Assessing age and breeding origin of wrecked Little Auks Alle alle: the use of biometrics and a variable under wing pattern

Biometrics and plumage characteristics of Little Auks Alle alle were evaluated to assess possibilities for the external ageing of individual birds. Age is important when biometrical data are used to assess the subspecific identity or probable breeding origin of the birds. Standard biometrics included bill length (feathers to tip), distance from nostril to lip, bill depth, head, wing (maximum flattened chord), and tarsus length, and body mass. The presence or absence of white or white-tipped feathers was checked in seven feather groups of the (grey) underwing. Bill depth and wing length were the most useful measurements to separate adult and juvenile Little Auks (when combined, classification accuracy 83%). In combination with body mass (only emaciated birds were used), the age was assigned correctly in 88% of the examined birds. White or white-tipped feathers in the lesser primary coverts (LPC) occurred more frequently in juveniles than in adults, while the reverse was true for the greater secondary coverts (GPC). Only 74% of the Little Auks were properly aged on the basis of a combination of LPC and GPC pigmentation. With body mass being a ‘difficult' measurement (an assessment of physical condition is required and incomplete corpses cannot be weighed), the combination of bill depth, wing length and white in LPC and GPC was evaluated (87% correctly assigned). A review of biometrics collected in breeding areas indicated that birds wrecked in The Netherlands were of the subspecies A. alle alle, with an overall size similar to for example birds of Bjørnøya (Bear Island) in the Barents Sea. It is recommended to use bill depth and wing length for ageing in combination with pigmentation patterns of LPC and GPC in future studies of wrecked birds. For comparisons with breeding populations, bill length and wing length are the most widely available and therefore useful measurements.