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unpublished).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unpublished), or to an intrinsic difference in stress responses between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.