Who wears the pants in a mute swan pair? Deciphering the effects of male and female age and identity on breeding success.

Traditionally, many breeding traits (e.g. the timing and size of clutches) were considered to be female-only traits in that males played little-to-no role in their expression. Although the contribution of males to such breeding traits, as well as other aspects of reproduction, is increasingly recognized, few studies have demonstrated the effects of male age and life history on breeding traits and, importantly, whether these effects are underlined by additive-genetic variation. Here, we take advantage of a long-term data set on mute swans (Cygnus olor) to demonstrate that the ages of both the male and female parents play significant roles in the timing and size of clutches, although recruitment success did not show similar effects. Individual males varied significantly in their influence on the timing of egg laying. We decomposed this variation using an 'animal model'; competing models that were the source of this variation as additive-genetic or permanent-environmental variation was not statistically distinguishable. Our results add to the growing evidence that reproductive performance should be considered as a product of the identity and condition of both parents.