Voorstel voor een leeftijdsindelingssysteem bij Zeekoet en Alk

Common Guillemots Uria aalge are by far the commonest seabird washed ashore in the Netherlands. Until recently, it was not possible to age these birds, so nothing was known about their age structure and its annual variation. However, two papers were recently published concerning age characters of both Guillemots and Razorbills Alca torda. In the first, Kuschert et. al. (1981) described a colour-contrast in the wing, caused by arrested moult in the greater wingcoverts in first winter birds (see Sandee in this issue). In the second, Anker-Nilssen & Røstad (1981) described the occurence of a bursa Fabricii (hereafter referred to as the bursa), a gland functional in the younger age classes of most birds, which disappears as they mature (see Van Franeker in this issue). The present paper combines these two ageing characters to produce four distinct age classes, and tests them on 83 beached Razorbills, which can be aged separately using bill characteristics (Anker-Nilssen & Røstad 1981). In addition, the classification method is applied to a sample of 200 Guillemots, beached with the Razorbills early 1983. The four distinct age-classes, developed in the proposed classification system (hereafter called the contrast/bursa-system, or c/b-system and c/b-age), are as follows: I : contrast in wing present, large bursa (index ≥50, see Van Franeker) IIa : no wing contrast, large bursa ÌIb : no wing contrast, small bursa (index <50) III : no wing contrast, no bursa There was a significant correlation of age, as defined by the c/b-system (c/b-age), with that defined by bill characteristics (bill-age) in the Razorbill (table 1.). In addition, there existed correlations of both ageing systems with gonadal development (see figures 3 and 4 in Van Franeker). Both systems of age classification showed increases in various parameters of bodysize (tables 2 and 3). From this is concluded that the proposed system is useful for ageing Razorbills, and tha it might be for Guillemots as well. Evidence for this applicability to Guillemots were correlations of age with gonadal development (figures 5 and 6 in Van Franeker), increases in body-size parameters (table 5), as for the Razorbill, and progress in acquiring summerplumage (table 6). Also shown were possible age-related differences in sexratio (juveniles have a small excess of females, table 4, see Camphuysen in this issue), and a possible sex-difference in the progress of acquiring summerplumage (females ahaid of males, table 6). There appeared to be hardly any age-related difference in the colour of the back, suggesting no great differences in origin between age groups (table 7). It is hoped that further research will be carried out to determine the exact relationship of age by the contrast/bursasystem to that on ringed, known-age birds, and that ringing recoveries will be re-analysed to determine survival rates for c/b-age groups, enabling us to relate the age-structure of Guillemots and Razorbills to the impact on populations.