This lecture was intended to give an idea of causal ethological methods used to analyse the functional organisation of the causal machinery underly- ing the behaviour of an animal. This approach is sketched on the basis of a special case: a study of the causal factors involved in the occurrence of different behaviour patterns in a herring gull incubating its eggs. It is shown how the concept of a functional organisation underlying behaviour makes it possible to interprete: 1. the apparently irrelevant occurrence of behaviour (re-settling, nest- building, preening) interrupting the principal, functionally relevant, behav- iour (incubation) which is going on; 2. variations in the values attributed by an animal under different circumstances to characteristics of objects releasing and directing certain behaviour patterns (e.g. a shift of the size of the egg models optimal in evoking a retrieval response when placed on the nest rim); 3. the evolution of behaviour elements serving a signal function. The further development of adequate methods for the causal study of the functional organisation of the behaviour of a species as well as of an indi- vidual, is considered to be one of the most important tasks for ethology in the near future, particularly with respect to the contribution of ethology in understanding human behaviour. Resume Par cette conference l'auteur a l'intention de presenter les methodes ethologiques, appliquees pour analyser l'organisation fonctionnelle du me- canisme causal, qui est a la base du comportement d'un animal.