APPLICABILITY OF EXTERNAL MEASUREMENTS TO SEXING OF THE CAPE PETREL DAPTION CAPENSE AT WITHIN-PAIR, WITHIN-POPULATION AND BETWEEN-POPULATION SCALES

Sexual size dimorphism in the Cape pet;el Daption capense was analysed to improve methods for sexing live birds in field studies. Samples or iginated from two geographically distant study populations in the Anta rctic: Nelson Island, South Shetland Islands (sample N: 83 M, 89 F), a nd Ardery and O' Connor Islands, Windmill Islands (sample A: 36 M, 27 F). Using single characters with cut points calculated from sexed indi viduals, about 59-76% (N; best separator tarsus length) and 61-86% (A; best separator tube length) of birds could be assigned to the correct sex. Combination of characters improved performance of sample-specifi c discriminant functions to a maximum of 84% in the N-sample (five cha racters) and 95% in the A-sample (four characters). The most useful ch aracter sets as well as ranking of individual characters (their weight s) differed between samples. At the same time, cross-testing revealed asymmetry in the applicability of sample-specific functions to the oth er sample. Hence, the method of a generalized discriminant (Auk 110: 4 92-502, 1993) was implemented and tested to develop a species-specific discriminant for the Cape petrel. For all examined character sets, re sults of their cross-application to the other sample were improved by using character weights estimated from combined samples instead of wei ghts derived only from the original sample. The generalized function ( tube length + 1.05bill depth at tube + 0.72*culmen length + 0.07*wing length) combined with a cut point calculated from a distribution of d iscriminant scores (assumed mixture of two normal distributions with p ossibly unequal variances) is suggested as currently the best compromi se for sexing Cape petrels in populations for which specific functions are not yet available: comparison with our best sample-specific funct ions shows a decrease in accuracy by only 1% to 3%. However, examinati on of other promising characters (head length, bill depth at gonys, mi d-toe length) is recommended to improve further a generalized discrimi nant. In aliases a larger percentage (N: 87-89%, 38 pairs; A: 100%, 27 pairs) of individuals was correctly sexed by within-pair comparison o f discriminant scores than according to the sample-specific cut point. No clear evidence for assortative mating with respect to body size ha s been found in this study. Available biometric data show little evide nce for geographical variation in the Cape petrel, which suggests good applicability of generalized sex-discriminants in other populations.