BOER P DE, VOSKAMP P & RIJN S VAN (2013) Wintering of Hen Harriers Circus cyaneus in Southern Limburg; do hamster reserves form an ecological trap?. LIMOSA 86 (3): 169-179.

The ecology of Hen Harriers wintering in the southern part of the province of Limburg, Netherlands, was studied in the winters of 2010/11 and 2011/12. Prior to 2004 Hen Harriers used to be scarce in the study area, dominated by intensive farming. The establishment of 400 ha of arable land with a special management for Hamsters Cricetus cricetus created unprecedentedly high numbers of Common Voles Microtus arvalis as a side effect. This food bonanza attracted many predators, including Hen Harriers. Initially the main focus of this study was on the occurrence of roosts, diet and habitat use. Habitat use was determined by tracking birds fitted with radio transmitters. Roosts were detected the same way and by simply following birds leaving the feeding grounds. Diet was determined by pellet analysis. Roosts were counted simultaneously where possible. In total a maximum of 48 Hen Harriers was counted on ten different roosts. Numbers on large roosts (8-10 birds) appeared to be more stable than those on smaller ones. One bird regularly roosted in trees. The diet showed little variation, 98% of the prey items being Common Voles. The radiotracked birds showed a preference to for hunting in hamster management areas, consisting of non-harvested crops like winter wheat and lucerne. Calcareous grassland reserves were visited frequently as well. A possible downside appeared with the finding of a depredated Hen Harrier in 2008 in one of the hamster reserves. Subsequently this the central part of the reserve was systematically searched for signs of depredation during four consecutive winters resulting in a total number of 10 dead Hen Harriers in 2007-2012. Seven birds were depredated, six probably by Red fox Vulpes vulpes and one by Goshawk Accipiter gentilis. Given the seasonal maximum numbers of roosting birds, this yields a mortality rate of ca. 18%, which raises the question whether an ecological trap was created. On the other hand modern intensive farming probably offers poor foraging conditions for wintering Hen Harriers wintering near safer roost sites in wetland habitats.

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