|Supplementary material – Ridley et al.: Interspecific audience effects on the alarm-calling behaviour of a kleptoparasitic bird.
Study site and population
We studied drongo-pied babbler interactions at the Kuruman River Reserve in the southern Kalahari Desert, South Africa (26º58’S 21º49’E) during March-June and September-November 2006. The study area is semi-arid grassland and acacia savanna (see Raihani & Ridley (in press) for a detailed site description). All drongos and pied babblers in the study population were individually recognisable by a unique combination of coloured rings, and were habituated to close approach (2 – 3 m), facilitating clear behavioural observations.
Predators present at the study site included pale chanting & gabar goshawk (Melierax canorus & M. gabar), black kite (Milvus migrans), yellow and slender mongoose (Cynictis penicillata & Galerella sanguinea), African wild cat (Felis silvestris), puff adder (Bitis arietans) and Cape cobra (Naja nivea). When a predator was first sighted, drongo behaviour and babbler response was recorded for the first sighting only. Subsequent sightings of the same predator were not recorded. Due to the open habitat prevalent at the study site visibility was good, and the likelihood of observers detecting predators less than 100m from the focal drongo was considered high.
Definitions of foraging modes
Foraging modes were defined as follows: (a) solitary: all foraging behaviour occurring in the absence of other species, including aerial hawking, pecking at a substrate, or picking prey off the ground (b) kleptoparasitic: when the focal drongo was less than five metres away from the most peripheral member of a babbler group, watching foraging individuals, and following the group between foraging areas. A kleptoparasitism attempt was considered to have occurred when a drongo swooped down upon a babbler carrying a food item and attempted to steal the item. During both foraging modes, drongos spent most of their time perched in an elevated position until opportunities to hawk or steal food items arose and were considered to be foraging terrestrially only when they were physically on the ground. Both foraging modes were mutually exclusive: focals on drongos foraging solitarily did not involve occasional attempts at kelptoparasitism. In cases where the foraging mode was unclear (i.e. where drongos appeared to follow babbler groups but remained peripheral and often left the group to self-forage) focals were excluded from analysis.
Focals on drongos in kleptoparasitic foraging mode were conducted only when a single drongo was present at a babbler group. Individuals were not focalled twice on the same day (mean interval between focals: 4.2 ± 1.1 days). On some occasions, more than one drongo followed a babbler group. These focals were excluded because of competitive and aggressive interactions between drongos affecting foraging behaviour.
Drongo alarm calls
Drongos primarily responded to predator sightings by either continuously alarm-calling or mobbing the predator in question. Drongos were never observed attempting to steal babbler food items following a true alarm call. To investigate whether drongo true and false alarm calls were similar, we made sample recordings of drongo alarm calls using a Sennheiser MKH416T directional microphone and a Marantz PMD670 solid-state sound recorder. We recorded true alarm calls (when a predator was present) and false alarm calls (when attempting to steal food from a babbler and no predator was apparent) from the same individual (Figure 1). No predators were observed for 15 minutes before or after a false call was recorded. Recordings were stored as Windows PCM files (sample rate: 44 kHz; resolution: 16 bit), and sonograms were generated using Cool Edit 2000 (Syntrillium Software Corporation, AZ, USA).
Figure 1. Sample sonogorams of true and false alarm calls from the same drongo. The true alarm call (a) was directed at a yellow mongoose. The false alarm call (b) was directed at a babbler who had just obtained a large beetle larva, which the drongo successfully stole.
Raihani, N.J. & Ridley, A.R. in press. Adult vocalisations during provisioning: offspring responses and post-fledging benefits in wild pied babblers. Anim. Behav.