Reproductive senescence and mating tactic interact and conflict to drive reproductive success in a passerine

An understanding of the drivers of individual fitness is a fundamental component of evolutionary ecology and life-history theory. Reproductive senescence, mate and mating tactic choice, and latent heterogeneity in individual quality interact to affect individual fitness. We sought to disentangle the effects of these fitness drivers, where longitudinal data are required to understand their respective impacts. We used reproductive allocation and success data from a long-term (1989-2018) study of white-throated dippers (Cinclus cinclus) in Switzerland to simultaneously examine the effects of female and male age, mating tactic, nest initiation date, and individual heterogeneity on reproductive performance. We modeled quadratic and categorical effects of age on reproductive parameters. The probability of polygyny increased with age in both sexes before declining in older age classes. Similarly, hatching probability in monogamous pairs and the number of nestlings hatched in both monogamous and polygynous pairs increased with female age before declining later in life. As predicted, offspring survival in monogamous pairs increased with male age before declining in older age classes, but male age had no effect on offspring survival in polygynous nesting attempts. Our results demonstrate that parental age, mating tactic, and individual heterogeneity all affect reproductive success, and that the impacts of senescent decline are expressed across different demographic components as a function of sex-specific senescent decline and mating tactic.

Methods
Demographic research on dippers was conducted at three small to medium-sized drainages (Ku¨snacht, Sihl, and Wehrenbach) near the city of Zurich, Switzerland. For 30 years (1989-2018) breeding dippers (30-70 pairs/yr) were marked with metal rings and plastic color markers and associated with nests through resighting of marked individuals and their behavior. Nests were located by carefully searching streams flowing into the northern portion of Lake Zurich from February to June. The mating tactic for each nest was determined through observation of male and female behavior. Previous research has demonstrated low extra-pair paternity in this (<0.01; Becker et al. 2015) and other (<0.02; Øigarden et al. 2010) dipper populations. Reproductive attempts were monitored to cessation, and observers recorded whether or not a nest hatched offspring (γ) and the number of hatchlings from each nest conditional on success (ξ). Nestlings were also marked with metal rings and plastic color markers prior to fledging at seven to 14 days of age. In the majority of years, all fledged offspring were marked across the three drainages. Offspring were monitored and reencountered as philopatric sub-adults and adults, allowing for the estimation of local recruitment probability (ψ). See Hegelbach (2001, 2004) and Becker et al. (2015) for further details regarding data collection.