Changes in endosteal cell layer number of long bones are not appropriate for ageing birds: Evidence from Baltic razorbills (Alca torda Linnaeus, 1758)

Estimating the age of birds by counting the number of cell layers within the circumferential lamellae of the bone cortex gave promising results in some species (e.g., fulmar Fulmarus glacialis; shag Phalacrocorax aristotelis) but was not confirmed for others (e.g., Canada goose Branta canadensis; sandhill crane Grus canadensis). Moreover, different results were received for different bones from the same individual and the factors influencing these results have not been identified. We have evaluated the ageing method for a non-migratory population of razorbills (Alca torda) from Baltic Sea colonies by examining humeri from 38 razorbills recorded dead on the Polish coastal zone of the Baltic Sea. Only humeri of 19 birds showed clearly defined borders between the osteonal matrix and the endosteal bone. Razorbills were divided into three age categories by bill ornament (juveniles, immature, adults) and showed a trend towards a gradual loss of cell layers in the endosteal bone with increasing age, where median values totalled 8.5, 7 and 6 layers respectively. Our results indicate that counting the number of cell layers within the circumferential lamellae of the bone cortex is not a reliable methodology for ageing razorbills.