# Title of Dataset: Partner retention as a mechanism to reduce sexual conflict over care in a seabird Set of csv files with data used to perform all analyses described in the paper. ## Description of the Data and file structure Each file is linked to one of the 5 hypotheses described in the paper. These data were used for principal component analysis and data derived from there were used in models to test our hypotheses. Below the variables used in each of the 5 analyses is described in detail: **File 1: Courtship PCA** PairID: unique identifier for the breeding pair DisplayRate: courtship displays per hour JulianDateFirstCourtship: Number of days between the first courtship observation in the colony was observed and the onset of focal pair's courtship JulianLayDate: Number of days between 1st April and the date female laid first egg DaysCourtLay: number of days between first courtship of a pair and first egg lay date PairBondDuration: number of years focal pair had breed together as partners prior to current breeding attempt EstablishedPair: binary coding for whether or not a pair had bred together at least once before current breeding attempt MaleAge: age in years of breeding male FemaleAge: age in years of breeding female MeanAge: mean age of male and female combined **File 2: Synchrony PCA impute** PairID: unique identifier for the breeding pair PropOneInc: proportion of incubation photos where a single parent was incubating on the nest PropMidBoutSwitch: number of brief parental swapping on nest as a proportion of all incubation photos taken PropSyncDisplay: proportion of all within-pair courtship displays that were synchronous, as described in paper Prop2BirdSync: proportion of within-pair courtship displays where both partners actively participated and were also synchronous, as described in paper PairBondDuration: number of years focal pair had breed together as partners prior to current breeding attempt EstablishedPair: binary coding for whether or not a pair had bred together at least once before current breeding attempt MaleAge: age in years of breeding male FemaleAge: age in years of breeding female MeanAge: mean age of male and female combined **File 3: Pair investment PCA impute** PairID: unique identifier for the breeding pair WeightedMeanVol: mean egg volume of focal pair, weighted by clutch size (number of eggs) IncShare: Absolute deviation of male contribution to incubation (calculated as proportion of incubation photos where male was incubating eggs) from 0.5. MaleRegurgitation: Number of times a focal male regurgitated for partner as a proportion of all observation hours performed PairBondDuration: number of years focal pair had breed together as partners prior to current breeding attempt EstablishedPair: binary coding for whether or not a pair had bred together at least once before current breeding attempt MaleAge: age in years of breeding male FemaleAge: age in years of breeding female MeanAge: mean age of male and female combined Other variables not used in this paper **File 4: Pair investment PCA impute** BirdID: unique identifier for individual bird PropNBSweeps: number of nest-building observations as a proportion of all the nest-building checks performed since nesting began DefenceRate: number of nest defence displays/total observation hours since nesting began CountALLDisplaysBirdInvolvedIn: number of courtship displays observed for focal individual CountEPDisplay: number of courtship displays by focal individual directed towards individual other than breeding partner PropEPDisplays: proportion of all displays recorded that were directed towards individual other than breeding partner PropParticipatedWP: proportion of all within-pair displays that the focal bird actively participated in ObsHours: total hours a bird was observed for Sex: binary coding for molecular sex (1=male, 0=female PairBondDuration: number of years focal pair had breed together as partners prior to current breeding attempt EstablishedPair: binary coding for whether or not a pair had bred together at least once before current breeding attempt Age: age in years of bird Other variables not directly used in paper **File 5: Pair investment PCA impute** PairID: unique identifier for the breeding pair InvestPC1: first principal component value from PCA on partner investment InvestPC2: second principal component value from PCA on partner investment CommitPC1Mean: first principal component value from PCA on partner commitment CommitPC2Mean: second principal component value from PCA on partner commitment DisplaySync: proportion of pair's courtship displays that were performed synchronously, as described in paper MeanAge: mean age of male and female combined

The costs of caring for offspring are predicted to lead to an evolutionary conflict between parents, where each parent benefits if the other provides the majority of care. However, in many biparental species, breeding partners remain together for multiple breeding attempts and their respective future reproductive fitness prospects are therefore intertwined. Since an increase in current care by a long-term partner reduces that partner’s future investment and longevity, individuals do not automatically only benefit when their partner provides care. We test whether selection will favour individuals that reduce the burden of care falling on long-term partners, thus decreasing evolutionary conflict over parental care. Using a semi-natural, captive colony of black-headed gulls (Chroicocephalus ridibundus), we show that benefits of long-term partner retention can indeed reduce sexual conflict. Long-term partners have less intense courtships and are more behaviourally compatible, and individuals in long-term pairs spare each other’s resources by increasing their own parental investment compared to those with new partners. Lastly, we demonstrate that high partner compatibility in newly-formed pairs can select for commitment to the pair bond. Our results highlight that compatibility benefits of long-term partnerships can increase selection for mate retention and increase parental investment.