Eerstejaars mannetje Wespendief Pernis apivorus op de voet gevolgd: gedrag van een gezenderde asielvogel voor en na vrijlating

In July 1997, a nestling Honey Buzzard of c. 3 weeks old was found on the forest floor and brought to the nearest rehabilitation centre. The bird was raised in captivity and fed on day-old chicks, bananas and larvae of mealworms. Its iris colour changed from brown in late summer 1997 to mid-grey on 8 November, slightly lighter grey on 27 November and 14 December, light grey on 13 January and 3 February and grey with a glimpse of yellow on 20 February'. It started moulting its flight feathers on 3 May 1998 and by 23 July had replaced its first five primaries (counted descendantly), the 6th was just emerging; of the secondaries S1-2, 5-7 and 10-11 were in various stages of moult. The rectrices had been partly replaced, both central feathers R1 being renewed (but still in blood shaft), R3 and 4 just emerging and R6 (outermost) half-way renewal. The male was transported to the northern Netherlands on 22 July 1998, in order to be radio-tagged, released and studied in a forested area on sandy soil. By then, its iris colour had changed into pale yellow, but its cere was still bright yellow. A 2-year old Honey Buzzard in captivity in England (born 1996) had a bright yellow iris and a pale-green cere in spring 1998 (pers. comm. R. Khan). During its stay in captivity in Drenthe, from 22 July through 1 August, and as a free-flying bird from 2 August through 10 September, the confiding behaviour of the bird permitted close study throughout captivity and after release, without any indication that the bird changed its behaviour because of the presence of an observer. Apart from sitting and watching the surrounding (and especially flying insects), much time was spent in feather maintenance. Both in captivity and after release the bird started sunning as soon as the sun touched its perch, turning its back towards the sun, angling its wings in delta-fashion and spreading all rectrices. Sunning was most often performed before 11:00 hr, timed sessions taking between 15 and 318 sec (length depending on sun disappearing behind clouds). After a rainy or misty night, the released bird often changed its position in such a way that the sun fell on its perch, apparently to dry its plumage (low-intensity sunning). After six days of fine weather, when it started raining on 28 July; the bird immediately got excited and adopted a high-intensity rainbath-posture with raised and fully spread wings and spread tail, and occasionally slowly rotating on its perch (Photo 5). After some time this changed into a low-intensity, stock-still posture with drooping wings and partly spread tail (Photo 6). Whenever the opportunity arose, it tried to take a normal bath, soaking its underparts, alternately dipping its head and wings in the water to wet the upperparts, and splashing water over its entire body. Unlike a tame, but free-roaming juvenile Honey Buzzard in 1997, it never bathed in shallow pools in the open, apparently because it avoided large open areas (unsafe?). During prolonged, heavy rainfall, it never took refuge (in captivity nor in the wild), taking the full impact of rain in a hunched, stock-still position unless rainfall was accompanied by high winds; in the latter case, it choose a more sheltered position. Despite its extremely confiding behaviour in captivity, it was quickly scared by unfamiliar people, dogs and avian predators in the wild. Actually, its confiding behaviour only remained towards the author (despite the fact that I caught the bird regularly to assess its body mass and moult score), probably because the receiver produced a sound not dissimilar to the ticking-call of Honey Buzzards;the bird always reacted by starting to produce begging sounds or the ticking-call when hearing the receiver. Low-flying Common Buzzards Buteo buteo were intently watched, but the first appearance of a Goshawk (adult male) resulted in a much more vehement reaction: it immediately froze on its perch, feathers of head and neck flattened, slimming the body and very slowly stretching its neck until it was almost horizontal. In this position it kept on staring at the Goshawk (which landed at a distance of some 50 m, apparently not aware of the presence of the Honey Buzzard), It recovered its normal stance only some 30 sec after the Goshawk had left the area. Apparently, Honey Buzzards are able to discriminate between dangerous and less dangerous avian predators without having ever seen or experienced one (as in Burp). Goshawks are serious predators of Honey Buzzards and their nestlings. For example, of 4 nests in my study area in 1998, three were depredated at various stages of the breeding cyle (lx egg stage, 1x nestling