Roosts and diurnal aggregations of Grey Herons (Ardea cinerea) were censused each fornight from September to April during five consecutive years (1991-96) in Navarra (northern Spain). The number of roosts and the peak number of wintering herons increased during the study period, from 19 to 21 localities and from 398 to 693 birds, respectively. The wintering population was mainly composed of young birds from September to October and from March to April, while adult birds predominated during the midwinter months. Total number of birds peaked several times along the study periods, apparently because of pre- and postbreeding migratory movements.
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