Boomvalken Falco subbuteo in Groningen: hoogspanning ten top

Groningen is a flat, mainly agricultural province in the northern Netherlands, with only 1.1 % of the total area (2078 km2) covered by woodland During 1996, some 110 km of high voltage transmission powerlines (with 293 transmission towers) were checked for the presence of disused nests and territorial Hobbies (Fig. 1). All in all, 57 old nests were located on 172 towers with two crossarms, and 53 old nests on towers with three crossarms. The nests, except a single Common Buzzard's Buteo buteo nest, had initially been built by Carrion Crows Corvus corone. Only 18% of the nests (n=l 10) were in excellent condition, 57% in moderate condition and 25% were delapidated Most old nests were found on the lower two crossarms in transmission towers with three crossarms (52 out of 53 nests); in towers with two crossarms, 28 out of 57 nests were situated on the lower crossarm All together, 67 nests had been built at the very tip of a crossarm, 36 in the central part of the tower and only 7 in the central section of a crossarm. Five successful Hobby pairs were located, of which at least four bred on disused crow’s nests in transmission towers At two of these nests, the nestlings were measured and weighed (Table 1), onset of laying being 12 and 23 June respectively Including non-systematic surveys and inquiries elsewhere in Groningen, a minimum of 24 territories was located in 1996 (Fig. 2), of which at least 4-5 pairs had been nesting in high voltage transmission towers. Choice of prey was studied by identifying pluckings and analysing pellets (n=40). Most prey items were typical of open farmland in Groningen, such as Hirundo rustica, Delichon urbica, Passer domesticus and P. montanus (Table 2). Several species occur only as migrants in the area, such as Tringa ochropus, Actitis hypoleucos, Oenanthe oenanthe and Ficedula hypoleuca. Of age-identified birds (n=74), 72% was in juvenile plumage. Apparently, some Bam Swallows had only very recently fledged before being caught by Hobbies, given their not yet fully grown outermost primaries and rectrices. A survey through the literature showed that nesting of Hobbies on high voltage transmission powerlines has become increasingly common over much of Western Europe since the late 1970s, especially in The Netherlands, Germany and E-France