Drivers of within- and among-individual variation in risk-taking behaviour during reproduction in a long-lived bird

Abstract

Plastic and selective mechanisms govern parental investment adjustments to predation threat. We investigated the relative importance of plasticity and selection in risk-taking propensity of incubating female common eiders Somateria mollissima facing unprecedented predation in SW Finland, Baltic Sea. Using a 12-year individual-based dataset, we examined within- and among-individual variation in flight initiation distance (FID), in relation to predation risk, nest detectability, individual traits and reproductive investment ( N FID = 1009; N individual = 559). We expected females nesting in riskier environments (higher predation risk, lower nest concealment) to mitigate environmentally imposed risk by exhibiting longer FIDs, and females investing more in current reproduction (older, in better condition or laying larger clutches) to display shorter FIDs. The target of predation—adult or offspring—affected the mechanisms adapting risk-taking propensity; females plastically increased their FID under higher adult predation risk, while risk-avoiding breeders were predominant on islands with higher nest predation risk. Risk-taking females selected thicker nest cover, consistent with personality-matching habitat choice. Females plastically attenuated their anti-predator response (shorter FIDs) with advancing age, and females in better body condition were more risk-taking, a result explained by selection processes. Future research should consider predator type when investigating the fitness consequences of risk-taking strategies.

Breeding under pressure: Corticosterone is associated with reproductive investment under fluctuating predation risk in a long‐lived sea duck

“…As baseline CORT is also positively associated with reproductive investment and hatching success in the study system (see 4.1), our results therefore suggest that a decrease in reproductive investment is associated with increased predation pressure (Ghalambor & Martin, 2000). This conclusion is further supported by a significant reduction of clutch size over the study period (Mohring et al, 2022), which is consistent with previous experimental work showing that clutch size and reproductive investment are reduced under higher predation risk (Doligez & Clobert, 2003; Zanette et al, 2011). Intriguingly, these decreases in reproductive investment and baseline CORT were not associated with a population‐level decrease in hatching success.…”

“…As long‐lived birds, female eiders are expected to prioritise their own survival at the expense of their current reproductive event (Ghalambor & Martin, 2001; Williams, 1966), resulting in stronger behavioural and physiological responses to adult predation risk. In line with this, adult predation risk, rather than nest predation risk, was associated with plastic adjustments of escape behaviour in eiders (Mohring et al, 2022).…”

“…Furthermore, breeding during years of high predator abundance or under higher island-specific adult predation risk was associated with an individually plastic decrease in female baseline CORT levels, although this latter effect was only evident on open islands. Besides predation risk, female body condition at hatching has concurrently increased in this population (Mohring et al, 2022;Öst et al, 2022), but the fact that body condition was unrelated to baseline CORT levels (this study) makes confounding by temporal trends in body condition unlikely. Food availability is another potential confounding factor, as it may affect allostatic load and hence baseline CORT levels (reviewed in Busch & Hayward, 2009).…”

“…Accordingly, there is ample experimental and theoretical evidence that reproductive performance increases with age and experience before declining at very old ages (e.g. in eiders; Mohring et al, 2022; Noreikienė et al, 2021). To our knowledge, our study provides the first evidence of a within‐individual decrease in baseline CORT levels at old age (but see Lendvai et al (2015) for an individual‐specific decrease in the CORT stress response with age), supporting the idea that CORT can mirror reproductive senescence in wild animals.…”

“…Actively incubated, depredated and already hatched nests during female monitoring were defined as breeding attempts. Because nest and adult predation are distinct processes that can trigger different antipredator responses (Hua et al, 2014; Mohring et al, 2022), we also calculated an island‐specific nest predation risk index. For each island and year, island‐specific nest predation risk index was calculated by dividing the number of failed breeding attempts on a given year and island by the total number of breeding attempts for which nest fate was known, on this island and during this year (4156 known nest fates out of 4501 nests, 92.3%).…”