Foundation of ethology, the scientific study of animal behavior. "It was based on a series of six lectures at Columbia University in 1947 and presented a general model of animal behavior. Basically, it was about methodology, about behavior as an outcome of conflicting 'drives,' about the hierarchical organization of behavior as a hierarchy of nervous centers, and about communication between animals" (Larry W. Swanson). 

In this paper Tinbergen defined Tinbergen's Four Questions, or complementary categories of explanations for animal behavior. that form the basis of ethology: Causation, Ontogeny, Survival Value, and Evolution. 

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."
See No.14138.
(Thanks to Juan Weiss for this reference and its interpretation.)