Bruine Kiekendieven Circus aeruginosus op Friese nazomerslaapplaatsen in het muizenrijke jaar 2019

During 2019, the Dutch province of Friesland was swamped by a serious outbreak of Common Voles Microtus arvalis. This resulted in high breeding success of vole-eating birds of prey, including Marsh Harriers. In August, many Marsh Harriers were recorded using nocturnal roosts, hence the instigation of a coordinated survey of Marsh Harriers on nocturnal roosts in the province in September. At 16 of 18 surveyed potential roost sites Marsh Harriers were found roosting (Table 1). Five roosts contained 40 or more individuals. Taking into account non-surveyed roosts, at least 300 birds are thought to have roosted in Friesland in August as well as September. Over the period August- December 2019, sex ratio and age were determined for 246 Marsh Harriers (Figure 1). The majority of roosting individuals consisted of juveniles (155 individuals, 63.0%), with far fewer males (N=29, 11.8%) and females (N=62, 25.2%). September was the only month with a decent sample of sexed and aged harriers (N=220), which showed a very similar ratio of juveniles (N=127, 61.2%), males (N=25, 12.0%) and females (N=56, 26.8%). At the roosts De Deelen and Bokkumermeer the sex of 20 juvenile Marsh Harriers was determined: 13 females (65%) and 7 males (35%). Marsh Harriers arrived almost an hour and a half before sunset at the roost, with most birds arriving between half an hour before and 20 minutes after sunset (Fig. 2). Morning departure at one roost started about half an hour before sunrise and lasted until 20 minutes after sunrise (Fig. 3). At two inland roosts, respectively 52 and 61 pellets were collected in September, containing the remains of 173 Common Voles Microtus arvalis, 3 Bank Voles Myodes glareolus and 2 Brown Rats Rattus norvegicus. Eight pellets contained feathers of unidentified bird species. At both roosts plucks of Common Snipe Gallinago gallinago and juvenile Starlings Sturnus vulgaris were found.