Evidence for Convergent Evolution in Gape Morphology of the Bat Hawk (Macheiramphus alcinus) with Swifts, Swallows, and Goatsuckers

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ABSTRACT

Gape morphology has been linked to feeding and breeding ecology in raptors, according to the ingestion rate hypothesis. Mammal feeding raptors have larger gapes, allowing them to ingest prey more rapidly than bird feeders, which have evolved smaller average body sizes and gapes to capture more agile prey. One highly derived raptor, however, the Bat Hawk (Macheiramphus alcinus), specializes on colonial bats and swiftlets concentrated daily in a limited temporal window by capturing and swallowing them whole in flight. We hypothesized that the gape of the Bat Hawk evolved to feed rapidly on agile vertebrates limited temporally. We predicted that the gape of the Bat Hawk would be significantly larger than the gape of other raptors, more closely resembling the gapes of swifts (Apodidae), swallows (Hirundinidae), and goatsuckers (Caprimulgiformes). We measured gape area of the lower mandible in museum specimens representing 138 bird species in six orders. We also compared gape area by prey type in over 100 raptor species in three orders. We predicted that insectivorous raptors would exhibit gapes similar to mammal feeders but would differ from bird feeders because insects are not agile prey. The Bat Hawk had the largest gape of any raptor and more closely resembled the gape of insectivorous birds, which also swallow prey whole in flight. The evolution of an enlarged gape may have permitted the Bat Hawk to exploit a previously unrealized ecological niche. Gapes of bird feeding raptors were smaller than in mammal and insect feeders, supporting the ingestion rate hypothesis.

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