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Impaired hatching success and male-biased embryo mortality in Tree Sparrows
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Evolution, Population and Conservation Biology.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Evolution, Population and Conservation Biology.
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2007 (English)In: Journal of Ornithology = Journal fur Ornithologie, ISSN 0021-8375, E-ISSN 1439-0361, Vol. 148, no 1, 117-122 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2007. Vol. 148, no 1, 117-122 p.
Keyword [en]
Eggs, Embryo mortality, Fertilisation, Passer montanus, Sex ratio
National Category
Biological Sciences
URN: urn:nbn:se:uu:diva-95302DOI: 10.1007/s10336-006-0112-2ISI: 000243854600016OAI: oai:DiVA.org:uu-95302DiVA: diva2:169467
Available from: 2006-12-21 Created: 2006-12-21 Last updated: 2011-02-16Bibliographically approved
In thesis
1. Embryonic Mortality and Sex Ratios in the Tree Sparrow
Open this publication in new window or tab >>Embryonic Mortality and Sex Ratios in the Tree Sparrow
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tree sparrows (Passer montanus) have been studied in two areas in Sweden since 1997. At both sites, tree sparrow eggs had remarkably low hatching success. On average only 60% of the eggs hatched. Analyses have shown that this was caused by embryonic mortality, which was highly sex biased. About 70 % of the dead embryos were males, while about 65 % of all fledged nestlings were females. Impaired hatching success here related to two factors. Hatching success was lower for pairs with a male in poor body condition, and it was lower in areas with a high local population density.

A sex bias in the mortality early in life has been demonstrated in several species. Since the competitive ability of males is determined by conditions early in life, parents with poor provisioning capacity should prefer to produce female offspring in broods reared under poor conditions. The body condition of a tree sparrow during the nestling stage was well correlated to the condition as an adult, and pairs in which the male parent was in poor condition produced chicks in poor condition. Since the breeding success of a pair depended more on the condition of the male, females appear less affected by conditions early in life. Parents with poor provisioning capacity appear to bias offspring survival towards females, and a difference in the early susceptibility may be adaptive.

Changes in birth sex ratios have in some cases been suspected to result from exposure to estrogenic environmental pollutants. This was examined by exposing great- and blue tit embryos to a synthetic estrogen. Although there was a difference in the mortality rate, the difference was present also in the control groups, why this could not be attributed to estrogen exposure.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 42 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 258
Ecology, Hatching success, Embryonic mortality, Sex ratio, Density dependence, Parasites, Ekologi, Passer montanus
urn:nbn:se:uu:diva-7426 (URN)91-554-6765-2 (ISBN)
Public defence
2007-01-12, Zootissalen, Evolutionary Biology Centre, Villavägen 9, 752 36 Uppsala, 10:00
Available from: 2006-12-21 Created: 2006-12-21Bibliographically approved

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