If you’re a pied babbler (Turdoides bicolor), you’ve got a bit to worry about while foraging. It’s hard to keep watch for predators and eat your fill simultaneously, and trying to do both yourself would mean that you’re either going to spend a lot of time looking for a predator that might not be there or that you might be oblivious to the predator that gets you. Fortunately for these birds there are individuals that act as sentinels, and as a new study in Current Biology reports, these sentinels appear to be boosting the feeding success of those they are watching over.
Like some other birds species, pied babblers forage in loose groups (about 3 – 15 individuals), looking for insects in the nooks & crannies on the ground. While they’re doing so they’re vulnerable to raptors (no, not those raptors), snakes, and mammalian predators, and about 30% of the time a sentinel is posted. All adult members in the group take turns being the sentinel, spending from less than a minute to over six minutes at their “post,” and birds posted as sentinels are usually the first to spot a predator and give an alarm call. The presence of such sentinels appears to make the group more “comfortable.” When a sentinel is looking out, the individuals spread out into the open and do not look up as often, but when there is no sentinel the individuals move closer together away from more open areas and look up from their foraging with greater frequency.
The influence of the sentinels begs the questions “How do the foragers know when a sentinel is watching?” In the pied babblers, sentinels give out relatively quiet “sentinel calls,” and playback experiments revealed that when these calls were played back foragers spread out and acted like they would if a sentinel was on guard. Foragers themselves may be communicating among each other as well, but vocal cues from the sentinel are certainly important in determining the behavior of the group. Although pied babblers live and forage in relatively open habitats, it’s impossible to feed and be keeping a line of sight on the sentinel to see that they’re there, so the vocal cues certainly serve to let the group know that someone is watching out for predators.
Given such behaviors, it’s not surprising that the presence of a sentinel benefits the foraging group. Since the sentinel calls are continuously reassuring them that someone is watching for predators, foragers can spend more time looking for food and actually increased their food intake when a sentinel was present. When there isn’t a sentinel, they not only have to keep looking up but also are in closer proximity (thus resources in a smaller area are depleted faster), so the activities of foragers definitely benefit from the presence of sentinels.
Why is this bit of natural history significant? Other birds and some mammal species also have sentinels to look out for predators, but there has long been a question of whether the sentinels are being “selfish” or “cooperative.” Signaling the end of a sentinel bout, the sentinel coming “off duty” would benefit from the vigilance of the next sentinel, potentially gaining a substantial return for a few minutes of watchfulness. With the pied babblers, however, the presence of the sentinel benefits the entire foraging group (which probably includes kin), the benefits to the group (i.e. better nutrition, which may be translated to better reproductive success) perhaps being more important in driving the evolution of this behavior. Whether the sentinel behavior of these birds is really “selfish” or “cooperative” is left somewhat murky in the paper (although the authors believe that it is cooperative), and more study is certainly warranted to determine how sentinel behavior might benefit group individuals.
HOLLEN, L., BELL, M., RADFORD, A. (2008). Cooperative Sentinel Calling? Foragers Gain Increased Biomass Intake. Current Biology, 18(8), 576-579. DOI: 10.1016/j.cub.2008.02.078