Fledgling sex ratios in relation to brood size in size-dimorphic altricial birds |
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Authors: | Dijkstra Cor; Daan Serge; Pen Ido |
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Institution: | aZoological Laboratory, University of Groningen PO Box 14, 9750 AA Haren, The Netherlands
bMinistry of Transport and Public Works, Institute for Inland Water Management and Waste Water Treatment (RIZA) PO Box 17, 8200 AA Lelystad, The Netherlands
cDepartment of Genetics, University of Groningen PO Box 14, 9750 AA Haren, The Netherlands |
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Abstract: | In six species of dimorphic raptors (females larger than males)and one passerine (males larger than females), the sex ratioat fledging varied systematically with brood size at fledging.In all species the strongest bias toward the smaller sex wasestablished in the largest as well as the smallest broods; amore even distribution of males and females was observed inbroods of intermediate size. We explored a specific differentialmortality explanation for this sex ratio variation. Our hypothesispostulates that variation in mortality is caused by differencesin food demand between broods of the same size, due to theirsex composition. Data from the marsh harrier Circus aeruginosuson gender-related food demand and overall nestling mortalitywere used to predict the frequency of surviving males and femalesat fledging, assuming an even sex ratio at hatching and randommortality with respect to both sexes within broods. The modelquantitatively fits the marsh harrier data well, especiallyin broods originating from large dutches. Although we anticipatethat other mechanisms are also involved, the results supportthe hypothesis of sex-ratio-dependent mortality, differentialbetween broods, as the process generating the observed brood-sizedependence of fledgling sex ratios in sexually dimorphic birds. |
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Keywords: | brood size marsh harrier sex ratio sex specific mortality sexual dimorphism |
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