Greenfinch plumage varies both between and within age and sex classes. This study looked at the possible causes and consequences of this variation in a colour-ringed population. Plumage colour was both repeatable and heritable. The function of colourful plumage in the breeding season was reviewed. Many aspects of the breeding biology of Greenfinches were studied and the effects of plumage on breeding success investigated. Brightly coloured birds seemed to have greater reproductive success than dull ones. Brightly coloured males were also more likely to ret urn to the study area in the following breeding season. Greenfinches are usually regarded as monogamous, but I found that over 25% of nests involved polygamy. Polygyny, polyandry and possible cases of polygynandry were recorded, but polygyny was the most common of the three. It was demonstrated that the experimental provision of food influenced the occurrence of polygyny. The literature generally considers polygyny to be bad for females, however in Greenfinches polygynous pairs were as successful at producing independent offspring as monogamous pairs. Polygynous birds recruited more offspring into the local population than monogamous birds, although this may reflect differences in dispersal. Since polygynous males were bright and had better survival and since colour was found to be heritable, females may have been choosing males for their good genes. If colour is an honest signal, there must be some cost preventing dull birds from becoming bright. Bright Greenfinches were more likely to be killed by Sparrowhawks during the summer than dull Greenfinches and they were also more likely to be injured. In comparison, dull birds were more likely to be killed by Tawny Owls in the winter. Whether Greenfinch plumage variation acted as a "badge of status" over the winter was investigated. The brighter a Greenfinch's plumage the more likely it was to win confrontations at a bird table in the winter, regardless of food type (contra Maynard Smith & Harper 1988). So what influenced a Greenfinch's plumage? Birds with damaged feathers only regrew bright plumage if they were in good body condition. Birds with low fat stores regrew paler feathers after damage, which is possibly related to the fact that the carotenoid pigment is stored in fat. Therefore, it is possible that after the breeding season good quality nales recovered faster, put on more fat and acquired brighter plumage in the moult. Plumage variation in the House Sparrow was also investigated. In hand estimates of bib size were correlated with spectrophotometric estimates of melanin content. Bib size was not related to organ size. The results of this study are compared with the literature on status signalling . It is argued that badges are handicaps i.e. uncheatable signals of individual quality, rather than being arbitrary signals or signalling Resource Holding Potential.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:334915 |
| Date | January 1991 |
| Creators | Eley, Caroline C. |
| Publisher | University of Sussex |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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