Numbers and hybridization of spotted eagles in Estonia as revealed by country-wide field observations and genetic analysis

Abstract

Spotted eagles include two medium-sized similar raptors breeding sympatrically in Estonia. In the current study I checked if population numbers derived from country-wide field observations over a 20-year period were in line with the estimate extrapolated from surveys in study plots (500-600 breeding territories of the Lesser Spotted Eagle and 10-30 territories of the Greater Spotted Eagle). I also analysed the structure of the hybridizing spotted eagle population using field descriptions and genetic analysis. The country-wide census enabled to distinguish 664 putative breeding territories, in 81% of which eagles were recorded in two or more years while in 53% breeding was confirmed. In 492 territories the species was identified in the field from morphological characters, and in 158 of them genetic analysis was used to verify the identification. The Greater Spotted Eagle was observed in 26 territories, in 54% of which breeding was confirmed. Altogether 10 pure-species Greater Spotted Eagle pairs and 14 pairs mixed with the Lesser Spotted Eagle were recorded whereas in 12 territories species compositions changed - mainly towards hybridization and Lesser Spotted Eagle pairs. Genetic analysis suggested breeding of an adult hybrid spotted eagle in eight territories and later generation backcrosses in five territories. The present study supported the plot-based estimates of population size but also showed extensive, partly hidden hybridization and decline of the Greater Spotted Eagle. It also showed that, despite several caveats, full census based on a large database of casual field observations can be a powerful tool for estimating the size and distribution of a rather numerous raptor population and is essential for discovering breeding sites of a rare species. In hybridizing species, however, more specialized approaches (comprehensive search of nests, detailed morphological descriptions, and genetic analysis) should be used as well.