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\begin{document}

\title{The trophic strategy of the European honey buzzard Pernis apivorus
during breeding: extravagant specialization or genius solution?}



\author[1]{Jorge Ángel Martín-Ávila}%
\author[1]{Salvador Rebollo}%
\author[2]{José M. Fernández-Pereira}%
\author[1]{Luisa Díaz-Aranda}%
\affil[1]{Universidad de Alcalá}%
\affil[2]{Independent field assistant}%


\vspace{-1em}



  
  \date{May 26, 2024}


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\selectlanguage{english}
\begin{abstract}
The study of the diet of extreme specialist species can help to
understand the selective pressures that modulate the trophic strategies
of raptors. This article provides insight into the trophic strategy of a
little-known top predator and allows for a better understanding of the
selective pressures that shape its diet. We studied the diet of European
honey-buzzards, a raptor specialized in the consumption of social wasp
brood. The ratio of predator to prey biomasses is one of the highest
among raptors. We studied some factors that affect the energy demand of
pairs which influence the diet composition and the daily rate of prey
delivery to the nest. We explore hypotheses about the role of native and
exotic vespids in the diet and the influence of the number and age of
nestlings on diet composition, daily rate of prey delivery, and size of
combs delivered. We installed trail cameras in 24 honey-buzzards nests
in north-western Spain in 2018-2021. We estimated the proportion and
daily rate of delivery of native common-wasps (Vespula vulgaris),
invasive Asian-hornets (Vespa velutina), reptiles, and birds and the
honey-buzzard\selectlanguage{ngerman}´s preferences for vespid species. We performed LMMs,
GLMMs, and MLRMs to analyse relationships between response variables and
predictors. We identified 4611 prey. Honey-buzzards mainly consumed
vespids (82\% of prey). Common-wasps and Asian- hornets were almost the
only two vespids consumed. The invasive hornet was the second most
important prey consumed, but common-wasps were preferred. Vespids were
more important as the age and number of nestlings increased. Our results
suggest that the honey-buzzard´s diet is the adaptive result of the
conflict between being a median-sized insect-eating migratory nidicolous
raptor and collecting enough morsels for the growth of its nestlings,
supporting the prey size and ingestion rate hypothesis. We discuss
implications of our findings for the management of the invasive wasp%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{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 \emph{et al.} 2008).

The genus \emph{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 \emph{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 \emph{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 (\emph{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
(\emph{Vespa velutina} Lepeletier, 1836, var\emph{. nigrithorax} du
Buysson, 1905) in the diet of European honey-buzzards arouses special
interest (Maci\selectlanguage{ngerman}à \emph{et al.} 2019, Rebollo \emph{et al.} 2019,
Rebollo\emph{et al} . 2023). Asian-hornets were introduced accidentally
in Europe in 2004, but have now expanded to large areas of southwestern
Europe (Arca \emph{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
(Laurino\emph{et 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 \emph{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 (\emph{e. g.} , a lizard)
or not (\emph{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 (\emph{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).

\textbf{2. Methods}

\emph{2.1. Study area}

The study area (about 670 km\textsuperscript{2}) is in the north-western
Spain (42º 20' N, 8º 47' W) (\textbf{Supplementary Figure S1} ). It
presents a mild humid-oceanic climate, with annual average precipitation
of 1402 mm and annual average temperature of 14.2°C (Rodríguez-Lado
\emph{et al} . 2018).

The landscape is mountainous with an average height of 213 m.s.n.m
(0--646 m.s.n.m) and interspersed valleys and plains, all conforming a
heterogeneous mosaic of forest patches and farms (Rebollo \emph{et al} .
2017). The 50.9\% of all the study area is covered by patches of forest
masses, mainly occupying the middle and upper parts of the slopes. These
forest patches are dominated by non-native eucalyptus plantations
(\emph{Eucalyptus globulus} Labill., 1800) but also there are mixed
patches with native species such as the oak (\emph{Quercus robur} L.) or
the maritime pine (\emph{Pinus pinaster} Ait., 1789).

\emph{2.2. Study species}

European honey-buzzards show the greatest distribution compared to other
species of the genus\emph{Pernis} , occupying most part of Europe and
some parts of Asia during the breeding season (Gamauf and Haring 2004).
Wintering areas are distributed throughout sub-Saharan Africa.
Post-nuptial migration occurs from late August and pre-nuptial migration
mainly from early May.

