CASTELIJNS H & WOUTERS P (2011) Hen Harriers Circus cyaneus in The Netherlands during the winters of 1985/86 - 1989/90. LIMOSA 84 (1): 1-11.

During five consecutive winters (1985 - 1986 - 1989 - 1990) Hen Harriers were counted on winter roosts in The Netherlands. Potential roosts were visited at least three times per winter (November, January and March; Table 1), and observers were asked to count from two hours before until half an hour after sunset. Following these recommendations, only a fraction of the roosting birds will have been missed (Fig. 2). A total of 223 (potential) roosts were surveyed, but country-wide coverage was not achieved. Roosts in the north were surveyed most intensively in the first years of the study, roosts in the southwest more frequently in later years. In general, the coverage was better for coastal than for inland roosts. Most birds roosted near water (Fig. 6) and 83% of roosts were located in reedbeds.
The proportion of roosts occupied by harriers was low in November and March, and high from December through February (Fig. 4, Table 2). The average number of Hen Harriers per roost varied between 3 and 8 (depending on month and year). On 19% of all roosts no birds were observed and in only 3% of the roosts the number exceeded 10 (Fig. 3). Mean numbers of harriers per roost per winter (Fig. 5) were not correlated with weather conditions, though two of the five winters were classified as cold (none with with relevant snow cover).
Observers were also asked to distinguish between adult males and ringtails (juvenile males and juvenile and adult females). 21.7 % of the counted birds were adult males (Table 3). The proportion of adult males was highest in November and lowest in December (Fig. 4c). It tended to be higher in the north and on inland roosts (Fig. 1). Males arrived about ten minutes later than ringtails on two well examined roosts (Fig. 2). The total number of wintering Hen Harriers in The Netherlands during the study period was estimated at 760-1360 individuals, 4-12% of the European population at that time. ####Hen Harrier Circus cyaneus

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