Abstract
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.
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Acknowledgements
M.S. and A.L. were supported by FORMAS and the Swedish NFR. PTR was supported by the Academy of Finland and Uppsala University. Data were collected and analysed with the help of E.J. Pellatt, A. Donald, P. Jacobsson, G. Engström, J. Nilsson, B. Rogell and T. Strand. A. Laurila and J. Höglund helped to improve the manuscript. We are also indebted to Alice Jakobsson, Aina and Tjelvar Lundberg and the staff at Ekhaga Försöksgård for providing access to the study areas.
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Communicated by F. Bairlein.
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Svensson, M., Rintamäki, P.T., Birkhead, T.R. et al. Impaired hatching success and male-biased embryo mortality in Tree Sparrows. J Ornithol 148, 117–122 (2007). https://doi.org/10.1007/s10336-006-0112-2
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DOI: https://doi.org/10.1007/s10336-006-0112-2