DOI: 10.5061/dryad.m63xsj44s
title: Disentangling the causes of age-assortative mating in bird populations with contrasting life-history strategies

description: 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
 assortment between years. However, we demonstrate that the drivers of
 age-assortment differ between the species, as expected from their
 life-histories and resultant demographic differences. In great tits, pair
 fidelity has a weak effect on age-assortative mating through pair
 retention; variation in age-assortment is primarily driven by fluctuations
 in age-structure from variable juvenile recruitment. Age-assortative
 mating is therefore largely passive, with no evidence consistent with
 active age-related mate selection. In mute swans, age-assortment is partly
 explained by pair retention, but not population age-structure, and
 evidence exists for active age-assortative pairing. This difference is
 likely to result from shorter life-spans in great tits compared to mute
 swans, leading to fundamental differences in their population
 age-structure, whereby a larger proportion of great tit populations
 consist of a single age-cohort. In mute swans, age-assortative pairing
 through mate selection may also be driven by greater age-dependent
 variation in fitness. The study highlights the importance of considering
 how different life-histories, and demographic differences arising from
 these, affect population processes that appear congruent across species.
 We suggest that future research should focus on uncovering the proximate
 mechanisms that lead to variation in active age-assortative mate selection
 (as seen in mute swans); and the consequences of variation in
 age-structure on the ecological and social functioning of wild
 populations.
description: <strong>Data Collection</strong>
 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, since 1989, a roundup has occurred every 2-years after the
 breeding season, where all birds in the colony are captured and marked,
 thus identifying non-breeding individuals present in the population. Age
 is based either on the date of hatching for resident birds or colouration
 of plumage and beak for immigrant birds, which can discriminate between
 birds of 1-, 2- and 3-years or older. Most breeding individuals hatched in
 the swannery (80% males and 93% females), and therefore age is known.
 Additionally, of the 13% that immigrate and breed in the population, 57%
 do so at an age when they can still be aged accurately.
 Data collection from both populations adhered to local guidelines
 for the use of animals in research, and all fieldworkers involved over the
 many years of fieldwork have held BTO ringing licenses (author BTO permit
 number: C / 6987). Additionally, the long-term monitoring in Wytham has
 been subject to review by the Department of Zoology ethics
 committee. <strong>Statistical
 Analyses</strong> We calculated the correlation of age in mates
 using Spearman’s rank correlation as a nonparametric test of association
 that calculates the direction and strength of correlation in partner
 ages. We used permutation analysis to simulate random
 pairing. We compared the
 observed number of pairs assorted by age against a distribution expected
 from random pairing with respect to age. We created this distribution
 through permutation analysis by randomly assigning partners together,
 depending on the birds breeding in a given year, and calculating the
 frequency of absolute age-differences in these randomly-paired
 populations. This was repeated 1000 times. From these 1000 iterations, we
 constructed a frequency distribution of the number of age-assorted pairs,
 against which we compared our observed number of age-assorted pairs to
 evaluate whether the number of pairs assorted by age would be expected
 through random pairing. We evaluated whether age-assortative mating is
 age-dependent and driven by assortative pairing within certain
 age-cohorts. We ran a Spearman’s rank correlation between the within-year
 proportion of ‘first-time breeders’ and the proportion
 assorted by age in both species. We also ran a Spearman's rank
 between the within-year proportion of ‘young breeders’ and the proportion
 assorted by age in both species.
 Further, we used permutation analysis to assess whether the relationship
 we see between these two variables would exist if the population were to
 pair randomly. We assigned partners randomly, depending on those breeding
 in the given year, and calculated the relationship between the proportion
 of young breeders and age-assorted pairs. We repeated this 1000 times,
 yielding a distribution of 1000 Spearman’s rank correlation coefficients
 when within-year pairing is random.
 We used
 Spearman’s rank correlation to assess the relationship between the
 within-year proportion of the population that consists of
 'newly-formed experienced pairs', and the proportion of the
 population assorted by age.
description: 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)