Discussion
Our results demonstrate a link between Cory’s Shearwater migratory
strategy and response on extraction from the nest. Links between
migratory strategy and behavioural traits have so far only been explored
in a handful of studies in very few taxa across the animal kingdom, and
only one bird species
(Blue Tit: Nilssonet al. 2010; Roach Rutilus rutilus : Chapman et al.2011; Wild Elk Cervus canadensis : Found & St. Clair 2017;
Hoverfly Episyrphus sps.: Odermatt et al. 2017). This
study provides valuable insights into the causes and consequences of
partial migration from a perspective that is not very well explored,
making within-population comparisons made possible by our large sample
size of simultaneously tracked individuals.
In Cory’s Shearwaters, migrants and residents have also been found to
differ in their physiological stress responses over the non-breeding
period, with migrants showing higher feather corticosterone
concentrations and greater tail feather fault bar intensity
(Pérez
et al., 2016, M. Gatt, unpublished data). Tail feather fault bar
intensity reported in Gatt et al (unpublished) reflected experienced
acute stress, such as antagonistic interindividual interactions and
inclement weather
(Jovani
& Rohwer 2016), over the period of tail feather growth between 2017 and
2018, which coincides with the period of behavioural assessments
presented here. Our current findings, that migrant and resident males
differ in their behavioural reaction to a standardised stressor at the
breeding colony, suggest that the differences in feather corticosterone
and fault bar intensity within the population may be, at least partly, a
result of variation in an endogenous threshold to a stress response
between males of different migratory strategies rather than variation in
exogenous triggers over winter.
The covariation in behavioural and physiological responses to a stressor
has been well explored in animal behaviour studies and described as
individual coping style
(Koolhaaset al. 1999). Most studies report a correlation between low
corticosterone responses to stressors and bold, aggressive, and/or fast
exploring personalities (“proactive” personalities), while high
corticosterone responses are associated with cautious, fearful, and slow
exploring personalities (“reactive” personalities)
(Cockrem
2007).
Our observations appear to contradict these if the greater reactivity on
extraction from the nest is interpreted as aggressiveness forming part
of a “proactive” personality. However, we argue that it is rather low
reactivity to extraction from the nest that reflects greater boldness,
similar to interpretations of Kittiwakes at the nest presented with a
novel object
(Harriset al.2019).
Under this interpretation, higher reactivity towards extraction would be
a sign of heightened stress response and self-defence, rather than nest
defence, and the correlation with higher activity of the parasympathetic
system recorded over winter would be in agreement with previous
literature. The authors have also repeatedly observed that the more
reactive individuals are also more likely to attempt to escape the nest
after handling, supporting this interpretation.
Coping style appears to determine individual prevalence and distribution
under different scenarios
(Koolhaaset al. 1999; Dingemanse et al. 2004; Cockrem 2012).
“Reactive” personalities with high corticosterone responses are more
successful in unpredictable conditions due to their higher plasticity in
behaviour and more thorough exploration. “Proactive” personalities
with low corticosterone responses are less neophobic but explore new
environments more superficially and are more repetitive in their
behaviour
(Benuset al. 1991; Cockrem
2007).
These correlated suites of characteristics have already been seen in the
foraging behaviour of Cory’s Shearwaters
(Krügeret al.2019).
Resilience to competition in bold individuals is also hypothesised to be
an important factor determining foraging movements in seabirds, as
suggested by their closer foraging distributions relative to the colony
(Patrick
& Weimerskirch 2014; Harris et al. 2019; Krüger et al.2019).
If these trends hold for the determination of migratory strategy in
Cory’s Shearwater, we could expect that male Cory’s Shearwaters
remaining resident at the Canary Current experience a more predictable
environment, similar to that experienced during the breeding season, but
perhaps also greater competition than migrants experience.
The novel environment test appears to be assessing different behavioural
traits that are not strongly linked to the patterns discussed above.
Unlike their response to extraction, Cory’s Shearwater’s exploratory
behaviour shows phenotypic plasticity, particularly across annual cycle
stages. The low likelihood to enter an unfamiliar environment during the
pre-laying season may have fitness consequences in accordance with
parental investment theory and nest defence intensity hypotheses
(Montgomerie
& Weatherhead 1988). In the pre-laying period, Cory’s Shearwaters
invest highly in nest defence and intense fights between individuals are
often observed at dense colonies such as on Selvagem Grande. Such fights
can result in injury, and occasionally even death (pers. obs.). Entering
unfamiliar environments may increase the risks of unnecessary fights
resulting in energy loss or physical damage. Additionally, during
incubation birds may experience a perceived urgency to return to the
nest due to increased motivation to provide parental care.
The novel environment test also exposed how young Cory’s Shearwaters,
which recruited as breeding birds during or in the years directly before
this study, emerged into the novel environment faster than older birds.
One hypothesis in the literature that could explain this is that stress
responses are suppressed in experienced breeders at ages at which the
potential for current reproductive success are high, but are not
suppressed in young, inexperienced birds which give greater value to
their immediate survival than their reproductive attempt
(Montgomerie &
Weatherhead 1988; Elliott et al. 2014).
We acknowledge that the timeframe of the analysis presented here is not
representative of the longevity of shearwaters and advise caution when
interpreting the permanence of behavioural type, which may change over
the long term, similar to individual quality
(Catryet al. 1999) and migratory strategy
(Perezet al. 2014). We encourage more long term individual observations
of long-lived species to improve our understanding of the role of
temperament in population ecology. The assessment of behaviour on
extraction from the nest may be a simple, straightforward and
informative measure of stress response for burrow nesting seabirds,
particularly in regularly monitored populations in which individuals
need to be handled for other purposes. However, the interpretation of
birds’ responses requires further investigation in order to be able to
extract more information. In particular, among the assessments which are
not considered here, some birds retreated as a reaction to an
approaching hand rather than staying put or attacking, which resulted in
some confusion as to where such a response would lie in an ordinal list
of categories of increasing reactivity
(Patricket al. 2013). An ability to classify more reactions could help
describe a greater proportion of a population.