Abstract
In partially migratory populations, individuals from a single breeding area experience a range of environments during the non-breeding season. If individuals show high within- and among- year fidelity to specific locations, any annual environmental effect on individual life histories could be reinforced, causing substantial demographic heterogeneity. Quantifying within- and among- individual variation and repeatability in non-breeding season location is therefore key to predicting broad-scale environmental impacts on the dynamics of partially migratory populations. We used field resightings of colour-ringed adult European shags known to have bred on the Isle of May, Scotland, to quantify individual variation and repeatability in winter location within and among three consecutive winters. In total, 3797 resightings of 882 individuals were recorded over 622 km of coastline, including the Isle of May. These individuals comprised over 50% of the known breeding population, and encompassed representative distributions of ages and sexes. The distances from the Isle of May at which individuals were resighted during winter varied substantially, up to 486 km and 136 km north and south respectively and including the breeding colony on the Isle of May. However, resighting distances were highly repeatable within individuals; within- and among-winter repeatabilities were >0.72 and >0.59 respectively across the full September-March observation period, and >0.95 and >0.79 respectively across more restricted mid-winter periods. Repeatability did not differ significantly between males and females or among different age classes, either within or among winters. These data demonstrate that the focal shag population is partially migratory, and moreover that individuals show highly repeatable variation in winter location and hence migration strategy across consecutive winters. Such high among-individual variation and within-individual repeatability, both within and among winters, could lead to substantial life history variation, and therefore influence population dynamics and future conservation management strategies.
Highlights
It is increasingly recognised that demographic heterogeneity, stemming from temporal and spatial variation in individual lifehistories, can substantially influence population dynamics [1,2,3]
Winter resighting data A total of 313 positive survey days were distributed across all main survey sites in all months during all three winters (Figure S1), the number of positive survey days varied among years (Table 1) and months (Figure S1)
There was no systematic bias in the sexes or ages of individual shags that were resighted in winter compared to the observed Isle of May breeding population the preceding summer
Summary
It is increasingly recognised that demographic heterogeneity, stemming from temporal and spatial variation in individual lifehistories, can substantially influence population dynamics [1,2,3]. The growth of stochastically-dispersing seeds depends on the physical characteristics of colonised sites [6] and small-scale environmental variation affects individual reproductive success in vertebrates [7,8,9]. Such environmental effects can extend beyond immediate life-history components and have carry-over [10] or delayed effects [11] on future reproductive success and survival, with further consequences for population dynamics [5,12,13]. One critical step towards understanding patterns of life-history and demographic heterogeneity and their population dynamic consequences is to quantify variation in the locations occupied by population members, and by extension the environments experienced
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