Eighty to 90 % of contacts demonstrate the validity of the functional unit concept: the winter quarter is divided into separate geographical sectors (the F. U.) exploited by distinct groups. Individuals of a group use the same diurnal roost, fly out
at night to exploit feeding grounds which are exclusive to the group and return to the same roost the following day at dawn. The strong social organisation of teal leads to the division of the population into several groups, each one exploiting exclusively