Geographic variation in size and sexual dimorphism across a hybrid zone between Carrion Crows (Corvus corone corone) and Hooded Crows (C. c. cornix)

Abstract

Sexual size dimorphism was analysed across a hybrid zone existing between allopatric populations of Carrion Crows and Hooded Crows (Corvus corone corone and C. c. cornix) in northwestern Italy. Twenty-five morphological (osteological and plumage) variables were measured in a sample of 1599 specimens obtained from allopatric parental areas and from the hybrid zone. For each morphological variable considered, sexual size dimorphism was measured as the ratio of males/females or the absolute difference between mean sizes of males and females within each subpopulation studied. Overall sexual size dimorphism was described by discriminant function analysis. All the sexual size dimorphism measures adopted showed variation across the hybrid zone according to elevation. Male/female ratios and male–female differences significantly differed between the parental allopatric "operational taxonomic units," Carrion Crows being more dimorphic than Hooded Crows. Across the hybrid zone, sexual size dimorphism was correlated with the phenotypic composition of the crow subpopulations and with size of males but not size of females. The data presented show that sexual size dimorphism is correlated with an ecological gradient and that its geographical variation is determined by variation in size of males only.