Large numbers of auks (Alcidae) have died in recent years in a variety of mortality incidents, many of which resulted from oil pollution. This thesis explores the background to determining age, sex and origin of Guillemots Uria aalge and Razorbills Alca torda involved in such incidents, and presents data resulting from the examination of these birds.
Ageing factors considered were cloacal bursa, supra-orbital ridge, gonad size, wing and bill measurements, and bill grooves in Razorbills. Most ageing factors were mutually supportive, though as yet there were only few confirmations from known-age birds.
Sex determination was usually straight-forward, based on gonadal inspection. Especially in adults, males were on average shorter-winged and bigger—billed than females in both, species; however, individual birds could not safely be sexed from external measurements. Sex ratios, from various mortality incidents, were mainly equal in Razorbills of all ages and in adult Guillemots.
Origins of Razorbills could be fixed morphometrically from size (as indicated by winglength and bill depth), which separated most birds of the form torda (breeding in Scandinavia and Russia) from those of the form islandica (breeding in France, Britain, Ireland, Faroe and Iceland). Guillemots could be fixed more closely using three parameters which increased clinally through western Europe towards the north: winglength, proportion of bridled birds, and degree of colour saturation of upperparts.
A knowledge of the age, sex and origin of auks involved in mortality incidents is important in contexts of conservation and of population ecology. Present knowledge could be particularly improved through the implementation of a minimum standard system of examination of auks killed in incidents; the storage and study of known-age (ringed) auk corpses; and the acquisition of more data on fine—grain taxonomy in European auks
