We conducted the monitoring surveys of wintering Eastern Marsh Harriers Circus spilonotus and Hen Harriers C. cyaneus at their roosting sites in Watarase Marsh, central Japan between the winters of 1994 and 2009. The Marsh Harrier used two to seven sites as roosts and the Hen Harrier one to four during the survey periods. The numbers of the two species fluctuated greatly from year to year at the roosting sites due to disturbance resulting from reed harvest in the neighborhood, and from environmental factors such as the changes of vegetation conditions resulting from changes in the water level at the roosting sites. Although there were no great differences in the characteristics of roosting habitat between Marsh and Hen Harriers, the roost sites of the two harriers were segregated.
The peak abundances of the two harriers were reached during different periods, with Marsh Harriers in January and Hen Harriers in March. This suggests that some Hen Harriers arrive at their wintering grounds later than Marsh Harriers. A mean abundance of the Marsh Harrier was 30.44(6.90 SD) for a period of 16 years, while that of the Hen Harrier was 10.44(4.00 SD) for a period of 14 years. There was no significant correlation in abundance between the two species.
Population indices increased from 1.48 to 1.71 during 2005-2007 for Marsh Harriers, and from 1.85 to 2.14 during 2001-2003 for Hen Harriers. These increases were not, however, significant. The population indices increased by 2.3% per year for Marsh Harriers but declined by 4.9% per year for Hen Harriers. No other long-term monitoring surveys of these harriers have been conducted at any other sites in Japan, so we have no data with which to compare our results. The lack of similar studies in other areas limited our ability to determine the potential effects of some regional factors on the abundance of the harriers wintering in the study site. Therefore, a nation wide long-term monitoring program of these harriers is urgently required to determine their wintering population trends in Japan.
2010 by Japan Bird Research Association