Martínez, A. E., Parra, E., Muellerklein, O. & V. T. Vredenburg. 2018. Fear-based niche shifts in neotropical birds. Ecology 99:1338-1346. https://doi.org/10.1002/ecy.2217
Animals use a variety of strategies to avoid predation; one is safety in numbers. Different species have even been observed to work together to decrease their collective likelihood of being captured by a common predator. Some species, however, put in more effort to protect the group than others. Sentinel species, like a group of birds called antshrikes (Thamnomanes), fulfil the role of protector by making alarm calls to warn other birds of an incoming predator. Neotropical birds can form flocks consisting of multiple species, but the birds that aren’t antshrikes benefit from the presence of these alarm-callers without returning the favor. How would they fare if the antshrikes weren’t around? An experimental study was conducted by Martinez et al. (2018) to determine just how much of an impact antshrikes have on the livelihood of other birds.
The antshrikes go on strike
The authors of the study hypothesized that the benefit of having antshrikes around is two-fold: First, the protection from antshrikes allows the mixed-species flock to make use of higher-risk habitat that they would normally avoid (i.e. more open, exposed habitat) and secondly, they allow the birds to remain in a conspicuous but protective flock for a greater amount of time. These hypotheses have been collectively deemed the fear-based niche shift hypothesis. To test their hypotheses, the study authors experimentally removed a species of antshrike, Thamnomanes ardesiacus, from mixed-species flocks found in lowland tropical forest in southeastern Peru. They chose flocks that consisted of the same 10 species of birds, including T. ardesiacus. The habitat use of the flocks, including shifts in their territory and the type of vegetation they used, and the amount of time the birds spent in their flocks was observed before and after T. ardesiacus was removed.
Strategies on strategies
Our sentinel species proved itself to be very useful. Flocks from which T. ardesiacus individuals were removed responded to this change by shifting their home range, presumably to safer habitat. The flocks spent more time in denser vegetation at lower heights in the forest, another indication of an attempt to reduce exposure to predators. Six bird species reduced the amount of time they spent in the flocks after T. ardesiacus was removed, indicating that it was no longer beneficial to spend time in a mixed-species flock without an alarm-caller. However, some flocks that moved to denser and lower vegetation stayed together and some birds in one flock traded in their disbanded flock for other flocks with different sentinel species. It appears that some of the non-sentinel birds don’t care who they’re eavesdropping on.
A landscape of fear
As this study highlights, birds that are not able to adapt and reform allegiances may find that their ecological niche has shrunk due to fear of predation. This study provides evidence of dependency on sentinel species. The relationship between sentinel species and those that benefit from them is not one-sided, however. There’s safety in numbers, and that goes for the sentinel species too. Every bird in a flock benefits from the presence of all other birds because the likelihood of being captured by a predator decreases with more potential prey around. A landscape of fear in which predators restrict the potential niche their prey can fill is further exacerbated by the loss of an avoidance strategy. Should anything happen to sentinel species, and there are many threats to neotropical birds, this combined with habitat destruction may leave other birds with nowhere to hide. The biodiversity of the neotropics must be sustained so that various species can play their important roles.