Na 80 jaar afwezigheid broedt de Raaf Corvus corax weer in Drenthe

The last known breeding attempt of a Common Raven pair in the province of Drenthe, northern Netherlands, was recorded in 1923. It was wiped out as a result of human persecution. Following reintroductions in The Netherlands in the 1970s, and the subsequent settlement and population increase on the Veluwe in the central Netherlands (main breeding range nowadays; Bijlsma et al. 2001, van Manen & Renssen 2002), Ravens were increasingly recorded in areas away from the Veluwe; in Drenthe from 1982 onwards. The presence of a pair in western Drenthe in 1995 led to behaviour that indicated settlement of a territory in an old Scots pine Pinus sylvestris stand in the winter of 1996/97 (Fig. 1). However, both members of this pair disappeared, presumably as a result of death (human caused?). Few Ravens were recorded in 1998-2002. A Raven pair transporting dead twigs was again recorded on 24 February 2003. Surprisingly, nest-building was seen three days later (27 February) in a Scots pine stand some 4 km away from the previous observation. Both adults collected dead twigs from trees and the ground, and transported the twigs to a flimsy platform in the crotch in the upper third of a Scots pine (Photo). Although this nest had not increased in size on 1 March, both adults were seen transporting a beakful of wool/hair towards its vicinity. It turned out that the actual nest had been built 60 m north of the flimsy platform, as evident from the persistently calling incubating female on 13 March. Her begging call had a frequency of once every 3-15 seconds (mean 7.7 ± 4.5, N=9) and soon attracted her mate (which had been away). This call proved to be an excellent method of tracing the nest site. The nest was built on top of a slanting Scots pine (Photo), completely unprotected by branches from above, in a stand of Scots pine near a small forest opening (13x15 m). The 12 nearest pines were on average 10.1 ± 2.8 m away from the nest tree (range 4.2-13.7 m), had a mean diameter at breast height (dbh) of 29.9 ± 5.6 cm (range 22-40 cm) and were on average 14.2 ± 1.4 m tall. The nest tree itself had a height of 14,5 m and a dbh of 26 cm. The local vegetation consisted of Empetrum nigrum and Deschampsia flexuosa, with an undergrowth of Rhamnus frangula (2-4 m) and some Quercus robur and Sorhus aucuparia. The nearest forest edge was 1100 m away. Nest content was checked on 24 April, when three nestlings were ringed, weighed and measured (Table 1, Photo 4). Onset of laying was back-calculated at 5 March. The nestlings were in good condition, but one of the three disappeared before fledging. Fledging of the remaining two nestlings occurred on 16 May and between 18 and 24 May; the Ravens, including the fledglings. were not seen anymore in the environment of the nest on 30 May. When the pair was relocated, some 8 km away on 7 June, only a single fledgling was left. The pair with fledgling were still together on 5 November. Parental care was measured as the frequency with which one or both parents attended the nest or the fledgling. The pair stayed together all the time during the pre-incubation stage. All visits to the nest site showed the female to be incubating, with the male in attendance on 3 out of 4 visits (Table 2). During 12 visits in the nestling stage, both parents were gone on two occasions (but reappeared within 21-35 minutes). On all other occasions, one or both parents attended the nest and started alarm-calling as soon as they caught sight of me (but not when other pedestrians passed, suggesting they were able to differentiate between potentially harmful and harmless individuals). After fledging, parents and fledging were always together, except once (begging young on 17 June, but parents may have been nearby). Foraging flights covered distances of 1000-5000 m from the nest (mean 2544 ± 1452 m, N=9) during incubation, and 3750-6500 m (mean 4754 ± 916 m, N=l2) during the nestling stage. These flights and foraging sites may not have been representative of home range size, as some records indicate the coverage of a much wider range (>7OOO ha during incubation, >9OOO ha during nestling stage). Food included meat from road casualties (including dog Canis lupus), unidentified fresh meat, insects, eggs of Mistle Trush Turdus viscivorus (cached in upper twigs of a Pinus nigra), and sheep (carrion). Both pair members were moulting primaries in mid-May, but the female had started moulting already in April and showed a rather high speed of feather replacement (Table 3: 7th primary lost by 14 June), indicating that she may have been a rather young bird (although not Juvenile or immature, given her shining plumage, dark-brown iris and well-feathered bill base). Moult of rectrices started well after primary moult, i.e. around 14 June (1 central tail feather lost in female, none lost in male). A second pair, apparently non-breeding, was recorded in April 2003, some 7 km north of the nest site of the successful Raven pair. The successful breeding of a Raven pair in this part of The Netherlands is astounding. The birds apparently mostly foraged outside the forest, i.e. in farmland and on heaths. Over the last few decades, farmland has been transformed into a sterile environment where farming practices have been industrialised and food abundance and – diversity have crashed. Moreover, deliberate poisoning of foxes Vulpes vulpes, corvids and raptors is still common practice, hence threatening the very existence of Ravens. It is therefore to be seen whether Ravens shall successfully colonise the province of Drenthe, or remain occasional breeding birds.