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, 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 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 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.