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Extra-pair copulations (EPCs) can create or intensify sexual selection and, provided that fertilisation success is related to phenotypic traits, help explain sexual dimorphism in socially monogamous species. Previous experimental manipulations of the ornamental coloration in male bluethroats, Luscinia s. svecica, have shown effects on their social mating success, mate-guarding behaviour, and within-pair- and extra-pair paternity. This study investigates the relationship between male characteristics (reflectance of the blue throat feathers, width of the chestnut breast band, wing length, body condition and age) and fertilisation success under natural, non-experimental conditions. Combining three breeding seasons, 29% of 720 offspring were sired by extra-pair males and 54% of 136 nests contained one or more extra-pair offspring. The chroma (spectral purity) of the blue throat feathers and the width of the chestnut band were positively related to paternity in own nest, and for blue chroma this translated into a significantly positive relation with total fertilisation success. This suggests that differential within-pair paternity success exerts directional selection on the colour signal. None of the throat colour measures or morphological traits were significantly related to overall extra-pair fertilisation (EPF) success. However, restricting the analysis to males with one or more EPFs, there was a positive relation between amount of extra-pair paternity and blue chroma. Old males were more successful than young ones in achieving EPFs. Pairwise comparisons showed no plumage differences between cuckolded males and the males that cuckolded them. The absence of phenotypic correlates of male EPC-success agrees with our recent finding that females improve offspring quality through individual choice of EPC partners with 'compatible genes' rather than 'good genes' in an absolute sense. Our results indicate that experiments where traits are manipulated outside the natural range should be interpreted with caution, and illustrate the importance of a dual approach (experimental and correlative) in studies of sexual selection in the wild.


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