Investigating a link between bill morphology, foraging ecology and kleptoparasitic behaviour in the fork-tailed drongo

Fork-tailed drongos, Dicrurus adsimilis, are facultative kleptoparasites of many bird and mammal species. They use complex alarm call strategies, a combination of both true and false alarm calls, to procure food items from their hosts. The evolution of this interspecific interaction may have been catalysed by a selective benefit for drongos that were able to procure a wider diversity of food resources through kleptoparasitism. We tested the hypothesis that drongos, which principally hawk their prey from a perch, employ kleptoparasitism to exploit a terrestrial foraging niche for which they do not possess the appropriate morphological adaptations, as indicated by bill morphology. We analysed the bill characteristics of the terrestrially foraging pied babbler, Turdoides bicolor, a primary host for the fork-tailed drongo, and both drongo and pied babbler museum specimens to establish potential links between differences in functional morphology and foraging ecology. We also observed the foraging behaviour of babblers and drongos in the Kalahari Desert. Drongo bills were significantly shorter, thicker and less curved than those of pied babblers, and large energy-rich subterranean prey items, such as insect larvae, were not procured by drongos while self-foraging. Furthermore, drongos preferentially attempted to kleptoparasitize prey types that they did not capture while self-foraging. We suggest that the calorific and nutrient content of a larger and more diverse set of prey, particularly buried larvae, which drongos did not excavate on their own, establishes a selection pressure for kleptoparasitism and may have contributed to the complex behavioural interactions between drongos and their hosts.