Disentangling the effect of genes, the environment and chance on sex ratio variation in a wild bird population

Erik Postma, Franziska Heinrich, Ursina Koller, Rebecca J. Sardell, Jane M. Reid, Peter Arcese, Lukas F. Keller

Research output: Contribution to journalArticlepeer-review

42 Citations (Scopus)


Sex ratio theory proposes that the equal sex ratio typically observed in birds and mammals is the result of natural selection. However, in species with chromosomal sex determination, the same 1 : 1 sex ratio is expected under random Mendelian segregation. Here, we present an analysis of 14 years of sex ratio data for a population of song sparrows (Melospiza melodia) on Mandarte Island, at the nestling stage and at independence from parental care. We test for the presence of variance in sex ratio over and above the binomial variance expected under Mendelian segregation, and thereby quantify the potential for selection to shape sex ratio. Furthermore, if sex ratio variation is to be shaped by selection, we expect some of this extra-binomial variation to have a genetic basis. Despite ample statistical power, we find no evidence for the existence of either genetic or environmentally induced variation in sex ratio, in the nest or at independence. Instead, the sex ratio variation observed matches that expected under random Mendelian segregation. Using one of the best datasets of its kind, we conclude that female song sparrows do not, and perhaps cannot, adjust the sex of their offspring. We discuss the implications of this finding and make suggestions for future research.

Original languageEnglish
Pages (from-to)2996-3002
Number of pages7
JournalProceedings of the Royal Society of London. B, Biological Sciences
Issue number1720
Early online date23 Feb 2011
Publication statusPublished - 7 Oct 2011


  • sex ratio
  • animal model
  • Markov Chain Monte Carlo
  • overdispersion
  • song sparrow
  • Melospiza melodia
  • sparrows melospiza-melodia
  • brown-headed cowbirds
  • song sparrows
  • reproductive success
  • R package
  • heritability
  • manipulation
  • repeatability
  • adjustment
  • kestrel


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