Sex allocation in Savi’s warblers <Emphasis Type="Italic">Locustella luscinioides</Emphasis>: multiple factors affect seasonal trends in brood sex ratios |
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Authors: | Júlio M Neto Bengt Hansson Dennis Hasselquist |
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Institution: | 1.CIBIO/UP-Centro de Investiga??o em Biodiversidade e Recursos Genéticos,Universidade do Porto,Vair?o,Portugal;2.Department of Animal Ecology,Ecology Building,Lund,Sweden |
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Abstract: | Sex allocation theory predicts that whenever the relative fitness of sons and daughters differ, females should invest more
in the sex with the greatest fitness return. In this study, we evaluated the influence of various ecological factors on the
brood sex ratio (BSR) of Savi’s warblers (Locustella luscinioides) across several breeding seasons. There was a slight but significant female production bias at the population level, which
is consistent with the ‘local resource competition’ hypothesis, as the breeding density is very high and females are more
prone to disperse. We found that there was a significant decline in BSR during the breeding season, but no influence of male
size, female size, social status nor extra-pair paternity were detected. The seasonal decline in BSR was further evaluated
by assessing the within- and between-female effects, which indicated that multiple factors were operating simultaneously in
our study population. First, there was a significant within-female decline in BSR, which was consistent with the decline in
female condition due to the reproductive effort associated with multiple brooding (supporting the Trivers and Willard hypothesis).
Second, a significant decline in BSR with the laying date of first clutches of different pairs indicated that male and/or
female qualities are also associated with the seasonal variation in BSR. Finally, a comparison between the sex of the youngest
nestling with the remaining ones did not suggest any bias, indicating that females do not compensate for the increased mortality
of the last nestling (caused by asynchronous hatching) by producing a male from the last laid egg. |
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