Blauwe Kiekendieven Circus cyaneus als broedvogel op Terschelling in 1996-98

The Hen Harrier colonised the Dutch Wadden Sea Islands in the 1940s, and its population peaked with 100-120 pairs in the 1970s and 100-130 pairs in the early 1990s. The species is in rampant decline since the mid-1990s, with only 29 pairs left in 2008. Recent research implied food shortage (following habitat change) and increased human disturbance as possible causes. The present author reflects on his findings in 1996-98, when he studied Hen Harriers on Terschelling, one of the former strongholds of this species. These findings suggest that several other factors may have played a role in the decline, i.e. predation, habitat change, competition for nest sites with Marsh Harriers Circus aeruginosus and food competition with other raptor species. Several nests were found having been depredated; although rats were suspected, this was not substantiated. Disturbance by man, or predation by corvids, was considered unlikely. Competition for nest sites with Marsh Harriers was observed twice, respectively in 1997 and 1998, when nest sites of Hen Harriers were taken over by Marsh Harriers. Competition for nest sites may have increased in the wake of habitat change (invasion of scrub in open dunes). Finally, food competition with other raptors may have increased: Marsh Harriers are common on most Wadden Sea Islands, whereas Buzzards Buteo buteo and Goshawks Accipiter gentilis have successfully colonised the islands in recent years.