The southern pied babbler, Turdoides bicolor. Photo by Blake Matheson.Pied babblers (Turdoides bicolor) are sociable South African songbirds, which live and forage for food in groups. During foraging, some adult babblers act as sentinels, perching above the ground to scan for predators, and alerting the rest of the foragers if any danger shows up. Sentinel behavior is cooperative—sentinels free the rest of the group to concentrate on feeding, but sentinels themselves cannot forage. The study's authors, Bell et al. hypothesized that sentinels might communicate how long they're willing to stand watch to the rest of the group, so as to prompt new birds to take up watch and given the current sentinels a break to feed.
The authors first established that how hungry a babbler is determines how long he or she is willing to stand watch, which they did by feeding the birds with meal worms immediately after they concluded a period of sentinel duty. Babblers receiving ten worms returned to duty faster than those receiving just one, and stayed on duty longer. Further feeding experiments and observations established that both foragers and sentinels called to each other less frequently when they were well fed, that sentinels called more frequently the longer they stayed on watch, and that sentinels who ultimately stayed on watch the longest also called the least frequently in their first minute on watch.
So call frequency is the babblers' signal for how badly they want to forage—foragers hearing higher-frequency calls from sentinels should take them as a call for relief; and sentinels hearing lower-frequency calls from foragers should take them as permission to leave the watch and start foraging. To test this hypothesis, the authors played recorded calls to foragers and sentinels, and found that the birds responded as I've just described. The apparent babble of the babblers, then, is actually a perpetual negotiation about who should be on sentinel duty—sentinels complaining when they get hungry, and foragers telling the sentinels, "Not yet! I just need to catch a few more worms."
Bell, M., Radford, A., Smith, R., Thompson, A., & Ridley, A. (2010). Bargaining babblers: vocal negotiation of cooperative behaviour in a social bird. Proc. Royal Soc. B DOI: 10.1098/rspb.2010.0643