Dabblings with Babblers: Revelations in social behaviour from the Pied Babbler Research Project

Southern Pied Babblers. Fledgling begging for food. Image by Robbie Hopper.

Robbie Hopper graduated from the University of Edinburgh in 2013 in Zoology. She worked for eight months at the Pied Babbler Research Project, and is now studying biology, gender and philosophy independently while she figures out what to do next. Blog coming soon at fibihopr.wordpress.com! 

It’s cold and still in the Kalahari desert before sunrise. Bright stars break through the darkness, dense with the calls of night-time insects. But a new chorus is rising with the light, and as the horizon begins to redden, bird calls fill the air in swift, darting cacophony. To searching ears, however, one laughing call rises above the others in distinction…

Southern pied babblers (Turdoides bicolor) are medium-sized black and white passerine birds, resident to the Kalahari desert. Living out in such wilderness, and standing at less than fifteen-centimetres tall, they might easily escape your notice. However, to those with an interest in social behaviour they provide a fascinating study system, which, thanks to the efforts of the Pied Babbler Research Project, has come under the global scientific spotlight.

The Pied Babbler Research Project sits nestled in the heart of the Kuruman River Reserve in the Northern Cape, South Africa. Today, ten years since its conception, it remains under the principle leadership of Associate Professor Amanda Ridley of the University of Western Australia, who founded the project with the help of her field assistant (and now research colleague), Dr Nichola Raihani in 2003. The project is a prolific international operation, with researchers coming from institutions in Australia, Switzerland, the United Kingdom and South Africa to study the babblers and their associated species.

Pied Babbler sentinel. Image by Alex Thompson, CC BY-SA 3.0.

The “white cat-laughers”, as they’re known in Afrikaans, certainly provide much matter for intellectual occupation. They are cooperatively breeding passerines, near-endemic to semi-arid acacia savannah habitats in this part of southern Africa. They live in stable groups that consist of a dominant male and female, their immature offspring and a number of subordinate adults – helpers, which are usually (but not always) related to the dominants, and assist in feeding and protecting the family. Within the confines of their permanent territories, which they defend from neighbouring groups by use of overt group vocal displays, they breed seasonally and forage daily, using a sentinel system, by which group members alternately stay on guard and warn the rest of the group of any approaching danger.

One key focus of babbler research is their intelligent vocal capabilities. They communicate constantly using a diverse repertoire of sounds. Whilst foraging for food, each individual gives low-pitch close calls, known as “chucks” to indicate its position to other group members. Chuck calls function as a spacing regulator whilst foraging, so that individuals don’t encroach on each others’ feeding patches[1]. Raising the pitch of the chuck a little tells the group that a large, shareable food source has been found[2].

Studying babblers in Kalahari. Image courtesy of Rute Mendonça.

Whilst others are foraging on the ground, often one babbler will take a higher position – up an Acacia tree, say – and act as sentinel, watching for approaching for predators. While all is clear the watching babbler gives regular sentinel calls, which reassure the feeding birds that they can continue at ease, spreading out widely from each other without needing to look up as often to check for danger[3]. On sight of a predator, such as a mongoose or a pale chanting goshawk, the sentinel gives an alarm call that informs the rest of the group of the threat so that they can seek cover. The higher the predation risk in their environment, the more sentinel activity the babblers will undertake[4].

The babblers’ vocal repertoire also includes “purr calls” that are directed from adults to offspring during both nestling and fledgling stages. Playback experiments revealed that nestlings respond to adult purr calls with begging, and are obliged with food[5]. Adults also use purr calls in direct conjunction with feeding fledglings, and use the calls to lure fledglings to foraging patches or away from predators[5][6][7]. These findings demonstrate that the babblers condition their young as nestlings to respond to the purr calls, and then they use this conditioning to teach older chicks essential skills in foraging and predator avoidance. Such deliberate and purposeful teaching of young by adults has been rarely demonstrated previously[8].

Researchers have also discovered that babbler fledglings communicate with their parents non-vocally: in what is thought to be a display of blackmail, fledglings spend more time on the ground, where there is a higher risk of predation, than in the safety of trees when hungry. Whilst on the ground, they are fed more by the foraging parents and helpers than when they are in the tree. Thompson et al cite this as support for Zahavi’s idea, that the fledglings risk their own mortality in order to attain higher provisioning rates by the parents[9][10].

