Schrikrui bij Wespendieven Pernis apivorus

On 20 July 2001, an adult male European Honey-buzzard in poor condition was captured in the eastern Netherlands; the bird was unable to fly (disabled left wing, function of right wing also slightly hampered, no fractures though; Fig. 1). During human activities in its cage, the bird froze on its sitting post for 10-20 minutes; he slept in a horizontal, lying position and didn’t fly at all until 1 September. While testing its flight abilities on 14 September (when 896 g, maximum wing chord 400 mm), he lost several body feathers and one rectrice during handling; the latter’s follicle was open and empty. That same day, when caught at night (and probably asleep), the bird lost another five rectrices (Fig. 2). During a second flight test on 1 October, it again lost some thigh feathers during handling. The follicles of the rectrices lost earlier appeared healthy, but whether new feathers were being formed was not to be seen. The male was released on 1 October (a broken rectrice was cut). Another rectrice and the 4th left primary were found afterwards in its cage. Ten other cases of -possible, probable and confirmed- fright moult in European Honey-buzzards were scraped from the literature and by asking around among Honeybuzzard aficionados, revealing observations of free-flying Honey-buzzards in the breeding season with most of their rectrices missing or growing simultaneously (cases 1, 2, 6, 7 and 9; see also Fig. 3 and Müskens 1997), rectrices found close together underneath a Honey-buzzard nest (case 3; 8 rectrices of two birds; see Photo 1 in Müskens 1997) and underneath a Goshawk nest (case 8; 3 undamaged rectrices, i.e. not a plucked Goshawk prey; W. van Manen), loss of the tag-carrying rectrice in an adult male within a few days of capture and radio-tagging (case 5, may have been normal moult; Ziesemer 1995), loss of the entire tail in an adult within minutes of capture on 22 July 2001 (case 4, this bird was tagged with a satellite-transmitter and arrived in Liberia on 20 November 2001; Dr. B.-U. Meyburg) and (near-) simultaneous loss of respectively most and 6 rectrices in two successive years in captivity (case 10, Fig. 4; the bird was handled each time when it happened). Fright moult in Honey-buzzards was exclusively recorded for rectrices and feathers from the thigh, belly and flank. Moreover, all ejected feathers were full-grown. Growing feathers (as in moult or in nestlings) were never fright-moulted. In these respects, Honeybuzzards show the same strategy as other bird taxa well-known for their fright moult, such as galliformes, doves and pigeons, parakeets and parrots, nightjars and a number of passerines. Until now fright moult had not been described for any species of raptor. However, Honey-buzzards are likely candidates to have evolved fright moult, as they spend considerable periods of time on the ground (up to 30% during daylight in the breeding season, based on radio-tagged birds; R.G. Bijlsma), excavate ground-nesting social wasps (during which they run a high predation risk from avian – and ground predators), hunt stationary prey (social wasps, bumblebees, naked nidiculous nestling birds, amphibians; successful hunting does therefore not depend on manoeuvrability) and are poorly equipped with weapons to defend themselves (bill and claws relatively powerless). The loss of tail and body feathers during acute stress has probably evolved as an anti-predator defence strategy. Honey-buzzards on the ground are most likely to be attacked and grabbed from behind, and the sudden ejection of tail feathers may distract a predator long enough to be able to escape. The loss of these feathers is not as serious as it would have been for avian predators actively hunting for adroit prey designed to escape. Honey-buzzards are furthermore noted for their ability to start migration with growing rectrices, as recorded in juveniles in the poor wasp-year of 1997 (R.G. Bijlsma). The histology and endocrinology of fright moult is still largely unknown.