1. Introduction
The diet of predators is influenced by strong selective pressures, as
the diet has direct repercussion in the fitness (Johnson and Agrawal
2003). There is a wide range of types of diets in raptors, but they can
be classified as generalists or specialists based on the range of prey
consumed (Newton 1979). The study of the diet of extreme specialist
species and the influence on the diet of some factors that affect the
energy demand of individuals, such as the number and age of nestlings,
may help to understand the selective pressures that modulate the trophic
strategies. Furthermore, knowing the diet is important to understand the
interactions between species in an ecosystem, which is necessary for
better management of species, both prey and specialist predators,
including conservation goals (Sergio et al. 2008).
The genus Pernis Cuvier, 1817 includes four species of raptors
(known as honey-buzzards), widely distributed along Eurasia and the
African continent. These species are highly specialised in the
consumption of social Hymenoptera, mainly vespids (Gamauf and Haring
2004, van Manen et al. 2011, Ziesemer and Meyburg 2015, Purroy
and Purroy 2016). In most ecosystems, Vespidae play important roles,
mostly regulating other populations of insects by predation but also
providing ecosystem services like pollination or carrion recycling
(Sumner et al. 2018). Therefore, the study of the interaction
between raptors and vespid species is uncommon and interesting from an
ecological and management perspective.
European honey-buzzards (Pernis apivorus L. 1758) (hereafter,
honey-buzzards) locate
underground colonies of eusocial vespids and digs out combs filled with
brood (larvae and pupae) (Ziesemer and Meyburg 2015). During the
breeding season, honey-buzzards carry these combs to the nest to feed
the nestlings. This insectivorous diet represents an exceptional case
for a medium-sized raptor that deserves investigation. Due to its
relatively high body weight (600-800 g), a diet consisting of individual
insects might not be effective in balancing the energy expenditure of
hunting and transporting. The fact that honey-buzzards feed on eusocial
insects would allow them to obtain a significant amount of biomass from
each prey (piece of comb with brood) that it delivers to the nest.
Additionally, the recent inclusion of the invasive species Asian-hornet
(Vespa velutina Lepeletier, 1836, var. nigrithorax du
Buysson, 1905) in the diet of European honey-buzzards arouses special
interest (Macià et al. 2019, Rebollo et al. 2019, Rebolloet al . 2023). Asian-hornets were introduced accidentally in
Europe in 2004, but have now expanded to large areas of southwestern
Europe (Arca et al . 2015). The European Union listed
Asian-hornets as an invasive alien species of concern in 2014
(UE1143/2014). Its presence is causing significant impacts on local
biodiversity and economic losses in agriculture and beekeeping (Laurinoet al . 2019, Rojas-Nossa and Calviño-Cancela 2020). Hence,
studying the response of a specialized raptor to the sudden appearance
of an exotic prey species is of both ecological and management interest.
The available studies on the diet of honey-buzzards are limited in
number and often based on small amounts of observations gathered by
collecting prey remains at the nest or analysing faecal samples and
stomach contents of hunted individuals (Itämies and Mikkola 1972,
Roberts and Coleman 2001, van Manen et al. 2011, Ziesemer and
Meyburg 2015, Purroy and Purroy
2016). The paucity of studies may
be influenced by the fact that the honey-buzzards is a species with
discrete habits. This species is
migratory, with a brief stay in Europe between May and August and it
prefers sparsely populated wooded habitats for both breeding and
hunting. These factors make it challenging to study, resulting in its
trophic biology and ecology being among the least investigated of
European diurnal raptors (Hagemeijer and Blair 1997). Therefore, more
information is needed about the trophic ecology, consumption
preferences, and reproductive biology of honey-buzzards, especially with
the recent addition of a new invasive exotic wasp to its diet.
The aim of the present study is to examine the trophic strategy of
honey-buzzards in a population of the south-western Europe. We address
the following specific objectives: (1) Assess the composition of the
honey-buzzard’s diet and the daily rate of delivery of prey items to the
nest during the breeding season. We defined an “item” as any prey that
adults delivered to the nest, whether intact (e. g. , a lizard) or
not (e. g. , a comb of a vespid colony). (2) Determine the
honey-buzzard’s preferences for native and exotic eusocial vespids
species in the ecosystem. (3) Investigate the changes in the diet
throughout the breeding season, considering the age of the nestlings,
number of nestlings, and study year. (4) Discuss the selective pressures
that modulated the trophic strategy of honey-buzzards and potential
implications of our findings on the management of the exotic
Asian-hornet. We expect a preference for Asian-hornets due to their high
abundance in the study area and larger larvae compared to native
common-wasps (Vespula vulgaris ) (Hypothesis 1, H1). We also
expect some increase in the proportion of Asian-hornets in the diet
along the years as honey-buzzards may learn how to take advantage of the
high availability of this new trophic resource (Hypothesis 2, H2). The
diet should vary throughout the breeding period due both to nestlings
demands and to the phenology of prey species, especially vespids, with
small colonies at early summer and high rates of production of workers
at late summer. The proportion of vertebrates in the diet of
honey-buzzards may be higher during the early stages of the breeding
period, when the abundance and colony sizes of Vespidae species are
still low (Hypothesis 3, H3). The rate of prey delivery to the nest is
expected to increase with both the number and age of chicks (Hypothesis
4, H4). The size of the vespid combs delivered to the nest should
increase with either the number or age of nestlings due to the higher
energetic demands of the nestlings and throughout the summer as the wasp
colonies grow (Hypothesis 5, H5).