https://phys.org/news/2018-05-amazonian-lookout-birds-species-dangerous.html

But scientists did have a few clues. One ubiquitous group of flock members are species whose calls alert their neighbors to the presence of threats like hawks or falcons. "People have thought for a while that maybe these 'sentinels' promote the ability of other species to use risky parts of the forest," said lead author Ari Martinez, who was a postdoctoral researcher at SF State during the study. By opening up new habitat to their neighbors, they might also bring together species that would normally flock on their own.

To put that idea to the test, the team captured alarm-calling dusky-throated antshrikes (Thamnomanes ardesiacus) from eight mixed-species flocks in southeastern Peru and kept each bird in an aviary for several days.