Elsevier

Animal Behaviour

Volume 83, Issue 2, February 2012, Pages 403-410
Animal Behaviour

Kleptoparasitism by attacks versus false alarm calls in fork-tailed drongos

https://doi.org/10.1016/j.anbehav.2011.11.009Get rights and content

Many animals steal food, termed kleptoparasitism, but attacks are frequently unsuccessful and may incur costs. Some species produce false alarm calls during kleptoparasitism, but the relative benefits of this versus physical attacks are not known. We investigated the kleptoparasitic behaviour of a population of individually recognizable and habituated fork-tailed drongos, Dicrurus adsimilis, which kleptoparasitize food using both false alarms and physical attacks. Drongos made false alarms more frequently when kleptoparasitizing small food items, probably because these can be more profitably stolen by false alarm than attack. False alarms were also more frequent when targets were larger species, which are likely to be more costly to attack since they more frequently defended food. Drongos were able to attack targets when a false alarm had failed but rarely did so, even though such attacks were no less successful than attacks when no false alarm had been made. It therefore appears that false alarms increase opportunities for kleptoparasitism because calls can be made when attacks are unprofitable. False alarm calls also appear to benefit drongos by increasing overall success in kleptoparasitism, because attacks could be made following failed false alarm calls. This is the first study to show the functional benefits of making false alarm calls, illustrating how they may be deployed strategically in kleptoparasitism when attacks would be less profitable or even unprofitable. More generally, false alarms may evolve where animals engage in physical conflict over resources because they can benefit individuals by decreasing the costs of competition.

Highlights

► We investigate the benefit of using false alarm calls to steal food. ► Study on fork-tailed drongos which steal food using attacks and false alarms. ► Made false alarms when attacks costly, increasing opportunities to steal food. ► Increased overall success as attacks could still be made when false alarms failed. ► Principle benefit of false alarms is that they reduce costs of conflict over food.

Section snippets

Study Site and Population

We studied a population of fork-tailed drongos in the South African Kalahari Desert (26°58′S, 21°49′E) from March to July 2008 and April to September 2009. Details of the habitat and climate have been published elsewhere (Clutton-Brock et al. 1999). The study population consisted of 64 drongos, habituated to observation at less than 5 m. Each individual was captured using walk-in traps baited with mealworms, triggered by an observer using a pull string to avoid injury from accidental trap

Use of False Alarms versus Attacks

Fork-tailed drongos made false alarm calls in 55 ± 4% of kleptoparasitism attempts (N = 822 kleptoparasitism attempts by 64 drongos; Fig. 2). They made false alarm calls more frequently in kleptoparasitism attempts for small food items than in attempts for large items (Appendix Table A1, Fig. 3a). Adult drongos were also more likely to make false alarm calls when targeting larger species than smaller species, although juvenile drongos did not differ (Appendix Table A1; Fig. 3b). Further analysis

Discussion

Fork-tailed drongos are likely to benefit from making false alarm calls when attempting to kleptoparasitize food because they avoid the costs of kleptoparasitic attacks. Drongos made false alarm calls more often in kleptoparasitism attempts for smaller food items despite false alarms being no more effective than attacks and food items of different sizes being no more or less frequently defended. They also made false alarms more often when targeting larger species, which defended their food more

Acknowledgments

Access to the study site and meerkats was kindly provided by the Kalahari Research Trust, T. Clutton-Brock and M. Manser. This work would not have been possible without the support of A. Ridley who initially established the drongo study population and provided access to the pied babblers. We thank two anonymous referees for their very helpful comments on the manuscript. Many thanks to N. Davies for supervision and M. Nelson-Flower, R. Sutcliffe, J. Sampson, M. Bell and the Bird Behavioural

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