How do eider females time their vigilance and diving in brood-rearing coalitions?
The timing of vigilance by prey animals in groups is important for detecting predators. While it has been thought that individuals do not respond to each other and scan at random, prey sometimes synchronize their scanning with each other. This is problematic since overlapping vigilance results in times when no one is watching for predators. There are also other open questions: does group size or nearby animals not part of the group influence the within-group timing of activities, and does offspring predation risk in socially breeding groups affect parental decisions? We studied the timing of vigilance among diving female eider ducks sharing brood care in groups subject to gull predation of ducklings. We found that females synchronized their bouts of vigilance, and this synchronization increased with female group size. However, this did not pose a threat to ducklings, since the proportion of time when at least one female was vigilant still increased with female group size. In contrast, there was less overlapping vigilance when the ratio of ducklings to tending adults decreased. The presence of nearby non-group females did not affect the within-group timing of vigilance, but broods with few ducklings, offering little dilution of predation risk, were more often associated with non-group females. The vulnerability of young to predation affects the mutual timing of vigilance of parents and enhances attraction to nearby conspecifics.
The results are published in the scientific journal Behavioral Ecology. The research was done by Markus Öst from the Aronia Coastal Zone Research Team and Tekla Tierala, and the work culminated in Tekla's M.Sc. thesis at the University of Helsinki.
Why are eiders changing nest site and does it matter?
What are the causes and consequences of changing the nest site? We set out to answer this simple question by studying eiders at Tvärminne in the Hanko archipelago.
The eider is a long-lived and site-faithful species, and eiders in Hanko moved on average only 21 meters between consecutive nesting events. Some individuals still moved kilometres away and previous nest fate, the age of the breeding female and nest density all together affected dispersal distances. Females moved farther after nest failure whereas they moved less with advancing age. They also moved farther if the density of surrounding nests was low. However, the hatching success of neighbouring nests had no effect on breeding dispersal distances.
Breeding dispersal distance did not affect hatching success in the current breeding season. Instead, the timing of breeding was progressively delayed with increasing dispersal distance. This delay is probably due to prolonged nest prospecting in new and unknown surroundings. Since early breeders are usually more productive, this dispersal-induced delay may reduce breeding success.
The results published in the scientific journal Oecologia show that female eiders receive no short-term benefits of breeding dispersal. However, changing to a new nest site may pay in the long run. Some islands at Tvärminne were consistently safer than others. Since eiders are long-lived, dispersal may be beneficial if a safer locality can be found. Eiders can breed successfully for many years in a row once a safe nest site is found.