Relationships between phenotype and fertilization success in male barn swallows.

Phenotypic variables were: testosterone concentration, ventral plumage colouration and length of the longest tail feather. Because testosterone concentration was correlated with plumage colour, the results of two seperate analyses are presented: one including the variables ‘testosterone concentration’ and ‘tail length’, and one including ‘plumage colouration’ and ‘tail length’. Fertilization success was split into i) cuckoldry rate, i.e. the proportion of extra-pair young in a male's own brood, ii) extra-pair fertilization success (EPF), and iii) total fertilization success, i.e. the total number of offspring produced in the entire breeding season. Extra-pair fertilization success (the total number of EPY gained in the study population in the entire breeding season) was analyzed in two steps; first using a binomial error distribution, and second using a normal error distribution, including only those males that gained extra-pair young (see Methods for rationale). The variance of the random factor ‘Barn’ was not significant in any model (all p>0.05). The variance of the random factor ‘Male’ was significant in all models (all p<0.001). Summaries derived from the mixed modelling procedure in MLwiN. All df = 1.