Neighbourhood watch among common eiders (Somateria mollissima) – Does group nesting reduce nest predation?

Abstract

Colony nesting is thought to be an example of the selfish herd strategy, where aggregating behaviour may be an evolutionary adaptation to reduce the risk of predation. The common eider (Somateria mollissima) is a ground nesting sea bird susceptible to high nest predation rates through the nesting period. They often nest in groups, yet some also choose to nest solitarily. I examined whether group nesting was associated to nest predation in a sub-arctic common eider colony on Grindøya, Northern Norway. Earlier studies relatively homogenous arctic habitats have shown reduced predation rates among high-density nesters, however the heterogenous and vegetation-rich habitat on Grindøya may reduce the advantage of group nesters over solitary nesters. Data containing GPS-location of nests, clutch size, laying-date, weight of the female and nesting success from 2011-2021 were analysed to see if the distance to nearest neighbour and number of neighbouring nests within a 20-meter radius was associated with predation rates. Both distance to nearest conspecific neighbour and number of neighbouring nests were significantly associated with predation in the models, indicating a strong group effect in reducing predation. Predation increased significantly during the study period, correlating with a significant increase in amount of group nesters. Thus, group nesting could be a behavioural adaption to increased predation pressure. However, the condition and experience of the nesting female seem to be the most important factors in reducing nest predation, as both an early egg laying-date and a larger clutch size are associated with reduced nest predation, the latter having the strongest relation to predation in the analyses. Nevertheless, group nesting seems to be an advantageous behavioural adaptation to high predation pressure, indicating an independent selfish herd effect among nesting common eiders.