stage, 1x two weeks after fledging), whereas in another study area with two pairs, an adult female was killed. Honey Buzzards are not well-adapted at handling vertebrate prey, especially mammals. This was clearly demonstrated by a juvenile Honey Buzzard in semi-captivity, whose handling time of dead Wood Mice Apodemus sylvaticus (each 17-20 g, provided by the author) was timed. The first mice offered took him >15 minutes to consume, which was reduced to <10 minutes after the 15th mouse and <6-7 minutes after the 20th mouse (Fig. 1). No further progression in dismembering and eating mice was discovered. Honey Buzzards, at least the two studied closely in semi-captivity, were singularly inept in pinning down vertebrate prey with their feet and claws. They simply stood on their prey (claws not in the prey but on top of it) or tried to clutch the mouse between the toes, first delicately nibbling at the head for some time before trying to rip off particles or open up the prey. Normally, the head was ignored and the body of the mouse opened at the throat, from where the Honey Buzzard tried to eat the insides with delicate bites. Hard parts, such as bones and feet, were ignored or removed more often than not. Parts of the skin were also stripped and discarded. Flesh and muscles were nibbled off the bones with very small bites. When electing and removing edible parts. Honey Buzzards showed increasing experience with a progressive number of mice handled, accounting for the decrease in handling time in due course. When given the choice, both Honey Buzzards preferred (in order of decreasing preference) wasp brood, peach, bananas (especially when ripe), frogs and mice. Edible frogs Rana esculenta were partly stripped before being eaten, probably because the serous glands on the dorsal surface of the skin (with toxic secretions?) caused irritation to the eyes as deduced from the rapid blinking and intermittent rubbing of the head against the shoulder. A TW-3 tag (9.5 g) with a tilt-switch activity sensor (manufactured by Biotrack, Wareham, UK) was glued to the base of the central rectrice of the first-year male (christened Burp) on 1 August 1998. Ground-to-ground signal ranged between 1.5 and 2 km in closed woodland and semi-open woodland respectively, using a Lintech flexible 3-element Yagi antenna. Tagging, weighing, measuring and moult scoring took 3 observers 70 minutes, during which the Honey Buzzard showed moderate stress. It was released in its cage, where it immediately resumed its quiet attitude and started preening for 65 minutes, not paying special attention to tag or ground-plane and main antennas. During the entire observation period, up to 10 September 1998, the Honey Buzzard ignored the presence of the tag, except for casual encounters while preening. The sideways tension of the wire of the main antenna, however, curved the central rectrice out of line with the other rectrices, after 3 weeks leading to a break in the feather (repaired by imping). Rectrices of Honey Buzzards with a protruding antenna are particularly prone to damage during digging when the bird -like a chicken- steps backwards (tail can get entangled in vegetation) after each digging action. Furthermore, the bird was reluctant to preen its central rectrice with antenna (because of the knotted strings?), resulting in a bedraggled feather. No reaction whatsoever was noticed regarding the ground-plane antenna. The first-year male was released on 2 August. It was constantly tracked until the day of its departure, 10 September. Flying performance steadily improved over a period of 3-4 weeks, but soaring and circling above tree-level was never witnessed throughout its stay; flying above tree-level was restricted to the latter part of its stay and never took more than a minute. Its activities were entirely restricted to 100 ha of mixed woodland, where he either sat for prolonged periods of time on low branches (scanning the environment), roamed from perch to perch or walked the ground. Between 2 August and 10 September, the Honey Buzzard located nine wasp nests (7x Vespula vulgaris, 2x V. germanica) and one bumblebee nest Bombulus sylvarum). One of the nests of Common Wasp had already been depredated by another Honey Buzzard (but workers still active), whereas one of the nests of German Wasp was in its last stage of decline and contained very few pupae. The Honey Buzzard located its first wasp nest on 9 August, i.e. seven days after its release. From then on, the bird was independent, completely ignoring the site of release and food offered there. Its body mass increased from a low of 735 g on 6 August to a peak of 895 g on 8 September, two days before its departure. Details on foraging methods and depredations of wasp nests will be published elsewhere.