Betekenis van Postduiven Columba livia voor Haviken Accipiter gentilis, en vice versa

In a study area of some 12x5 km, including 1394 ha of woodland, in the dunes of the northwestern Netherlands, Goshawks were absent as a breeding bird in the 1980s, but settled in the early 1990s to increase to a maximum of 12 pairs in 2000. Since then, numbers have declined to 7-8 pairs in 2006-09. This population has been closely studied since 1995, especially with regards to breeding and feeding ecology. Special attention was paid to the Racing Pigeon as prey of breeding Goshawks, mainly by systematically collecting rings of depredated pigeons near Goshawks nests. In 1995- 2009, a total of 1364 pigeon rings were collected, with an upsurge in numbers from 2006 onwards when a metal detector became part of the study routine. The information on the rings was used to track down the owners (in other words: the origin of pigeons). The pigeons killed by Goshawks were then categorized as ‘local’ (originating from pigeon fanciers living in villages bordering the study site), ‘en route during a race’ (usually pigeons from homes to the north of the study site), and ‘lost’ (pigeons from lofts to the south of the study site, overshooting their destination). Of 1364 pigeons, 7.8% came from local lofts, 11.1% was en route during a race and the remaining 81.1% had lost their way to the loft. In several other study sites in The Netherlands and Belgium, the proportion of local Racing Pigeons among Goshawk kills varied between 27 and 43%, often associated with regions with a higher density of pigeon fanciers. The age-ratio of captured pigeons was 50.4% first-year, 26.4% second-year and 23.2% older than two years (N=825, Table 3). This ratio was very similar for local, en route and lost pigeons. Data from other parts of The Netherlands showed an even higher preponderance of first-years among Racing Pigeons killed by Goshawks (54-66%, Table 5).The importance of Racing Pigeons for Goshawks during summer time (the racing season, coinciding with the breeding season of Goshawks when the males – weighing only some 700 g – are responsible for the food provisioning) is borne out by the proportion of Racing Pigeons in diets of Goshawks. In The Netherlands, this proportion varies geographically from 32-34% in the south (where most pigeon fanciers live) to 14-19% in the northeast (fewest fanciers) (Fig. 6). The high proportion in the northwestern Netherlands (32%) is most likely caused by a pigeon-biased prey collection (includes the present study). Annually, on average 15.5 Racing Pigeon rings were collected per occupied Goshawk territory (including rings found with a metal detector). If typical for Dutch Goshawks, the annual loss then amounts to 1500 pairs x 15.5 rings/pair = 23,250 Racing Pigeons (given the estimated total of 20,000 fanciers, about 1 pigeon/fancier, though with large variations depending on site). The Dutch number of Racing Pigeons varied between 2 and 3 million in the 2000s. Losses caused by Goshawks amount to at least 0.8-1.2% per year, i.e. a very small proportion compared to the total losses (40-60%). The importance of Racing Pigeons for Goshawks was clearly demonstrated in the reproductive output per Goshawk pair (chicks fledged), which showed a positive correlation with the number of pigeon rings found at nests (Fig. 4, Table 4).