Predation risk influences daily decisions by potential prey to avoid predators. Indeed, minimizing predation risk is a primary hypothesis explaining why animals live in mixed-species groups. For example, birds that eavesdrop on alarm calling heterospecifics have been widely documented in a number of forest ecosystems. Thus alarm calling birds may play a central role in 1) maintaining flock cohesion and 2) allow a flock to forage in open areas within the forest that otherwise incur high predation risk. Alternatively, the dilution effect, where “safety in numbers” may drive species to forage together in groups may underlie flock cohesion and microhabitat use. In order to address the question “What mechanisms underlie the cohesion and habitat use of mixed species flocks?” we conducted a short-term removal experiment to test whether flock cohesion and micro-habitat use by flock members was influenced by the presence of alarm calling birds, the dilution effect or both. Antshrikes in the genus Thamnomanes play a major role in Amazonian mixed species flocks by providing alarm calls that are widely used by other flock members. These alarm calls typically provide threat information about ambush predators such as hawks and falcons.
Results/Conclusions
We quantified the percent occurrence of other flocking species, their foraging heights and the vegetation profile of the forest used by the flocks both before and after the removal of antshrikes from the flock. We show that the majority of main flock members decrease the percent of time they spend in the flock, and that the flock microhabitat reflects increased vegetation cover when the alarm calling antshrikes are temporarily removed. The flocks did not completely dissolve, however, suggesting that the dilution effect likely plays a significant role underlying flock cohesion and microhabitat use in this system. We conclude that several mechanisms may operate simultaneously (alarm calling birds and the dilution effect) to maintain flock cohesion and allow birds to forage in “high risk” habitats in lowland Amazonian rainforests.