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.