Age and sex ratios of wintering Long-tailed Ducks Clangula hyemalis can be determined by analysis of photos of flying flocks at sea: A method description

The West Siberian/North European population of Long-tailed Duck Clangula hyemalis that breeds in Fennoscandia and Northwestern Russia, and winters in the Baltic Sea, has declined by at least 65% since the 1990s and is classified as globally vulnerable. To propose effective management actions to stop the decline, knowledge about demographic parameters is required. A photo survey method by which it is possible, in winter, to collect data on sex ratios and production of first-winter birds is presented here. The plumage traits and bill patterns, which are detectable in photos taken at a distance, are described. The traits can be used to discriminate between three categories of birds: adult males, first-winter males, and females. To extrapolate results from several photo surveys and obtain population-wide estimates of demographic parameters, knowledge about non-random distributions of different bird categories is needed. It was found that different age and sex categories were distributed differently across flocks of different sizes. The required sample sizes and the possible constraints and biases related to the photo survey method are discussed.