Diet composition and extreme specialization
Honey-buzzards behaved as an extreme specialist raptor that predate mainly upon underground nests of social wasps. More than 80% of the diet was comprised by this type of vespid and the first dominant (common-wasp) and the second dominant (Asian-hornet) accounted for 57.0% and 24.2% of prey items, respectively. Previous studies have also reported a high proportion of wasps with underground nests in the diet although with some variation in the composition of species depending on the study (Itämies and Mikkola 1972, Gamauf 1999, van Manenet al. 2011, Ziesemer and Meyburg 2015). Van Manen et al.(2011) found an even higher proportion of social wasps (92%) in The Netherlands, but they caution about the possibility of bias in their samples because they estimated the diet composition based only on remains collected at the nests. As far as we know, the present study and Rebollo et al. (2023) are the firsts to provide a reliable method (trail cameras installed in the nests) that allows to study a minimally biased diet during the breeding period.
Vertebrates comprised only 16.7% of the honey-buzzard´s diet (Table 2 ): reptiles (9%), birds (4.9%), amphibians (0.3%), mammals (0.1%) and unidentifiable vertebrates (2.1%). Contrarily to other studies (Itämies and Mikkola 1972, Gamauf 1999, Roberts and Coleman 2001), which declare amphibians (mainly frogs) playing an important role in the diet, in our study small reptiles and chicks of birds represented more than the 95% of the identified vertebrates. Differences may be due to the varying abundance of these vertebrate groups among different regions, as there are environmental differences between our study area and other European areas.
The specialization ofPernis species as predators of underground nesting social wasps can be confirmed by studying their anatomical adaptations. European honey-buzzards show scale-like feathers, beak, nostrils, tarsi, and nails specifically adapted for excavation of underground wasp nests (Vansteelant and Agostini 2021). See also Sievwright and Higuchi (2011 and 2016) for the Oriental Honey-buzzard, Pernis ptilorhynchus , a similar species to the European honey-buzzard.