During the past few years the ecology of the populations of Bramblings (Fringilla montifringilla) wintering in western Europe has undergone a rapid and important evolution due in large measure to the cultivation of maize (Zea maïs). The results is an increasingly frequent formation of flocks containing several million individuals. From 1965 to 1972 studies have been made of a roost in Pau (Pyrénées-Atlantiques) which has regrouped up to twenty million Bramblings. The purpose of the present study is to propose a general model relating the quantity of energy expended daily (QD) to the behaviour of wintering Bramblings and to the habitat variables. In the first part of this study different levels of energy metabolism are defined in function of the conditions in which the measures of energy expended occur. The general relationships linking these types of metabolism to body weight, and the relationship 9 v/b of flight metabolism to basal metabolism are indicated. In the second part of this paper the preceding general conclusions and the information concerning the daily cycle of activity are applied to the case of Bramblings wintering in the Pyrénées-Atlantiques. For example, in the month of December, an individual flying a distance of 80 km a day expends 28 kcal. In the third part of this study the relationship between the amount of energy expended (QD), the distance flown each day between the roost and the feeding grounds (D), the photoperiod qp and the time given to each activity (Ti) are established. These relationships enable us to deal with the problem of the maximal distance of dispersion from the center of the roost. In particular, the time lost reaching the feeding grounds constitutes the principal factor limiting daily dispersal, whereas energy loss is relatively unimportant. The last part of this paper proposes a general model (figure 9) for the total wintering area, giving the amount of energy expended (Qd) and the maximal quantity of energy assimilable (QA max) in relation to the population behaviour, the habitat characteristics, and the period of the year. Introduction into the model of the variables D and VN (value of the food ingested = quantity of energy assimilable by unit of time present on the feeding grounds) allows us to relate the potential number of individuals which can be supported by a given area to the different forms of maize cultivation. The sensitivity analysis indicates the preponderant role of the photoperiod in the determination of the QD. Finally, three examples of simulation are given.