Introduction

Differential migration, whereby members of the same species differ in migratory behaviour, be it in timings, migratory routes, stop-over sites or non-breeding areas, has been increasingly reported with the widening applicability of tracking technologies (McKinnon & Love 2018). This diversity in migratory strategy within species and populations could impact animal population survivability under the environmental changes and unpredictability brought about by the climate crisis. Species with wider non-breeding ranges and/or partially migratory populations appear to be less susceptible to the effects of environmental change due to the capacity for faster adaptability and/or less acute risks experienced by a portion of the population (Gilroyet al. 2016; Briedis & Bauer 2018). As a result, investigating the causes of differential migration may help us understand implications to population survivability in a changing world.
Over the past few decades, inter-individual variation in temperament has been recognised as a major driver of population ecology; individual responses in exploratory behaviour, boldness, and aggression, among others, have been related to each other, to survival and fitness, and explained in terms of evolutionary stability (Dingemanse & Réale 2005; Réale et al. 2007; Nilsson et al. 2010; Chapman et al. 2011; Wolf & Weissing 2012). Despite the ubiquity of partial migration in the animal kingdom and our increasing ability to track animal movements, investigations into temperamental correlates of migratory tendency are few and the subject remains ill explored (Nilssonet al. 2010; Chapman et al. 2011; Found & St. Clair 2017; Odermatt et al.2017).
The Cory’s Shearwater (Calonectris borealis ) is a long-lived, monogamous, migratory seabird in which males are partial migrants. Despite an apparent flexibility in over-winter movements and destinations, individuals are largely faithful to a particular non-breeding area (Diaset al. 2011). Migratory males return to the colony showing signs of higher overwinter stress levels than residents, as assessed by feather corticosterone (Pérezet al. 2016) and tail feather fault bar intensity (M. Gatt, unpublished). This difference could either be attributed to harsher environmental conditions experienced at distant non-breeding areas, although evidence of carry-over effects from this is absent (M. Gatt, unpublished), or to an intrinsic difference in stress responses between migrants and residents (Nilssonet al. 2014).
To date, there has been some research relating temperament to foraging movements in seabirds (Black-browed Albatross Thalassarche melanophris : Patrick & Weimerskirch 2014; Kittiwake Rissa tridactyla : Harris et al. 2019; Cory’s Shearwater Calonectris borealis : Krüger et al. 2019). The general indication is that bold individuals are more superficial explorers, less flexible in their foraging movements, and fare better in conditions with high competition and high predictability. Similar behavioural traits have been seen in migratory individuals of Roach (Rutilus rutilus ) and Blue Tits (Cyanistes caeruleus ) (Nilssonet al. 2010; Chapman et al.2011). It is not yet known whether this trend persists in partially migratory seabirds.
Here we explored whether migratory strategy is linked to behavioural traits in the Cory’s Shearwater. We assessed exploration of a novel environment and response to extraction from the nest in males, the partially migratory sex, during the pre-laying and incubation periods, and tracked their overwinter movements using GLS loggers. Based on the current scientific knowledge, and given the predominant medium-term, individual persistence in migratory strategy (Diaset al. 2013; Perez et al. 2014), we expected migrants and residents to differ in their temperament, reflecting evolutionary correlates between migratory strategy and the behavioural adaptations required in the different environmental and biotic pressures experienced. In support of this hypothesis, we found that migrant and resident Cory’s Shearwaters differed in their response to extraction from the nest. Together with previous findings that migrants display higher physiological stress over winter, this suggests that migrants and residents may be distinguishable by their stress threshold.