Age determination of Corn Buntings Emberiza calandra by skull ossification after the autumn moult

Abstract

When determining the age of fully grown passerines, ringers can often rely on plumage characteristics or other subtle colour characteristics such as iris coloration (eg in Sylvia spp) or tongue-marks in some warblers (Svensson 1992). For most species, plumage differences between adults (EURING age codes 4 or greater) and birds hatched in the current calendar year (EURING code 3, referred to here as first-year birds) are applicable due to different moult strategies in these two age categories. In late summer, juveniles often undergo a partial post-juvenile moult and adults a complete post-breeding moult. Thus, the degree of abrasion and the presence of moult limits in wings (and tail) indicate the age, and there is often no need to check for other features. However, there are some passerine species where adults and juveniles both undergo a complete late-summer moult (eg Alaudidae, Moustached Warbler Acrocephalus melanopogon, Long-tailed Tit Aegithalos caudatus, Sturnus spp, Passer spp (Svensson 1992, Jenni & Winkler 1994)), which makes it impossible to use plumage criteria for ageing afterwards (except when an occasional juvenile flight feather is retained). Therefore we need to resort to other characteristics, such as skull ossification. As in other vertebrates, juvenile birds immediately after fledging initially have only one layer of skull bone and as development proceeds a second layer grows below the first one. The degree of development of this second layer can often be observed as a ‘line’ between the doublelayered (whitish with spots) and the single-layered (pinkish without spots) skull. The growth duration of this second layer of bone is often genusor species-specific and some species complete skull ossification within two months after hatching (eg Northern Wheatear Oenanthe oenanthe), while other species (such as European Nuthatch Sitta europaea) can retain certain one-layer ‘windows’ even as adults (Svensson 1992, Jenni & Winkler 1994). For some species the progress of skull ossification is well known – with practice it is straightforward to carry