Evolution in the House Sparrow. III. Variation in Size and Sexual Dimorphism in Europe and North and South America

Geographic variation in size of skin variables in house sparrows Passer domesticus parallels the known variation in skeletal variables; size varies from small to large with latitude in North America but from large to small with latitude in Europe; there is no clinal pattern in our South American samples. Secondary sexual size dimorphism is wholly characteristic of house sparrows of all three continents. For North American and European samples there is smoothly clinal variation in the degree to which the sexes differ in size, with the greatest differences occurring at more northerly localities. The degree of size dimorphism increases northerly irrespective of geographic trends in overall size in either sex. Differential sexual size dimorphism may reflect differences in dominance between the sexes at winter feeding sites. Almost all Western Hemisphere populations of house sparrows show differences in size variables relative to those of probable ancestral European populations, but these are not sufficient by rigorous taxonomic standards to warrant nomenclatural recognition.