Sexing great SkuasCatharacta skuaby discriminant analysis using external measurements

A discriminant function used to sex Great Skuas at Foula, Shetland correctly classified 87.5% of the sample from which it was derived, and several lines of evidence suggest that it performed similarly with a sample of live birds caught over a three‐year period. There was no evidence that any of the variables used in the analysis changed during the period of the study or were affected by conditions at the colony during birds’ development. However, this may not be the case with birds caught in future years. Culmen length was correlated with age, and so was not suitable for inclusion in the analysis. The discriminant function derived by one worker performed comparatively poorly with a sample of birds measured by a different worker, despite the fact that their measurements were in close agreement with each other. We advocate great caution in the use of discriminant functions, particularly when a function has been derived by a different worker.