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Factors Affecting the Breeding Success of the Mute Swan Cygnus olor
Mike Birkhead, Philip J. Bacon and Peter Walter
Journal of Animal Ecology
Vol. 52, No. 3 (Oct., 1983), pp. 727-741
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/4450
Page Count: 15
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(1) This paper examines 5 years' data on the breeding biology of the mute swan Cygnus olor Gmelin., to determine what factors affect our measures of breeding success, laying date, clutch size and the number of cygnets fledged. (2) These measures of breeding success (the response variables) were examined against a number of predictor variables such as age, genotype, river types, year and mean winter temperature. (3) The data were examined in two ways. In preliminary analyses each of the response variables was tested separately against a number of relevant predictor variables. Secondly, the predictor variables were tested simultaneously against each of the response variables. (4) As adult survival was high and the same birds nested on the same territories in successive years, it was statistically invalid to combine data from all the years and then use standard linear regression. We present an alternative method for analysing data of this type. (5) Taking all the results of the restricted maximum likelihood estimations together, a causal chain was produced where; (a) female genotype, winter temperature and a pair effect significantly affected the date of laying; (b) date of laying and a pair effect significantly affected clutch size and (c) clutch size was the most significant factor determining the number of cygnets fledged. (6) Finally we considered our results in relation to a model which tries to explain the proximate regulation of laying date and clutch size.
Journal of Animal Ecology © 1983 British Ecological Society