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Arthur T Knackerbracket has found the following story:

Usually, birds of a feather flock together—but in the Amazon, some flocks feature dozens of species of all shapes and colors. A new study by San Francisco State University biologists singles out one reason why these unusually diverse flocks exist: lookout species that call in alarm when they spot dangerous predators.

[...] 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.

After the team removed the antshrikes, birds in each flock responded in a matter of hours. In three flocks, birds retreated to areas of denser cover at the same vertical level in the forest, while in another the members joined new flocks high in the canopy, another area that affords more cover from predators. Birds in control flocks, where the researchers captured antshrikes but immediately released them, tended to stay out in the open. The team reported their results today in the journal Ecology.

The results support the idea that alarm-calling species might allow their neighbors to live in dangerous neighborhoods. "These flocks occupy a middle layer of the rainforest that's not quite the ground and not quite the canopy," explained coauthor Eliseo Parra, a lecturer and researcher at San Francisco State. "A lot of literature suggests that this area is more dangerous. There are more opportunities for a predator to be hidden and still have a quick flight path." Remove the antshrikes and their former flockmates are left exposed, so they retreat to safer habitats.

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