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higher feather corticosterone concentrations and greater tail feather fault bar intensity (Pérez et al., 2016, M. Gatt, unpublished data). Tail feather fault bar intensity reported in Gatt et al (unpublished) reflected experienced acute stress, such as antagonistic interindividual interactions and inclement weather (Jovani & Rohwer 2016), over the period of tail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.