DOI: 10.5061/dryad.ffbg79d2t
title: Data from: Carry-over effects of seasonal migration on reproductive success through breeding site retention in a partially migratory bird

description: Understanding the maintenance and dynamics of phenotypic polymorphisms
 requires unpicking key ecological mechanisms shaping the fitness costs and
 benefits of expressing alternative phenotypes, generating selection.
 Seasonal migration versus year-round residence expressed in partially
 migratory populations represents one common polymorphism that can
 experience strong selection through differential reproductive success.
 Yet, key hypothesised pathways that could generate such selection remain
 to be empirically tested. One hypothesis is that migratory tactics affect
 subsequent reproductive success through carry-over effects on breeding
 site retention and resulting breeding dispersal. By remaining in breeding
 areas all year round, residents could retain their preferred breeding site
 between years, and consequently have higher reproductive success.
 Conversely, migrants that escape harsh non-breeding season conditions
 could return in better condition, with high resource holding potential,
 and outcompete residents to retain their site. Such effects could further
 depend on migration timing and vary between years. Yet, such pathways have
 not been quantified, precluding empirical parameterisation of partial
 migration theory.  We used four years of breeding and
 non-breeding season data from partially migratory European shags (Gulosus
 aristotelis) to test whether the three most frequent migratory tactics in
 this population (full resident, early migrant departing soon after
 breeding, and late migrant departing in late autumn) differed in their
 breeding site retention; whether site retention predicted reproductive
 success; and hence whether effects of migratory tactic on reproductive
 success were explicable through site retention. Overall, residents were
 much more likely to retain their breeding site between years than both
 early and late migrants, and site retention was associated with increased
 reproductive success. Yet, these effects varied somewhat among years: late
 migrants were always least likely to retain their site but had variable
 relative reproductive success. Path analyses revealed that effects of
 migratory tactic on reproductive success were only partly attributable to
 breeding site retention. These results indicate that multiple mechanisms
 underlie reproductive selection on migratory tactics, potentially
 contributing to maintaining behavioural polymorphisms. Yet, the clear
 associations between migratory tactics and local breeding dispersal reveal
 that these movements can be strongly interlinked across seasons, shaping
 overall spatio-seasonal dynamics in partially migratory systems.
description: This data consists in observational data that have been collected
 in the field based on bird rings. Migratory tactic estimation are based on
 a modelling approach, as detailed in the article. 
description: # Data from: Carry-over effects of seasonal migration on reproductive
 success through breeding site retention in a partially migratory bird
 [https://doi.org/10.5061/dryad.ffbg79d2t](https://doi.org/10.5061/dryad.ffbg79d2t) These datasets were used to study the link between migratory tactic, dispersal behaviour and reproductive success in a partially migratory seabird, the European shag. This work is published in the Journal of Animal Ecology. Thanks to observations of ringed shags along the coast of Scotland year-round, we had access to nesting and non-breeding location of many ringed birds. ## Description of the data and file structure The data consists in two files: "migratory_tactic_2017-2021.csv" and "dispersal_2018-2021.csv". We describe below the meaning of each column. All missing data are coded as *NA*. ### migratory\_tactic\_2017-2021.csv This file is set up at the non-breeding season level, providing migratory tactic estimation, individual age, and for most, reproductive success and breeding site retention after this non-breeding season. ##### General data:  * **season**: season over wich migratory tactic was assessed. [*2017-2018, 2018-2019, 2019-2020, 2020-2021*]. * **ind_ID**: unique identifier of the individual (colour ring combination). * **sex**: [*M]* for males, [*F*] for females. * **ring.age**: whether the bird was ringed as an adult [*A*] or as a chick [*P*]. * **age**: age of the individual (in years) during the breeding season following the estimation of migratory tactic (e.g. in 2018 for the season 2017-2018). This age was known precisely for individuals ringed as chicks, but was estimated for individuals ringed as adults (considering a minimum age at first breeding of 3 years; see main text for the article).  * **age.category**: age as a categorical factor: [*old*] if &gt;= 5 years, [*young*] if &lt;= 4 years. Individuals ringed as adult, for which this assignement was uncertain in the first two years following ringing, were excluded from analyses and are therefore not in this dataset. ##### Data related to the migratory tactic during the non-breeding season: Then comes the individual’s simplex of class propensities (in %). Classes are resident (R), early migrant (EM), and late migrant (LM). * **prop.R**: propensity to be classified as Resident. * **prop.EM**: propensity to be classified as an Early Migrant.  * **prop.LM**: propensity to be classified as a Late Migrant.  * **migr.tactic**: tactic ([*R*] for Resident, [*EM*] for Early Migrants, and [*LM*] for Late Migrants) wich obtained the higher percentage in the three preceeding colums prop.R, prop.EM and prop.LM.  ##### Data related to the breeding season following migratory tactic assignment (reproductive success and dispersal): * **breeding_ID**: unique identifier of the breeding attempt in the breeding season following the estimation of migratory tactic. This concatenates the season and the nest ID. It is therefore identical for both pair members during their joint breeding attempt. This is not unique when the precise nest location was actually unknown (*NA*). In that later case, the reproductive success was also unknown and these lines discarded from analyses relating reproductive success, dispersal, ad migratory tactics. These individuals were however kept in the path analyses, when investigating the effect of age on migrator tactic alone (see main text for details). * **repro.success**: reproductive success estimated as the number of fledglings produced in the breeding season following the estimation of migratory tactic.  * **nest.before**: identifier of the nest used in the year preceeding the migratory tactic assessment. Each nest location has a unique identifier and nests are reused over the years. * **nest.after**: identifier of the nest used in the year following the migratory tactic assessment. Each nest location has a unique identifier and nest are reused over the years. * **retain.nest**: whether the individual bred in the same nest in both breeding seasons referred to in the season column. [*0*]= no, [*1*]= yes. * **retain.subsite**: whether the individual bred in the same subsite in both breeding seasons referred to in the season column. [*0*]= no, [*1*]= yes. * **distance.type**: method used to estimate dispersal distance. [*zero*]= the bird retained its nest, distance is therefore zero. [*picture*]: distance estimated from high resolution pictures. [*qgis*]= distances at sea estimated using QGIS and the middle of each subsite as a reference. See Supporting Information of the article for details.  * **dispersal.dist**: estimated dispersal distance (in m) * **dispersal.dist.category**: dispersal distance category as used in the statistical analyses. [*50*]=  from 0 to 50m; [*100*]= from 50 to 100m; [*200*]= from 100 to 200m; [*250*]= from 200 to 250m; [*400*]= from 250 to 400; [*Inf*]= more than 400m.  ### dispersal\_2018-2021.csv This file is set up at the breeding season level, providing dispersal estimation (breeding site retention), individual age, and reproductive success. ##### General data:  * **season**: season over wich dispersal was assessed. [*2018, 2019, 2020, 2021*]. * **ind_ID**: unique identifier of the individual (colour ring combination). * **sex**: [*M]* for males, [*F*] for females. * **ring.age**: whether the bird was ringed as an adult [*A*] or as a chick [*P*]. * **age**: age of the individual (in years) during the breeding season following the migratory tactic assessment (e.g. in 2018 for the season 2017-2018). This age was known precisely for individuals ringed as chicks, but was estimated for individuals ringed as adults (considering a minimum age at first breeding of 3 years; see main text for the article).  * **age.category**: age as a categorical factor: [*old*] if &gt;= 5 years, [*young*] if &lt;= 4 years. Individuals ringed as adult, for which this assignment was uncertain in the first two years following ringing, were excluded from analyses and are therefore not in this dataset. ##### Data related to the breeding season following migratory tactic assignment (reproductive success and dispersal): * **breeding_ID**: unique identifier of the breeding attempt. This concatenates the nest ID and the season. It is therefore identical for both pair members during their joint breeding attempt. * **repro.success**: reproductive success estimated as the number of fledglings produced that season.  * **nest**: identifier of the nest used this season. Each nest location has a unique identifier and nests are reused over the years. * **nest.before**: identifier of the nest used in the preceding season. Each nest location has a unique identifier and nest are reused over the years. * **retain.nest**: whether the individual bred in the same nest in both the preceding and the current season. [*0*]= no, [*1*]= yes. * **retain.subsite**: whether the individual bred in the same subsite in both the preceding and the current season. [*0*]= no, [*1*]= yes.