Among-individual and within-individual variation in seasonal migration covaries with subsequent reproductive success in a partially migratory bird

Abstract

Within-individual and among-individual variation in expression of key environmentally sensitive traits, and associated variation in fitness components occurring within and between years, determine the extents of phenotypic plasticity and selection and shape population responses to changing environments. Reversible seasonal migration is one key trait that directly mediates spatial escape from seasonally deteriorating environments, causing spatio-seasonal population dynamics. Yet, within-individual and among-individual variation in seasonal migration versus residence, and dynamic associations with subsequent reproductive success, have not been fully quantified. We used novel capture-mark-recapture mixture models to assign individual European shags ( Phalacrocorax aristotelis ) to ‘resident’, ‘early migrant’, or ‘late migrant’ strategies in two consecutive years, using year-round local resightings. We demonstrate substantial among-individual variation in strategy within years, and directional within-individual change between years. Furthermore, subsequent reproductive success varied substantially among strategies, and relationships differed between years; residents and late migrants had highest success in the 2 years, respectively, matching the years in which these strategies were most frequently expressed. These results imply that migratory strategies can experience fluctuating reproductive selection, and that flexible expression of migration can be partially aligned with reproductive outcomes. Plastic seasonal migration could then potentially contribute to adaptive population responses to currently changing forms of environmental seasonality.

Episodes of opposing survival and reproductive selection cause strong fluctuating selection on seasonal migration versus residence

“…However, such effects will also depend on the degree of phenotypic plasticity following selection episodes, and on forms of selection acting before recruitment, which remain to be quantified in our system. Moreover, while our focus on individual full-winter migration versus residence captures considerable annual phenotypic and fitness variation, subtle forms of selection might act on the full diversity of possible phenotypic sequences of seasonal residence versus migration [24,48] and on the joint phenotypes of breeding pairs [31]. Future analyses with further data and methodological developments, including analyses of paired capture-resighting histories, will allow us to quantify these components and thereby reveal the full form and consequences of varying and fluctuating selection on liability for migration.…”

Variation in winter site fidelity within and among individuals influences movement behavior in a partially migratory ungulate

“…For some species, exposure to predation risk, hunting pressure, pathogens, and anthropogenic or natural barriers may be greater for migrants than resident individuals [ 8 , 11 , 12 ]. This balance of costs and benefits can influence fitness rates of animals [ 13 , 14 ], leading to tradeoffs in migratory versus residency behavior.…”

“…Even within partially migratory populations, simple classification into migrant and resident behavior is complicated by a wide array of migration behaviors, such as variability in the distance of migration [ 11 ] and decisions about when, where, and whether to migrate [ 18 ]. These can lead to fitness differences among individuals that adopt different strategies [ 11 , 13 ]. Fluctuating environmental conditions may further influence behavioral choices and their relative fitness impacts [ 13 ].…”

“…These can lead to fitness differences among individuals that adopt different strategies [ 11 , 13 ]. Fluctuating environmental conditions may further influence behavioral choices and their relative fitness impacts [ 13 ]. For example, conditional migration has been reported in some ungulate species, in which environmental conditions such as winter severity in a given year influence the likelihood that individuals migrate [ 19 ].…”

“…For example, conditional migration has been reported in some ungulate species, in which environmental conditions such as winter severity in a given year influence the likelihood that individuals migrate [ 19 ]. Such factors can result in changes in the prevalence of different behaviors within partially migratory populations [ 13 ].…”