Preferences towards native and exotic vespid species
In the study area, honey-buzzards mainly preyed upon colonies of common-wasps and Asian-hornets (Table 2 ). The proportion of other wasp species in the diet was negligible. This finding was in line with the abundance of wasp species in the study area, where common-wasps and Asian-hornets were by far the most abundant species (more than 97.5%, Table 3 ). Other authors (Roberts et al. 1999 and Roberts and Coleman 2001) also found in Britain that, despite the variety of wasp species in the area, the diet of honey-buzzards was constituted mainly by only two species while others were poorly represented. Ziesemer and Meyburg (2015) suggested that not all species of wasps have the same strategy or phenology, so honey-buzzards would be choosing to prey on those species that offer more resources for hunting effort.
Honey-buzzards showed strong preference towards common-wasps and rejected Asian-hornets. The high abundance of Asian-hornets in our study area and its larger brood led us to hypothesize (H1) that honey-buzzards would preferentially use this new trophic resource to feed their nestlings. This rejection could be due to the relocation of secondary nests in Asian-hornet, making these colonies less accessible by honey-buzzards. Gamauf (1999) also found that large underground colonies of Vespula spp . were preferred over their abundance whereas big European-hornets (similar in size to Asian-hornets) were rejected. This could be explained because European-hornets construct nests in tree cavities, making them practically impregnable for honey-buzzards (Nadolski 2012).
It is remarkable how fast an invasive exotic species has become the second most abundant prey species in the diet of this raptor. This result was shown in Rebollo et al. (2023), where Asian-hornet was consumed by all pairs of honey-buzzards and even became the main trophic resource in terms of biomass in one of the two years studied. Carlssonet al. (2009) discussed how the genetic diversity of natural populations of native predators, as well as their phenotypic plasticity, enhance their adaptation and learning capacity to include an exotic invasive species as prey in a rapid manner. This can only happen if the exotic prey species is within the feeding capabilities of the native predator population. In this sense, we expected an increase in the proportion of Asian-hornets in the honey-buzzard´s diet within the four-year study period (H2) but results did not show it. The time that honey-buzzards and Asian-hornets have been in coexistence in Europe is very short. If the incorporation of this prey into the honey-buzzard’s diet is providing some ecological advantage, it is expected that these preferences will change adaptively and, therefore, the importance of Asian-hornet in honey-buzzard’s diet will change in the future as well.