Morphometric Comparison of Skeletons of the Western Grebe Complex Aechmophorus of the United States and Canada

Abstract

Skeletal specimens of Aechmophorus--210 occidentalis (dark "phase") and 77 clarkii (light "phase")--were analyzed morphometrically using univariate and multivariate techniques to assess interspecific and intersexual skeletal variation; variation due to age and locale also was tested. A. occidentalis generally exceeded A. clarkii in skeletal dimensions; mean differences were less than 1 mm for widths of elements and approximated 5 mm for lengths of major wing elements. Also, 5-vertebral notaria were more frequent in the larger A. occidentalis than in A. clarkii. Univariate sexual dimorphism was highly significant in all skeletal measurements, dimensions of males exceeding those of females. Stepwise canonical analyses of the four species-sex groups of Aechomorphorus documented significant interspecific and intersexual differences in skeletons; interspecific differences in proportions within the pelvic limb and bill suggest differential refinement for locomotion and foraging. Multivariate sexual differences were more than twice as great as multivariate differences between species, and sexual dimorphism was significantly greater in A. clarkii than in A. occidentalis. Classification functions for skeletons of Aechmophorus of known sex permitted species identification for 85% of specimens. Identification of single elements to species does not appear to be feasible using mensural criteria. Lesser but significant multivariate variation in skeletons was attributable to age and locale of collection. Magnitude of interspecific differentiation in skeletal dimensions in Aechmophorus is small compared to that among species in other genera studied previously.