Ethology
Lorenz is famous for his 'longitudinal' studies: he lived with the animals he researched, and made observations throughout their lives. Then he wrote about what he had discovered in long, very readable papers. He researched instinctual behaviour, which he called 'fixed action patterns' (FAPs). Lorenz popularized FAPs as instinctive responses that would occur reliably in the presence of specific stimuli (called sign stimuli or releasing stimuli). von Frisch worked on communication in honey bees: the famous 'bee dances'. Tinbergen experimented with the stimuli which trigger fixed action patterns. He found out that artificial super-stimuli could often work better than the natural stimuli.