Evolution in the House Sparrow—;VI. Variability and Niche Width

ABSTR•CT.--Degree of secondary sexual size dimorphism in House Sparrows, Passer domesticus, varies from small at low latitudes to large at high latitudes.This organization of size dimorphism is an expression of geographic variation of locality variance in body size.For North American populations variance in body size is inversely correlated with number of interspecific competitors, and may result from the interplay between interspecific competition tending to decrease size variation and intraspecific competition tending to increase size variation.For European populations variance in body size is inversely related to number of congeneric species of Passer, but possible causal factors seem to be more complicated than they are for North American House Sparrows.Nevertheless, in either set of continental populations, degree of body size variation at a locality may reflect relative niche width.