Disentangling the causes of age-assortative mating in bird populations with contrasting life-history strategies

Age shapes fundamental processes related to behaviour, survival, and reproduction. Where age influences reproductive success, non-random mating with respect to age can magnify or mitigate such effects. Consequently, the correlation in partners’ age across a population may influence its productivity. Despite widespread evidence for age-assortative mating, little is known about what drives this assortment and its variation. Specifically, the relative importance of active (same-age mate preference) and passive processes (assortment as a consequence of other spatial or temporal effects) in driving age-assortment is not well understood.
In this paper, we compare breeding data from a great tit and mute swan population (51- and 31-year datasets respectively) to tease apart the contributions of pair retention, cohort age-structure, and active age-related mate selection to age-assortment in species with contrasting life-histories.
Both species show age-assortative mating, and variable assortmen..., Data Collection
Data used here are from a long-term study population of great tits in Wytham Woods, Oxford (51°46’N, 1°20’W). This population has been monitored since 1947, where breeding adults and their chicks have been marked with unique British Trust for Ornithology (BTO) rings since the 1960s. All chicks reared in nest-boxes are ringed at 14 days of age, while adults are trapped at boxes during the nestling phase, and identified by ring number or marked with a new ring if they are immigrant birds. Parent age is based on the year of hatching for local birds, or plumage characteristics for immigrants. Although immigration rates are high (53%), most are first caught as yearlings (76%) and therefore can be aged accurately.
We also used data from Abbotsbury Swannery and Chesil Fleet (50°35’N, 2°30’W), where swans have been breeding since at least the fourteenth century. Since 1977, all individuals that breed and hatch at the colony have been marked with a unique BTO ring. Additionally, ..., This includes:
1. Age-assortative_Mating.RData (all dataframes required to run the analyses can be found here)
2. Disentangling_the_causes_of_age-assortative_mating_-_ANALYSIS.R (R code to perform the analyses),