Natural selection favours those individuals capable of biasing investment in male and female offspring when the reproductive value of each sex differs. One way in which parental investment can be skewed is through altering the sex ratio, i.e. the numbers of sons and daughters produced. In birds, this can potentially be achieved at the egg stage or through subsequent modification of the numbers of male and female offspring post-hatching. I investigate both sex ratio biasing mechanism in <i>Parus major</i> and <i>P. caeruleus</i> breeding on the Swedish island of Gotland. From deserted<i> P. major</i> nests where eggs had been numbered as they were laid, egg sex ratio with laying sequence increased from approximately 0.50 to 0.75. However, sample sizes were small and the effect was not statistically significant when analyses were limited to modal clutch sizes. A cross-species analysis of sex ratio variation with laying sequence suggests that females lay eggs of the larger sex first, whether that is males or females. This relationship is expected to be associated with increased levels of brood reduction. Biasing egg sex ratios in this way might increase the reproductive value of a brood if, by reducing nestling competition through mortality, the quality of remaining offspring is enhanced. There was some evidence that this may only be true for small-brooded species. Female body condition of both <i>P. major</i> and <i>P. caeruleus</i> was experimentally manipulated through heating and cooling nest boxes prior to and during egg laying. There was no evidence of alterations to egg sex ratios in relation to female body condition in either species. After hatching, higher mortality of larger male <i>P. major </i>nestlings in poor nesting conditions is expected to result in female biased sex ratios.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:660158 |
| Date | January 2000 |
| Creators | Oddie, Kate |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/15551 |
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