The phrase "birds of a feather flock together" is used often because, well, it’s true. Bird species usually stick to their own. Occasionally, however, birds of many different feathers flock together in little multi-species gangs. And we may know why.
Multi-species flocks are not unheard of. In a place like the Amazon, there are so many different creatures living in close proximity to one another, it’s not surprising some have found a way of co-existing.
Researchers at San Francisco State University, however, were curious why these unusual groupings work, and perhaps more unusually, why they seem so stable.
"You come back to the same habitat after 20 years, and the same flocks are using the same areas of the forest," explained study author Vance Vredenburg. "It defies a lot of expectations."
Now, after studying Amazonian mixed-species flocks, reporting their findings in the journal Ecology, the team think they have discovered a key reason why they work so well: “Lookout” birds call out warnings to the rest of the gang when danger approaches, making them integral to these ecosystems, and, consequently, a target for conservation efforts.
One thing these multi-species flocks have in common are “sentinel” birds, those that “promote the ability of other species to use risky parts of the forest," according to lead author Ari Martinez.
By acting as an alarm sounding system, these birds open up parts of the forest to other bird species that may have previously found the habitat too risky, in turn allowing species that may not have previously met to mix and mingle.
To test this, the researchers captured alarm-calling dusky-throated antshrikes from eight mixed flocks found in Peru, keeping them separate for a few days and monitoring the effect it had on the rest of the flock. The response was almost immediate.
Three of the eight groups retreated into denser covered, more protected parts of the canopy, while many others flew higher into the trees to join new flocks. The control groups, however, the ones where the antshrikes were released immediately back into the flock, were less disrupted, sticking together and staying out in the open.
This, the researchers say, supports the theory that if you remove the sentinel birds from these mixed-species flocks the rest retreat to safer habitats, making them a key feature of these ecosystems. They also found that without these key birds the mixed flocks started to separate.
Although not technically a “keystone species” – those that have an integral position within an ecosystem and an unusually large influence on it, like beavers or wolves – the authors argue that these alarm-calling species are vital, and that conservation efforts should focus on them in the future.