Impaired hatching success and male-biased embryo mortality in Tree Sparrows

Magnus Svensson*, Pekka T. Rintamäki, Tim R. Birkhead, Simon C. Griffith, Arne Lundberg

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Citations (Scopus)


During the past 30 years, many species of farmland birds have declined dramatically in numbers in Northern Europe, a trend coinciding with a tremendous intensification of agriculture, although the exact causes of these declines remain unclear. One of the worst affected species is the Tree Sparrow (Passer montanus). We studied two Swedish Tree Sparrow populations during the years 1996-2004 and found that in both populations, almost half of all laid eggs remained unhatched. This led us to investigate whether the eggs failed to hatch because of: (1) eggs not being fertilised or (2) embryo mortality. Our analyses showed that all of the eggs investigated contained sufficient number of sperm for fertilisation and that they also had other visible signs indicating that fertilisation had occurred. Hatching failure was instead shown to result from embryo mortality. Using molecular techniques, we were able to determine that embryo mortality is more likely to affect male embryos than females and that the fledgling sex ratio was consequently highly female biased. The cause of this sex-biased embryo mortality remains unknown, but various potential explanations are discussed.

Original languageEnglish
Pages (from-to)117-122
Number of pages6
JournalJournal of Ornithology
Issue number1
Publication statusPublished - Jan 2007
Externally publishedYes


  • Eggs
  • Embryo mortality
  • Fertilisation
  • Passer montanus
  • Sex ratio


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