtic value and, consequently, the New World nine-primaried oscines group might gain from a new denotation.