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.