At the Iberian Peninsula, honey-buzzards breed at low densities
populations, mainly at forest masses in the northern Spain and Portugal
(Purroy and Purroy 2016). A census by SEO-Birdlife in 2009--2010
estimated a total of 1850 breeding pairs in Spain (Palomino Nantón and
Valls 2011). Adults begin the construction of the nest at early May and
the first eggs are laid on the second half of May (Rebollo \emph{et
al.}2023). Usually, two eggs are laid in alternating days. Chicks hatch
after 33-35 days of incubation. Once the nestlings are 35-40 days old,
they start the firsts short flights, coming back for eating until its
complete independence around 65 days old (van Manen \emph{et al.} 2011,
Ziesemer and Meyburg 2015).

At the study site, eleven species of eusocial Vespidae wasps can be
found (Vega \emph{et al} . 2022), all of them show annual biological
cycles. The surviving queens, which mated before hibernation, emerge at
the end of the winter, and begin the cycle building new colonies (Archer
2012). The first generation of workers is raised by the queen/s until
they complete their development, and the production of reproductive
individuals (males and gynes) takes place at the beginning of autumn.
After the mating season, males die and gynes take refuge for wintering
(Archer 2012). Asian-hornets usually begin the cycle with a primary
underground nest, but in mid-summer, when the colony grows and they need
more space, they usually move to the tree canopy and build an aerial
secondary nest (Diéguez-Antón et al 2022).

\emph{2.3. Tracing honey-buzzard nests and installation of trail-cameras
in the nests}

The forest patches at the study area have been surveyed systematically
for four years (2018-2021) to find all honey-buzzard nests
(Rebollo\emph{et al.} 2023). We installed trail-cameras in 24 nests of
honey-buzzards: 5 in 2018, 4 in 2019, 6 in 2020 and 9 in 2021. The
cameras (Browning SPEC OPS and SPEC OPS EDGE) were connected to an extra
battery to ensure they could cover the whole breeding period. Cameras
were placed in the surrounding branches at 1.5--2 m from the nest and
programmed to take only one picture when triggered, with a delay of 20
seconds between pictures. Accessing to the nest and installation of
trail-cameras was performed once the nestlings were 16 days old, moment
in which they can thermoregulate by themselves.

At the time of trail-camera installation we estimated the breeding
phenology from the age of nestlings, based on the length of the wing and
body mass (Bijlsma 1997). Laying dates were estimated by subtracting the
incubation time of a single egg (35 days) from the hatching date of the
oldest nestling in each nest (Roberts \emph{et al} . 1999). Laying
phenology was estimated in julian days, with January
1\textsuperscript{st} defined as day 1.

\emph{2.4. Identification of wasp combs}

The images taken by the trail cameras allowed us to differentiate
whether the comb had large cells typical of the genus \emph{Vespa}
and\emph{Dolichovespula} , or small cells of the genus \emph{Vespula}
or\emph{Polistes} . We were able to differentiate the vespid
subfamilies\emph{Polistinae} from \emph{Vespinae} based on the
configuration of the combs (see details in Rebollo \emph{et al} . 2023).

In order to check the identification carried out in the photographic
register, the 24 honey-buzzard´s nests were visited regularly each 7 or
14 days to collect the remains of the consumed wasp combs that had
fallen to the ground. We also collected wasp combs directly in the
honey-buzzard´s nests during the installation (July) and deinstallation
(September) of the cameras. The wasp combs were stored at -20 ºC until
laboratory analyses, in which we identified the combs using the
morphology of the mandibles of the larval exuviae (see details in
Rebollo \emph{et al} . 2023).

\emph{2.5. Estimating the abundance of vespid species in the field}

During 2020 and 2021 we installed baited traps (Avispa'clac®) to
estimate the abundance of vespid species in the nesting territories of
honey-buzzards. We surveyed 6 nesting territories in 2020 and 4 in 2021.
Seven baited traps were installed per territory as follows: 1 trap at 0
m from the nest, 3 traps at 500 m from the nest, and 3 traps at 1000 m
from the nest (\textbf{Supplementary Figure 2} ). The traps were always
restricted to forest patches as it is the main hunting habitat of
honey-buzzards (van Manen \emph{et al} . 2011). We used water, sugar and
yeast (5 / 2 / 0.06 kg, respectively) as a bait. Traps remained active
for 7 days in each survey. In 2020 we performed one survey in mid-July
and one in mid-August. In 2021, we performed two surveys in July and two
in August. Trapped insects were preserved in 70\% ethanol until
identification.

