Causes and consequences of adaptive seasonal sex ratio variation in house sparrows


Bernt-Erik Sæther, Department of Biology, Realfagbygget, Norwegian University of Science and Technology, N-7491 Trondheim, Norway. Tel.: +47 73590584. Fax: +4773596100. E-mail:


  • 1Here we examine how sex ratio variation in house sparrow broods interacts with other demographic traits and parental characteristics to improve the understanding of adaptive significance and demographic effects on variation in sex ratio.
  • 2The sex ratio in complete broods did not deviate significantly from parity (54·9% males).
  • 3There was sex-specific seasonal variation in the probability of recruitment. Male nestlings that hatched late in the breeding season had larger probability of surviving than early hatched males.
  • 4An adaptive adjustment of sex ratio should favour production of an excess of males late in the breeding season. Accordingly, the proportion of male offspring increased throughout the breeding season.
  • 5A significant nonlinear relationship was present between sex ratio and age of the female. However, there was no relationship between parental phenotype and standardized hatch day that could explain the observed seasonal change in sex ratio.
  • 6The sex-specific number of offspring recruited by a pair to subsequent generations was closely related to the brood sex ratio.
  • 7These results indicate an adaptive adjustment of sex ratio to seasonal variation in environmental conditions that affects the offspring fitness of the two sexes differently. Our results also suggest that such a sex ratio variation can strongly influence the demography and structural composition of small passerine populations.