Der Kumpan in der Umwelt des Vogels - Der Artgenosse als auslösendes Moment sozialer Verhaltensweisen.

This paper is often considered the foundational text and conceptual bedrock for ethology, the scientific study of animal behavior. Lorenz concentrated his studies in the bird populations. He analyzed social behavior in birds, and demonstrated the scientific concept of imprinting. 

In 1973 the Nobel Prize in Physiology or Medicine was awarded jointly to Karl von Frisch, Konrad Lorenz and Nikolaas Tinbergen "for their discoveries concerning organization and elicitation of individual and social behaviour patterns."

(Thanks to Juan Weiss for this reference and its interpretation.)