Variation in measurements of Cory's shearwater between populations and sexing by discriminant analysis

Abstract

Biometric data were collected at three colonies of the Atlantic race of Cory's Shearwater Calonectris diomedea borealis. Significant differences were found between sexes and between study sites. For each colony, a discriminant function was derived in order to achieve external sex identification. Using three measurements, all functions correctly classified more than 95% of the birds of known sex, and the biometrical variables investigated showed no variation with age. Marked differences in plumage and bill colour, in vocalization and in size, have led some authors to suggest that C.d. edwardsii could be considered as a distinct species. Comparison of morphological data from several colonies of Cory's Shearwater confirmed that the Cape Verde subspecies C.d. edwardsii occupies a very distinct position relative to the Atlantic and the Mediterranean (nominate C.d. diomedea) races. Birds from Mediterranean and Atlantic colonies show clinal variation in size, increasing from east to west. This morphological pattern is probably related to biological and physical conditions of the foraging areas of the different colonies.