Historical Ornithological Sources
"The science of ornithology has a long history and studies on birds have helped develop several key concepts in evolution, behaviour and ecology such as the definition of species, the process of speciation, instinct, learning, ecological niches, guilds, island biogeography, phylogeography and conservation. While early ornithology was principally concerned with descriptions and distributions of species, ornithologists today seek answers to very specific questions, often using birds as models to test hypotheses or predictions based on theories. Most modern biological theories apply across taxonomic groups and the number of professional scientists who identify themselves as "ornithologists" has therefore declined. A wide range of tools and techniques are used in ornithology and innovations are constantly made". (Quoted from: Wikipedia
Various German scientists have described the Aquila pomarina during the 19th century, some of their findings and conclusions can be still found in publications of that time. Find various historical texts from encyclopaedias, publications and journals in this topic.
For hints and information on how to find (additional) literature, please look here: http://www.looduskalender.ee/forum/viewtopic.php?f=46&t=234