DOI: 10.5061/dryad.532j0
title: Data from: Reproductive performance of resident and migrant males, females and pairs in a partially migratory bird

description: 1. Quantifying among-individual variation in life-history strategies, and
 associated variation in reproductive performance and resulting demographic
 structure, is key to understanding and predicting population dynamics and
 life-history evolution. Partial migration, where populations comprise a
 mixture of resident and seasonally-migrant individuals, constitutes a
 dimension of life-history variation that could be associated with
 substantial variation in reproductive performance. However, such variation
 has rarely been quantified due to the challenge of measuring reproduction
 and migration across a sufficient number of seasonally-mobile males and
 females. 2. We used intensive winter (non-breeding season) resightings of
 colour-ringed adult European shags (Phalacrocorax aristotelis) from a
 known breeding colony to identify resident and migrant individuals. We
 tested whether two aspects of annual reproductive performance, brood hatch
 date and breeding success, differed between resident and migrant males,
 females and breeding pairs observed across three consecutive winters and
 breeding seasons. 3. The sex ratios of observed resident and migrant shags
 did not significantly differ from each other or from 1:1, suggesting that
 both sexes are partially migratory and that migration was not sex-biased
 across surveyed areas. 4. Individual resident males and females hatched
 their broods 6 days earlier and fledged 0.2 more chicks per year than
 migrant males and females on average. Resident individuals of both sexes
 therefore had higher breeding success than migrants. 5. Hatch date and
 breeding success also varied with a pair’s joint migratory strategy such
 that resident-resident pairs hatched their broods 12 days earlier than
 migrant-migrant pairs, and fledged 0.7 more chicks per year on average.
 However, there was no evidence of assortative pairing with respect to
 migratory strategy: observed frequencies of migrant-migrant and
 resident-resident pairs did not differ from those expected given random
 pairing. 6. These data demonstrate substantial variation in two key
 aspects of reproductive performance associated with the migratory
 strategies of males, females and breeding pairs within a partially
 migratory population. These patterns could reflect direct and/or indirect
 mechanisms, but imply that individual variation in migratory strategy, and
 variation in pairing among residents and migrants, could influence
 selection on migration and drive complex population and evolutionary
 dynamics.
description: Individual reproductive
 performance and migratory strategy dataThese breeding data are
 part of the long-term monitoring of the European shag population on the
 Isle of May by the Centre for Ecology & Hydrology. The winter
 colour-ring resighting data were collected by staff and volunteers across
 the UK.Shag_Resighting_Data_Grist.txtPair reproductive performance and migratory strategyThe breeding data were collected as part of a long-term monitoring study on the Isle of May by the Centre for Ecology & Hydrology. The winter colour-ring resighting data were collected by staff and volunteers across the UK.Shag_Pair_Data_Grist.txt