\emph{2.7. Estimation of the daily rate of prey delivery in the nests}

We estimated the daily rate of delivery for each type of prey item
between 16 and 34 days old. We did not consider prey consumed by
nestlings older than 35 days because nestlings can move to the branches
(Roberts \emph{et al} . 1999, Purroy and Purroy 2016) and may consume
some prey outside the nest. To minimise bias in the estimates, we only
considered nests with at least five complete days of data available.

\emph{2.8. Statistical analyses}

We studied the size of wasp combs delivered to the nest (recorded by the
cameras) in terms of the number of cells, based on the number of cells
of the largest diameter (D) and the smallest diameter (d), using the
ellipse area formula:

\begin{equation}
N\ =\ D/2\ \times\ d/2\ \times\ \pi\nonumber \\
 \end{equation}

We used Linear Mixed Models (LMM) to study the variations in size of
common-wasp and Asian-hornet combs. Year, breeding phenology (julian
date), vespid species, number of nestlings, and age of the nestlings
were predictors. Nesting territory was the random factor. To avoid
overdispersion in the response variable, we calculated the mean values
of cells of the wasp combs delivered each day to each nest.

To assess honey-buzzard prey preferences for vespids species, we used
Ivlev's selectivity index (E) to relate the proportion of prey delivered
to nests to the proportion of the same prey available to the environment
(Rebollo \emph{et al} . 2017):

\begin{equation}
E=\frac{r-p}{r+p}\nonumber \\
 \end{equation}

Where \emph{r} was the proportion of cells of each wasp species
delivered to the nest. This proportion was estimated with the total of
comb items recorded by the cameras. The proportion of wasps available in
the field (\emph{p} ) was estimated as the average percentage of workers
of each wasp species captured within the 7 traps of each territory
sampled. The E index ranges from --1 to + 1. Positive values indicate
that honey-buzzards prey upon a species above its availability. We
considered a ``preferred prey'' the species with an E index above zero
(Rebollo \emph{et al} . 2017).

We performed Cumulative Link Mixed Models (CLMM) to study the variations
in the proportion of prey items delivered to the nests. We
differentiated the 4 main types of prey items (common-wasp,
Asian-hornet, reptiles, and birds) as response variables. CLMM models
estimate the change in the proportion between the categories of the
response variable directly from the number of observations. We explored
the age of the nestlings, the study year, and the number of nestlings as
predictors.

Finally, we analysed changes in the rate of prey delivery using GLMM
with a Poisson error distribution and log link function. The response
variable was the daily rate of prey delivery, measured as the account of
prey items delivered to the nest each day. We explored the number of
nestlings, the age of the nestlings, and the study year as predictors.
The random effect was the nesting territory.

In all cases, Akaike's (1987) information criterion (AIC) was calculated
for each model for the model selection process; a smaller AIC indicates
a better-fitting model as determined from the parsimony in the number of
parameters. We used the cut-off of \selectlanguage{greek}Δ\selectlanguage{english}AIC \textgreater{} 2 units to
differentiate models with better explanatory power (Burnham and Anderson
2002). When faced with models having comparable AIC values after the
selection process, the model with the lowest AIC value was preferred,
even if it was more complex. This decision aimed to enhance the clarity
of result interpretation and discussion. In all statistical procedures
we considered a level of significance of p \textless{} 0.05. All
statistical analyses were performed with R software v. 4.2.1 (R Core
Team 2022). LMM, CLRM and GLMM analysis were performed with packages
stats (R Core Team 2022), lme4 (Bates \emph{et al} . 2014), nlme
(Pinheiro \emph{et al} . 2022), nnet (Venables and Ripley 2002), ordinal
(Christensen 2022) and MuMIn (Barton 2022).

