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 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 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 (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 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 V. velutina larva is 413 mg and a V. vulgaris larva is 65 mg (Rebollo 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.