Groei van hongerende Wespendieven Pernis apivorus

Honey Buzzards experienced an extreme shortage of wasps in 1997 (the lowest wasp level in The Netherlands in at least 25 years), resulting in a dismal breeding season: many pairs did not start breeding, many nestlings died, adults left the breeding area in July and early August (earlier than usual; the radio-tagged adult female of nest 2 left the breeding area two days before her young fledged, although the male stayed behind for another week) and moulting was delayed or not started at all in the breeding area. The growth of nestlings in 3 nests was monitored. All nests contained 2 eggs, with onset of laying being 4 June (nest 1), 26 May (nest 2) and 25 May (nest 3). All eggs hatched, but the smallest nestlings died of undernourishment at ages of 13, 15 and 1 day old (hatching day = 0). The other nestlings survived till fledging, despite facing extreme food shortage. All three nestlings showed, in one degree or another, the effects of undernourishment, first and foremost in a retarded increase in body mass (Fig. 1). With the food shortage continuing and even further deteriorating, also the increase in wing length dropped behind the expected growth rate (Fig. 2). The Smilde-bird eventually fledged with a very low body mass (Fig, 1), and was found emaciated at a distance of 33 km 7 days after fledging (direction 280°). Another nestling, from nest 1 (christined Warp), was 400 g under its presumed normal weight at the time of fledging (Fig. 1) and showed a severely retarded growth of flight feathers (Fig. 2). This bird was found emaciated underneath the nest at an age of 50 days old. It was taken in custody and hand-reared in a free-flying state, while provisioning food ad libitum (mostly Wood Mice Apodemus sylvaticus, and bananas). This bird recovered very fast, with an increase in body mass in a single day from 455 g in the early morning to 615 g in the evening. Growth in wing length was also rapidly normalised, first increasing 5-6 mm/day up to wing length 310 mm, then by 3-4 mm/day up to wing length 330 mm, and finally by 1-2 mm/day up to day 78, the last day that measurements were taken (Fig. 2). Although showing a remarkable ability to recover from food stress, all nestlings were heavily infested with fault bars (Table 1), The majority of flight feathers showed a varying number of fault bars, which was even more evident in tertials and coverts. Warp, the bird from nest 1, had an extra handicap, as it had developed weak spots in the shafts of its tail feathers during the period of most extreme food shortage. As soon as this bird started to exercise flying, all rectrices broke and were eventually lost (artificial restoration took place by means of imping). This proves that, although recovery can be apparently complete, feather damage is more permanent till the first complete moult cycle. This will have an adverse effect on flight behaviour, especially in a long-distance migrant such as the Honey Buzzard.