\textbf{3. Results}

\selectlanguage{ngerman}\emph{3.1 Honey-buzzard´s diet composition}

A total of 2381 wasp comb fragments were collected in the 24
honey-buzzard´s nests and surroundings, accounting for 61533 large cells
and 150847 small cells analysed. We identified combs from 6 wasp
species: three with small cells \emph{(Vespula vulgaris} , \emph{Vespula
germanica} , \emph{Polistes sp.)} and three with large cells
(\emph{Vespa velutina, Vespa crabro} , \emph{Dolichovespula media)} .
The two most abundant species were common-wasps \emph{(Vespula
vulgaris)} and Asian-hornets, accounting for the 99.2\% of the small
cells and the 99.0\% of the large cells, respectively (\textbf{Table 1}
). This allowed us to simplify our statistical analyses by assuming that
all small cell combs could be associated to common-wasps and all large
cell combs to be Asian-hornets with minimum error\emph{.}

We obtained a total of 752678 pictures in the 24 nests from which 4611
prey items were recorded, accounting for 3909 vespid items, 607
vertebrate items, and 95 unidentifiable prey items. 81.9\% of the prey
items were vespids. Common-wasps were the most abundant species
(57.0\%), and Asian-hornets the second (24.2\%) (\textbf{Table 2} ).

The best-fitting model explaining comb size recorded by the cameras
included the interaction between wasp species and number of nestlings
(\textbf{Supplementary material Table 1} ). Combs of common-wasps had
more cells than those of Asian-hornets (t = 16.26; p \textless{} 0.001)
and this difference was greater in nests with one nestling (t = -3.49; p
\textless{} 0.001). There was not an isolated effect of the number of
nestlings on comb size (\textbf{Figure 1} ). Neither the year nor the
nestling age influenced the size of the combs delivered to the nest,
which means that the size of delivered combs remained stable during the
study period.

Baited traps captured a total of 15803 wasps, 5709 in 2020 and 10094 in
2021. In both years, the most abundant species was Asian-hornets (68.1\%
of all vespids captured: 63.9\% in 2020 and 74.3\% in 2021). The second
most abundant species was common-wasps (30.3\%: 34.7\% in 2020 and
23.5\% in 2021). The other vespids species (\emph{V. germanica}
,\emph{Vespa crabro, Dolichovespula media, D. sylvestris, Polistes
dominula, P. nimpha} ) represented the 1.6\% (1.3\% in 2020 and 2.1\% in
2021). This resulted in a mean Ivlev´s selectivity index value (E) of
-0.71 for Asian-hornets and 0.52 for common-wasps\emph{,} which
indicated a preference of honey-buzzards for the native species and a
rejection for the exotic species (\textbf{Table 3} )\emph{.}

Vertebrates accounted for 16.7\% of all prey items (\textbf{Table 2} ).
Reptile was the most important group of vertebrates (9\%). Lacertidae
(8.3\%) were predominant compared to Anguidae (0.3\%). The images did
not allow us to identify up to the species level all vertebrate items.
However, it was remarkable the abundance of Iberian emerald lizard
(\emph{Lacerta schreiberi} Bedriaga 1878) accounting for the 56.9\% of
all reptiles. Birds were the second most important group of vertebrates
(4.9\%). Chicks without feathers (4.2\%) were predominant respect to
feathered birds (adults, fledglings, and older nestlings) that accounted
for 0.7\%. In the case of avian prey, the common wood pigeon
(\emph{Columba palumbus} L. 1758) was the most abundant species (50.5\%
of all avian prey). Amphibians (all frogs) and mammals (mainly volves)
were rare in the diet accounting for 0.3\% and 0.1\% of all prey items,
respectively.

The proportions of the four main types of prey (common-wasp,
Asian-hornet, reptiles, and birds) changed significantly with the age of
the nestlings and the number of nestlings, but not with the study year.
The best-fitting model included the interaction between the age and the
number of nestlings and the additive effect of the study year
(\textbf{Supplementary material Table S1} ). Our results show an
increase in the mean proportion of vespids with the age of nestlings
(from 70.8\% at 16 days of age to 91.9\% at 45 days) due to the increase
of common-wasp (from 38.4\% to 74.6\%) while Asian-hornet proportions
decreased (from 32.4\% to 17.4\%). This caused an important reduction in
the mean proportions of vertebrates with the age of nestlings, which
constituted the 29.2\% of the diet at 16 days of age and 8.1\% at 45
days, showing a drastic reduction of reptiles (from 17.7\% to 5.4\%) and
avian prey (from 11.5\% to 2.7\%). The interaction of the number of
nestlings and age also showed a significant effect over the proportions
of prey groups delivered (Likelihood ratio = 47.63, df = 2, p
\textless{} 0.001), wherein the proportion of common-wasps increased
more intensely with the age of the nestlings in nests with two nestlings
(\textbf{Figure 2} ). We observed a non-significant effect of year over
the mean proportion of each prey group (Likelihood ratio = 7.14, df = 3,
p = 0.067, \textbf{Figure 3)} .

