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Offspring sex ratio variation in relation to brood size and mortality in a promiscuous species: the Aquatic Warbler Acrocephalus paludicola
Author(s) -
Dyrcz Andrzej,
SauerGürth Hedi,
Tkadlec Emil,
Wink Michael
Publication year - 2004
Publication title -
ibis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.933
H-Index - 80
eISSN - 1474-919X
pISSN - 0019-1019
DOI - 10.1046/j.1474-919x.2004.00256.x
Subject(s) - acrocephalus , sex ratio , biology , brood , sex allocation , offspring , zoology , ecology , juvenile , demography , population , pregnancy , genetics , sociology
We analyse nestling sex ratio variation in the Aquatic Warbler Acrocephalus paludicola to test for predictions from sex allocation theory that the brood sex ratio is close to parity. We also tested Fiala's (1980) prediction that there is no difference in sex ratio between broods affected and not affected by mortality, and whether a shift in primary sex ratios or simple differential mortality by sex underlies that difference. Furthermore, we explore additional analytical possibilities for inferring proximate mechanisms through simulation modelling. In the Aquatic Warbler, which is promiscuous, the overall sex ratio determined by molecular sexing of nestlings at 8–11 days of age did not deviate significantly from parity (proportion of females 0.509), nor did we find any predictive effect of brood size, maternal body mass, fat condition, wing and bill length, laying date, mean daily temperature, and multiple‐male mating. However, extensive simulation suggested that the whole pattern of sex ratio variation is unlikely to arise purely by chance: (1) there is a diverging sex ratio between complete and partial broods, (2) large broods tended to be female‐biased and small broods male‐biased, and (3) low ambient temperature prior to the laying period seemed to increase the proportion of female offspring in complete broods. We conclude that most variation in nestling sex ratio is non‐adaptive in nature, and results from variation in female nestlings mortality dependent on brood size and sex ratio.

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