Predatie van roofvogels door andere roofvogels in het Noord-Kennemerlands Duin

The number of Goshawk Accipiter gentilis territories in the dunes between Camperduin and Wijk aan Zee (7189 ha) increased from 9 in 1993 to 21-25 in 2000-09. Since then, numbers slightly declined to 17-20 territories in 2009-11. It was probably because of the rise of the Goshawk that the Hobby Falco subbuteo disappeared from the dunes, retreating to the polder area. The number of Hobby territories in the dunes between Camperduin and Wijk aan Zee decreased from 18 in 1993 to 2-6 in 2000-2004, and to 0-2 in 2005-2011. Since 2001 these have been only hunting territories. The last successful breeding attempt was in 1998. The number of Sparrowhawk Accipiter nisus territories also decreased – probably because of the rise of the Goshawk – from 20-22 in 1993-1995 to 14-16 in 1996-1998. After that there was an increase to 21-26 territories in 1999-2001, and a gradual decrease to 9-11 territories in 2009-2011. It is unclear how the temporary increase in the number of Sparrowhawk territories in 1999- 2001 can be explained. Did Sparrowhawks hide their nests better and behave less conspicuously than before? Or was this temporary increase the result of an abundance of food for the Goshawk in 1999-2001 and a shortage of food in the following years? The Pearson correlations between the number of successful breeding attempts for the Sparrowhawk in the dunes near Castricum (1058 ha) and the number of successful breeding attempts in this area for the Goshawk, the Buzzard Buteo buteo and one specific female Buzzard were -.20, -.86 and -.67, respectively. Does this mean that the influence of the Goshawk (especially predators of adult Sparrowhawks and older chicks) is overestimated and the influence of the Buzzard (possibly predators of eggs and/or younger chicks – the remains of which are more difficult to find) is underestimated?