\emph{3.2 Rates of prey delivery in the} honey-buzzard\emph{´s nests}

Honey-buzzards delivered, on average, 7.13 prey items per day: 5.5 were
vespids, 1.57 were vertebrates, and 0.1 were unidentified prey
(\textbf{Table 4} ). The best-fitting model to explain variations in the
daily rate of prey delivery included the number and age of nestlings
(\textbf{Supplementary material Table S1} ) but just the number of
nestlings showed a significant effect on the response variable (z =
5.34; p \textless{} 0.001). Nests with two nestlings received a mean of
2.57 prey items per day more than nests with only one nestling
(\textbf{Figure 4} ). The daily rate of prey delivery did not vary
significantly neither with the age of nestlings nor year.

\textbf{4. Discussion}

This study analysed the trophic strategy of an unusual forest
insect-eating medium-sized raptor specialised in consuming social
hymenopterans. We studied 4611 prey items obtained in 4 years in a total
of 24 honey-buzzard´s nests. Our results suggest that the
honey-buzzard´s diet is the adaptive result of the conflict between
being an insect-eating median-sized migratory nidicolous raptor and
collecting enough number of morsels for the growth of their nestlings,
supporting the prey size and ingestion rate hypotheses.

\emph{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
Manen\emph{et al.} 2011, Ziesemer and Meyburg 2015). Van Manen \emph{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 \emph{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
(\textbf{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 of\emph{Pernis} 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,
\emph{Pernis ptilorhynchus} , a similar species to the European
honey-buzzard.

\emph{Preferences towards native and exotic vespid species}

In the study area, honey-buzzards mainly preyed upon colonies of
common-wasps and Asian-hornets (\textbf{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\%, \textbf{Table 3} ). Other authors (Roberts \emph{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 \emph{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 \emph{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.
Carlsson\emph{et 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.

\emph{Changes in diet composition and rate of prey delivery with the age
and number of nestlings}

Proportions of each type of prey in the diet as the breeding season and
the age of the nestlings progressed showed significant differences
attributable to the phenology of each group of prey. As expected (H3),
the daily rate of vertebrate delivery to the nest and the proportion of
them in the diet decreased as the breeding season and the age of the
nestlings progressed. This was probably due to the increase in
detectability of wasp colonies throughout the summer, which increased
the daily rate of comb delivery and the proportion of comb items in the
diet over time. As other authors suggested (Kostrzewa 1998, Roberts and
Coleman 2001), the role of vertebrates on the diet could be even higher
immediately after prenuptial migration and prior to egg laying when it
is important to replenish their fat reserves. Thus, our findings
suggests that honey-buzzards consume vertebrates to complement the diet
when wasp colonies are not abundant, although we cannot discard that a
higher consume of vertebrates at the beginning of the nestling phase
responds to nutritional needs of nestlings in the initial stages of
their growth (maybe calcium or phosphorus), an option that deserves
future research.

There was an unexpected decrease in the daily rate of Asian-hornet comb
delivery and in the proportion of this species in the diet as the
breeding season and the age of the nestlings progressed. This reduction
could be explained because the relocation of underground colonies to the
tree canopy during mid-summer (Diéguez-Antón \emph{et al.} 2022)
avoiding honey-buzzards attacks.

There was an increase in the proportion of common-wasps in the diet as
breeding season and age of nestlings progressed, a pattern previously
observed by other authors (Itämies and Mikkola 1972, van Manen \emph{et
al} . 2011, Ziesemer and Meyburg 2015). The increase in the proportion
of common-wasps was more pronounced in nests with two nestlings than
nests with one nestling (as we expected in H4). Thus, honey-buzzards
became more specialized in consuming its preferred prey as energy
demands increased (\emph{i. e.} the number of nestlings increased). This
suggests that common-wasp provides the highest energy return relative to
the effort of searching for and processing the wasp nests and provides a
more effective strategy for feeding honey-buzzard´s nestlings.

