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).