The adaptive capacity of common eider ducks (Somateria mollissima) to respond to a changing predator profile in the Canadian arctic

Background/Question/Methods

Several predator-prey systems are in flux as an indirect result of climate change. In the Arctic, earlier sea-ice loss is driving polar bears (Ursus maritimus) onto land when many colonial nesting seabirds are breeding. The result is a higher threat of nest predation by historically infrequent predators. Responses of incubating birds can vary depending on the relative predation risk, as different predators pose different levels of risk. Selection should therefore favour the ability of individuals to assess types and levels of risks and make appropriate behavioural decisions in a given environmental context. We assessed the responses of incubating common eider ducks (Somateria mollissima) in the Canadian Arctic to polar-bear egg predation; in particular, their ability to perceive risk posed by polar bears, and to use predatory cues during bear foraging events. Combining remotely deployed heart-rate monitoring systems with an array of trail cameras that simultaneously monitored polar bear activity, we quantified eider heart rate as a metric for perceived predation threat in a large common eider breeding colony in Nunavut. We also used drone videography to film nest flushing behaviours of common eiders in response to foraging polar bears to assess how cue use influenced flushing behaviour and nest fate.

Results/Conclusions

Eiders exhibited mild tachycardia when bears were present closer to their nests, but were insensitive to the duration of bear exposure. Moreover, eiders were more sensitive (i.e., relatively greater heart rate) to variation in components of their own intrinsic state (degree of investment in incubation) and abiotic conditions (temperature and wind speed) than to variation in threat of bear predation. In a path analysis model resulting from observed eiders from drone footage, our greatest supported model found that more direct angles of visual gaze and travel angle by polar bears resulted in conspicuous nest flushes by eiders, whereas the presence of herring gulls (Larus argentatus) resulted in more discrete flushes of hens walking from their nests. Shorter flush distances between eiders and approaching bears resulted in greater nest predation. Findings suggest that while eiders are capable of recognizing and assessing risk, behavioural-physiological responses alone appear to be insufficient to mitigate polar bear predation at the population level. The effect of polar-bear predation on the reproductive fitness value of hens in this colony has been dramatic, and fluctuations in eider population size may now be driven by more external factors beyond the traditional driver of annual productivity.