Contrary to expectations (H5), the size of wasp comb items delivered to
the nest (number of cells) did not change with either the number and age
of nestlings. The increase in comb size over age of nestlings was
expected due to wasp colony growing during the summer. This result could
indicate that honey-buzzards exert some selection on the sizes of
colonies it preys upon, avoiding the small colonies. It is likely that
at the beginning of the breeding season, only the most developed
colonies are detectable by honey-buzzards, which appear to locate
colonies by following the movement of foraging wasp workers (Birkhead
1974). It is also important to consider the flight burden that
honey-buzzards can sustain during a trip back to the nest. Wasp colonies
can contain a high brood biomass, but honey-buzzards fragment the combs
and transports them to the nest in multiple trips (van Manen \emph{et
al} . 2011, Ziesemer and Meyburg 2015). The average comb items of
Asian-hornets and common-wasps contained 141 and 345 cells,
respectively. Assuming that all items are fully loaded with brood and
considering that the average weight of a \emph{V. velutina} larva is 413
mg and a \emph{V. vulgaris} larva is 65 mg (Rebollo \emph{et al} .
2023), we found that for each comb item, a honey buzzard would carry at
most 58.2 g of Asian-hornet brood and 22.4 g of common-wasp brood. The
weight of the inedible paper paste was not considered in these
calculations, but the weight of the comb item does not appear to be a
limiting factor for the choice of comb size provided in each item.

\emph{Selective pressures that modulated the trophic strategy of
honey-buzzards: extravagant specialization or genius solution?}

In seasonal environments, nidicolous and migratory birds must
efficiently manage the limited time available for breeding. Raptors
usually feed on prey relatively large compared to the bite size that
nestlings consume. It has been suggested that raptors have been under
strong evolutionary pressure to manage this conflict between collecting
and processing food for nestlings, \emph{i. e.} , the prey size and
ingestion rate hypotheses (Slagsvold and Sonerud 2007). Larger prey
items provide more bites but require more time to collect and process,
which may result in a relatively low ingestion rate compares to smaller
prey. Raptors have limitations in feeding on individual insects during
the breeding season despite insects being small-sized prey easy to
process. The energy yield of insects is low if the raptor carries a
single prey and must move long distances many times between the capture
site of insects and the nest. Hence, such a non-social insect-based diet
is unlikely in raptors (Sonerud \emph{et al} . 2014).

Honey-buzzard´s diet would be the result of the evolutionary pressure to
manage the conflict between collecting and processing food for
nestlings. They found a genius solution; to feed on a small-sized prey
easy to process (comb with brood) which rends a high nutritional value
and a high amount of edible biomass per delivery for an insect-eating
raptor. The ratio between the size of raptor (between 600 and 800 g) and
the size of prey (less than 1 g) is possibly one of the highest among
the raptors in the world. Larvae and pupae are individualized in the
cells of combs and their size allows nestling to swallow them in a few
seconds. Thus, provisioning for nestlings does not need to prepare prey
items into pieces of suitable morsels. Nestlings would start feeding
unassisted at a younger age (van Manen \emph{et al.} 2011, Ziesemer and
Meyburg 2015) allowing the female to resume hunting at an earlier stage
of nestlings. The vertebrates consumed by honey-buzzards also support
this interpretation because they are mainly small reptiles with short
appendices and easy to swallow in one bite. In the case of birds, they
are mainly immature individuals without feathers and easy to process.

