Use of Nest Shelters by American Common Eiders (Somateria mollissima dresseri): Occupancy Rates and Effects of Shelter Type on Nest Microclimate

Artificial nest shelters have long been used as a conservation tool for waterfowl management to provide safe nesting sites and improve breeding success of local populations. We examined occupancy rates, temperature, and humidity in 2 nest-shelter types provided for Somateria mollissima dresseri (American Common Eider) on coastal islands in the Eastern Shore Islands Wildlife Management Area, NS, Canada. We placed temperature and humidity loggers inside existing plastic barrel shelters, newly designed wooden A-frame shelters, and in natural nesting habitat on 3 islands. The annual proportion of shelters used for each shelter type varied considerably among islands, but we found no difference between the occupancy rates of wooden A-frame shelters (40%) and plastic barrels (39%). We found that wooden A-frame shelters closely mimicked daily mean temperature and humidity in natural nesting habitat, whereas barrel shelters created much hotter, drier conditions compared to natural nesting habitat and A-frame shelters. We suspect microclimate conditions under plastic barrels may cause dehydration for nesting eiders and lead to adverse effects. Therefore, we recommend the removal of plastic barrel shelters from eider colonies. Further research is needed to confirm whether A-frame shelters reduce rates of predation on eider hens and their eggs by large gulls and eagles compared to natural nesting habitat.