Oskar Heinroth (1 March 1871 – 31 May 1945) was a German biologist who was one of the first to apply the methods of comparative morphology to animal behaviour, and was thus one of the founders of ethology. His extensive studies of behaviour in the Anatidae (ducks and geese) showed that instinctive behaviour patterns correlated with taxonomic relationships determined on the basis of morphological features. He also rediscovered the phenomenon of imprinting, reported in the 19th century by Douglas Spalding but not followed up at the time. His results were popularised by Konrad Lorenz, whose mentor he was. Lorenz regarded Heinroth as the true founder of the study of animal behaviour seen as a branch of zoology.