\emph{Implications for the management of the exotic Vespa velutina}

The European Union listed Asian-hornets as an invasive alien species of
concern in 2014 (UE1143/2014). Our results agree with Rebollo \emph{et
al} . (2023) regarding the predation pressure of honey-buzzards on this
species, which could be of interest for the management of the exotic
wasp, at least on a small scale in the proximity of honey-buzzards´
nests. Four years after colonization, the Asian hornet became the second
most important prey of honey-buzzards in the study area. Our findings
allow us to identify two aspects of the Asian-hornet honey-buzzard
interaction that could limit the magnitude of honey-buzzards´ effects on
the exotic wasp. On one hand, honey-buzzards did not appear to show a
preference for Asian-hornets compared to the native
common-wasps\emph{,}likely because the annual life cycle of Asian-hornet
colonies is not entirely underground, and this predator has specific
adaptations to primarily target upon underground nests. It would be
expected that in those years in which the phenology of Asian-hornets is
delayed due to weather conditions, honey-buzzards would include a higher
proportion of the exotic wasp in their diet. On the other hand, the
degree of specialization exhibited by honey-buzzards on common-wasps may
make them particularly vulnerable to invasion by Asian-hornets. Since
its arrival, Asian-hornets have become the most abundant social wasps in
the study area, and their presence may have a direct impact on the
populations of common-wasps, both through direct predation and
competition for resources. We observed Asian-hornets preying on
common-wasp workers at the entrance of their nest, especially after a
honey-buzzard attack on common-wasp colony, which could have a
demographic effect on common-wasps. If the exotic hornet manages to
significantly reduce the populations of common-wasps, honey-buzzard´s
populations could decrease unless they adapt their trophic preferences
and hunting behaviour to compensate for the decrease in its staple prey.
There are examples where an invasive species affects the staple prey of
a raptor, leading to its local extinction (Spencer \emph{et al} . 1991).
Future long-term research should study the evolution of honey-buzzards´
preferences for Asian-hornets\emph{.} Additionally, the effects of this
species on ecosystem services provided by European native wasps and on
the density of predators specialized in the consumption of native wasps,
such as honey-buzzards, should be considered.

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\textbf{Figure and table captions}

\textbf{Table 1:} Abundance of vespid species in the comb remains
collected in honey-buzzard nests and surroundings. The abundance was
estimated as a percentage of cells of each species relative to the total
of large and small cells collected. The annual data for the period
2018-2021 and total are shown.

\textbf{Table 2:} Percentages of prey items in honey-buzzard's diet.
Large cell combs were considered ``Asian-hornet'' and small cell combs
``common-wasp'' (see \textbf{Table 1} ). For vertebrates, we
distinguished Lacertidae and Anguidae for reptile items, and feathered
(adults, fledglings, and older nestlings) and chicks (without feathers)
for avian items. The annual data for the period 2018-2021 and total are
shown.

\textbf{Table 3:} Relative abundance of vespid species in the diet of
the studied nests of honey-buzzards estimated from the number of cells
consumed, and relative abundances of vespids species in the nesting
territories estimated by baited traps. Ivlev selectivity index (E) was
calculated for Asian-hornet and common-wasp in 2020 and 2021.

\textbf{Table 4:} Daily rate of prey items delivered to honey-buzzard's
nests. Large cell combs were considered ``Asian-hornet'' and small cell
combs ``common-wasp'' (see \textbf{Table 1} ). For vertebrates, we
distinguished Lacertidae and Anguidae for reptile items, and feathered
(adults, fledglings, and older nestlings) and chicks (without feathers)
for avian items. The annual data for the period 2018-2021 and total are
shown.

\textbf{Figure 1:} Interaction effect of vespid species and number of
nestlings over the number of cells of the combs delivered to
honey-buzzard's nests. Common-wasps show more cells per comb than
Asian-hornets and this difference is greater in nests of one nestling (a
- a' / b'- b). The isolated effect of the number of nestlings was not
significant.

\textbf{Figure 2:} Interaction effect of the age and number of nestlings
in the proportions of the four main types of prey delivered to the
honey-buzzard nests: common-wasp (light grey), Asian-hornet (median
grey), reptile (dark grey), and birds (black). Proportions are shown
stacked. A significant effect was observed in the interaction of age and
number of nestlings.

\textbf{Figure 3:} Effect of the year in the proportions of the four
main types of prey delivered to the honey-buzzard nests: common-wasp
(light grey), Asian-hornet (median grey), reptile (dark grey), and birds
(black). Proportions are shown stacked. Note that the proportions are
slightly different from \textbf{Table 2} as here the model weighs the
random effect of the nest.

\textbf{Figure 4:} Effect of the number and the age of nestlings over
the number of prey delivered daily to honey-buzzard nests. Age did not
show a significant effect in the rate of prey delivered, only the number
of nestlings. The decrease in the rate of prey delivered to the nests
with the age was not significant.

\textbf{Table 1:}

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