Elsevier

Animal Behaviour

Volume 84, Issue 4, October 2012, Pages 1013-1022
Animal Behaviour

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

https://doi.org/10.1016/j.anbehav.2012.07.027Get rights and content

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.

Highlights

► Fork-tailed drongos kleptoparasitize pied babblers. ► We analyse bill morphology and foraging differences between drongos and babblers. ► Drongos target excavated prey when kleptoparasitizing babblers. ► Drongos possess significantly shorter, thicker bills than babblers. ► Procuring subterranean prey may partially incentivize kleptoparasitic behaviour.

Section snippets

Study Site and Field Methodology

Field work was carried out at the Kuruman River Reserve, Northern Cape Province, South Africa (26°58′S, 21°49′E) between 28 April and 28 May 2006. The study site is located in the southern Kalahari Desert and is characterized by semiarid grassland and acacia savannah, with an average rainfall of 217 mm per annum (see Ridley & Thompson 2011 for a detailed description of the habitat). All drongos and babblers in the study population were habituated to allow observation at a distance of 2–3 m

Bill Morphology

The differences between drongo and babbler museum specimens and specimens from the study site were qualitatively similar (average differences between museum specimens and between study site specimens: bill depth: 0.4 and 1.2 mm; gape width: 1.2 and 0.8 mm; bill width: 1.9 and 2.2 mm; head width: 0.4 and 1.7 mm; bill length: −2.8 and −2.9 mm; bill slope: 9.4 and 10.5°). The study site specimens were thus included in the analyses (all results were checked for qualitative differences by running the

Discussion

Our results show that drongo and babbler bill morphology is significantly different, with drongos possessing shorter, stouter bills and wider gapes and babblers possessing longer, decurved and thinner bills. By combining morphometric data with detailed foraging observations we have demonstrated that pied babblers and fork-tailed drongos occupy considerably different foraging niches, and that this might provide an explanation for the targeted kleptoparasitic attacks on buried larvae and pupae

Acknowledgments

We thank the Northern Cape Conservation Authority for research permits. The Kalahari Research Trust, Mr and Mrs H Kotze and Mr and Mrs de Bruin kindly allowed us access to their land in the southern Kalahari. We are grateful to Denise Hamerton and Vincent Bartnik of the South African National Museum as well as Tamar Cassidy of the Transvaal Museum for their hospitality and help. We thank all researchers at the Kuruman River Reserve for support and encouragement over the duration of our study

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