Use of Ratio Estimators to Estimate the Size of Common Eider Populations in Winter

Surveys of common eiders (Somateria mollissima) are often conducted in winter because eiders tend to congregate in small discrete areas during this period. Evaluations of population size are usually based on visual estimates, the reliability of which is difficult to assess. In winter 1989, we tested a method of surveying eiders in the Mingan Archipelago and Anticosti Island region in Quebec. The survey consisted of 2 consecutive overflights of the study area at high and low altitude where photographs were taken and visual estimates were recorded. We estimated the size of the population using a 3-step process. First, we estimated the number of adult males using a ratio estimator that combined high-altitude photo counts and visual estimates; the former was considered an accurate count. The ratio of females and immatures to adult males was then estimated from photos taken from low altitude. Subsequently, a capture-recapture model was used to estimate the number of eiders in small flocks. The proposed method produced estimates with an acceptable level of precision: 77,627 eiders (CV = 13%) on 2 February and 92,247 eiders (CV = 21%) on 28 February. This precision suggests that the method can be useful in monitoring common eider populations in winter, thus facilitating the management of this species. The proposed methodology is unusual in field biology because sampling cannot be controlled; thus, the estimation is relying of a model instead of a predefined sampling design.