Journal
JOURNAL OF EVOLUTIONARY BIOLOGY
Volume 14, Issue 5, Pages 829-834Publisher
BLACKWELL SCIENCE LTD
DOI: 10.1046/j.1420-9101.2001.00320.x
Keywords
differential mortality; early breeding; egg size; parental investment; sex ratio; Sturnus unicolor; spotless starting
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We investigated possible pre-hatching mechanisms of sex-differential investment by females that may contribute to offspring sex-ratio adjustment enhancing the fitness return from reproductive effort in the spotless starling (Sturnus unicolor). We found a seasonal shift in sex ratio from daughters to sons as the season advances. Furthermore, the probability of breeding at 1-year old and recruitment into the breeding population in daughters is associated with laying date but not with mass at fledging. The reverse is true for males which rarely bred at 1-year old. We also found that eggs containing female embryos are significantly heavier than those containing males in spite of the slight sexual dimorphism in favour of males. This suggests maternal control of provisioning, favouring daughters that may balance sibling mortality and competition with their brothers. Our results on seasonal variation in sex ratio and differential ego, provisioning are consistent with an adaptive tactic in which mothers increase their reproductive return by enhancing the probability that daughters survive and breed in their first year of life.
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