Temporal dynamics of selection on early-life phenotypic plasticity in seasonal migration versus residence

In this study, we quantified selection on early-life plasticity in the ecologically critical trait of seasonal migration versus residence, by fitting a novel multi-state model to spatio-seasonal resighting data from 13 newly-fledged cohorts of partially migratory European shags (Gulosus aristotelis).

To obtain the required data, during the 2010-2022 breeding seasons (April-August; 13 cohorts), all breeding attempts on IoM were monitored, and >95% of fledglings were marked with uniquely coded metal and colour rings. During the 2010-2024 non-breeding seasons, we undertook regular (approximately biweekly) resighting surveys on IoM (and adjacent day roosts) to detect current residents, and at core roost sites spanning the north-east UK coast (predominantly ca. 100-500 km from IoM) to detect current migrants.

We formulated individual encounter histories as occasion-specific summaries of resightings of 10,788 colour-ringed shags fledged during 2010-2022.

We defined five primary temporal ‘occasions’ spanning the natal breeding season (June-July, when chicks are typically ringed before fledging) to the following March (~8 months post-fledging, Figure 2), and an 'ever after' occasion.

The model outputs, i.e. posterior samples of the model parameters and derived parameters, are the primary results of our analyses.

Methods
Data collection

A partially migratory shag population breeding on Isle of May National Nature Reserve (hereafter ‘IoM’, Scotland, 56°11′N, 2°33′W) provides a highly relevant and tractable system to quantify the temporal dynamics of survival selection on early-life plasticity in migration versus residence.

To obtain the required data, during the 2010-2022 breeding seasons (April-August; 13 cohorts), all breeding attempts on IoM were monitored, and >95% of fledglings were marked with uniquely coded metal and colour rings, field-readable from ≤150m with a telescope (533–1064 ringed individuals/year, mean=818).

Since shags return to shore daily, marked individuals can be observed at coastal roost sites throughout the year, allowing direct observation of individuals’ current locations, and hence current resident or migrant status. 

Accordingly, during the 2010-2024 non-breeding seasons, we undertook regular (approximately biweekly) resighting surveys on IoM (and adjacent day roosts) to detect current residents (defined as individuals roosting on IoM at night), and at core roost sites spanning the north-east UK coast (predominantly ca. 100-500 km from IoM) to detect current migrants (defined as not returning to IoM at night; ESM A1). These migrant sites are reachable within 1-2 days by juvenile shags, and encompass their main winter range. Ad hoc resightings at other sites (spanning ca. 800km) were also collected, including citizen science contributions (ESM A1).

Overall, this generated a dataset of 32,376 first-year resightings spanning 10,788 colour-ringed shags fledged during 2010-2022.

Model analyses

The Bayesian MS-CMR model was implemented in Stan v. 2.26.1, using package rstan v. 2.26.13 in R 4.2.2. We ran n=4 chains each comprising 1,000 warm-up and 2,000 monitored iterations, yielding 8,000 posterior samples for inference.