Communal roosting and the seasonal dynamics of the hen harrier Circus cyaneus in the Záhorie region

A count of hen harriers Circus cyaneus was carried out at roost sites in south-western Slovakia from 1 992-2013. The number of sites and also the frequency of survey visits were different in different years. A total of 1 03 counts were carried out. Hen harriers were not seen roosting at the sites in 13 cases. In all, 10 localities with 14 roost sites were checked. Hen harriers roosted in sedge (5) or in ruderal associations (6) and meadows (3). A total of 634 individuals were recorded roosting. There were 483 brown birds (adult females and immature birds in the first calendar year) and 151 grey birds (males in the second calendar year and older). The average number of hen harriers at the roost sites ranged from 3 to 9 individual birds. The maximum number was at least 27 individuals (December 15, 2011 ; a minimum of 18 brown birds and 9 grey birds, Moravský Svätý Ján, Pacitov locality). The percentage of roosting males was 23.76%. The time period of roosting was also recorded. The last individuals were observed flying over the site from six minutes before sunset to 40 minutes after sunset. The average time of the last observation was approximately 1 7 minutes after sunset. The average luminosity at the last observation of an active individual bird was 28 lux. Hen harriers migrate from October to March in the study area. Occurrences in September, respectively in April and May, were very rare. Most registration and the highest number of individual birds were recorded in October, but especially in December and February.