Why females engage in social polygyny remains an unresolved question in species where the resources provided by
males maximize female fitness. In these systems, the ability of males to access several females, as well as the willingness
of females to mate with an already mated male, and the benefits of this choice, may be constrained by the
socio‑ecological factors experienced at the local scale. Here, we used a 19‑year dataset from an individual‑monitored
population of pied flycatchers (Ficedula hypoleuca) to establish local networks of breeding pairs. Then, we examined
whether the probability of becoming socially polygynous and of mating with an already mated male (thus becoming
a secondary female) is influenced by morphological and sexual traits as proxies of individual quality relative to the
neighbours. We also evaluated whether social polygyny is adaptive for females by examining the effect of females’
mating status (polygamously‑mated vs monogamously‑mated) on direct (number of recruits in a given season) and
indirect (lifetime number of fledglings produced by these recruits) fitness benefits. The phenotypic quality of individuals,
by influencing their breeding asynchrony relative to their neighbours, mediated the probability of being involved
in a polygynous event. Individuals in middle‑age (2–3 years), with large wings and, in the case of males, with conspicuous
sexual traits, started to breed earlier than their neighbours. By breeding locally early, males increased their
chances of becoming polygynous, while females reduced their chances of mating with an already mated male. Our
results suggest that secondary females may compensate the fitness costs, if any, of sharing a mate, since their number
of descendants did not differ from monogamous females. We emphasize the need of accounting for local breeding
settings (ecological, social, spatial, and temporal) and the phenotypic composition of neighbours to understand
individual mating decisions.
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