Among-raptor comparisons of gross morphology have been focused on fast-moving, bird-catchers and slow-moving mammal- and carrion-eating species. Very little information is available on the insectivorous European Honey-buzzard, which forages on stationary prey (mainly wasp grubs). We dissected four Honey-buzzards, two adult females (causes of death: starvation and killed by Northern Goshawk Accipiter gentilis) and two juvenile, recently fledged females (killed by Goshawk and marten) (Table 1). Body mass was adjusted for crop and stomach content. Organs were weighed till the nearest 0.1 g; the stomach was emptied before being weighed (wet weights only). Intestine length was measured from the cutoff point at the gizzard through the rectum. The rectum in bird D was 4 cm long, and small intestine length is therefore obtained by subtracting 4 cm of total intestine length. Gizzards of Honey-buzzards constitute only 0.58-1.18% of their body mass, as compared with 0.95-2.62% of body mass in bird- and mammal-eating raptors (Barton & Houston 1996). The soft-bodied wasp grubs are easily crushed and fermented; a light stomach therefore suffices to handle such prey. Equally, small intestines were very short in comparison with bird-, mammal- and carrion-eating raptors (Fig. 2). Although a short intestine is thought to reflect a less efficient digestion (Hilton et al. 1999), this is presumably not the case in Honey-buzzards. Its specific diet of wasp larvae probably enables an efficient digestion in the gastrointestinal tract. This is also exemplified by the fact that free-living Honey-buzzards do not produce pellets. A short intestinal tract and light stomach may furthermore reduce mass load of digesta, important in reducing the susceptibility of Honey-buzzards to predation (risky foraging on the ground, better manoeuvrability to escape avian predators such as Northern Goshawks). The juveniles showed a relatively shorter intestinal tract than the adults; up to their demise both fledglings had been almost exclusively fed on wasp larvae, whereas adult Honey-buzzards are known to also feed on other insects, amphibians and nestling birds. Moreover, fledgling mass was high with both juveniles having extensive tracts of fat on stomach and breast. Inner organs of the juveniles were also slightly heavier than in the adults (as proportion of body mass, cf. Table 1). It is assumed that juveniles, which become independent shortly after fledging, profit from having large organs and a heavy fat-load (despite running a considerably higher predation risk), whereas adult and non-breeding Honey-buzzards wait with fattening till shortly before departure towards the wintering grounds (Fig. 3). Adults with nestlings to attend may profit from low body masses during the breeding cycle, thus reducing flight costs during their extensive foraging forays.

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Werkgroep Roofvogels Nederland

Rob G. Bijlsma, & Theunis Piersma. (2002). Inwendige organen en maag-darmkanaal van Wespendieven Pernis apivorus in vergelijking met vleesetende roofvogels. De Takkeling, 10(3), 214–224.