The study deals with characteristics of behaviour, individual roost sites selections used by Eurasian Marsh Harrier (Circus aeruginosus) during autumnal migrations on the first known communal roost in Poland over the years 2003-2005. Eurasian Marsh Harriers communal roosts contained from 1 to 23 individuals which were spend nights in: Reed beds (Phragmites australis), Purple moor grasses Molinia cearulea, Small reeds (Calamagrostis sp.), Sedges (Carex sp.) thistles (Cirsium sp.). Adult birds were found to prefer centrally located individual roost sites, while juveniles rather the ones distributed regularly over the area of the roost. It was observed that Eurasian Marsh harriers competes for the centrally located individual roost sites. During pre-roost activities unsuccessful day foragers tend to hunt in the vicinity of the roost area. Some individual roost sites of Eurasian marsh harrier were also adopted by other vertebrates. The collected data indicate the informative and defence function of the roost.