Fork-Tailed Drongo. Image by Robbie Hopper.

The babblers’ communications, however, are not only of interest to humans: Fork-Tailed Drongos (Dicrurus adsimilis), another passerine bird which often lives in association with the babblers, pay close attention to the babblers’ vocal exchanges. The two bird species form a distinct relationship, in which each can benefit from associating with the other (although not always do). The drongos sentinel for foraging babblers groups whilst awaiting opportunities to steal their spoils. Normally solitary foragers, the drongos use their association with the babblers to gain more energy-rich meals than they would normally be able to – drongos weave impressive flight patterns to pick insects out of the air, but do not have the babblers’ digging expertise to enable them to uncover big prey items like burrowing skinks or scorpions. Therefore, they gain the babblers’ trust by acting as trustworthy sentinels, truthfully alerting babblers to the presence of both aerial and ground predators by copying their alarm calls. Once in a while, however, if a tasty morsel has been found, the drongos will give a fake alarm so that the babblers dive for cover, leaving their food open for the taking by the drongo sentinel.[11]

Babblers tolerate drongo presence with a number of conditions. Firstly, a drongo will probably not be tolerated to accompany a babbler group if they are already large enough in number to provide good sentinel coverage themselves. In these instances, drongos will be driven away from the group. In smaller groups, however, the babblers benefit from having drongo company, and so accept them as sentinels while they hunt for food. Secondly, if a drongo abuses the understanding and fake alarms too often, the babblers will cease to respond to it, and it will be driven away from the group. Babblers also sometimes fight back for their stolen food, resulting in mid-air tug-of-war. Older, more wiley babblers may not even respond to the drongos at all, and check for themselves whether or not the alarm was genuine before leaving their food vulnerable[11].

This is but a taster of the work that has been going on at Babbler Project over the years. If this article has sparked your interest, check out the project website for news and publications.

References

[1] Radford A.N. & Ridley A.R. 2008. Close calling regulates spacing between foraging competitors in the group-living pied babbler. Animal Behaviour 75: 519-527.

[2] Golabek K. 2011. Vocal communication and the facilitation of social behaviour in the Southern Pied Babbler (Turdoides bicolor). PhD Thesis, University of Bristol, Bristol.

[3]Hollén L.I., Bell M.B.V. & Radford, A.N. 2008. Cooperative sentinel calling? Foragers gain increased biomass intake. Current Biology 18: 576-579

[4]Ridley A.R., Raihani N. & Bell M.B.V. 2010. Experimental evidence that sentinel behaviour is affected by risk. Biol. Lett. 2010 6, doi.10.1098/rsbl.2010.0023

[5]Raihani N.J. & Ridley A.R. 2007. Adult vocalizations during provisioning: offspring response and postfledgling benefits in wild pied babblers. Animal Behaviour 74: 1303-1309

[6]Radford A.N. & Ridley A.R. 2006. Recruitment calling: A novel form of extended parental care in an altricial species. Current Biology 16: 1700-1704

[7]Raihani N.J. & Ridley A.R. 2008. Experimental evidence for teaching in the wild pied babbler. Animal Behaviour 75: 3-11

[8]Rapaport L.G. 2006. Parenting behaviour: Babbling bird teachers? Current Biology 16(17) DOI: 10.1016/j.cub.2006.08.012

[9]Zahavi A.1977. Reliability in communication systems and the evolution of altruism. In Evolutionary ecology (eds B Stenhouse, C Perrins), pp 253-259. Baltimore, MA: University Parks Press.

[10]Thompson A.M., Raihani N. J., Hockey P. A. R., Britton A., Finch F. M. & Ridley A. R. 2013. The influence of fledgling location on adult provisioning: a test of the blackmail hypothesis. Proc. R. Soc. B 280, 20130558.

[11]Ridley A.R. & Raihani N.J. 2007. Facultative response to a kleptoparasite by the cooperatively breeding pied babbler. Behavioural Ecology, doi:10/1093/bebeco/ar1092