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The saying “birds of the same feather flock together” takes on new meaning thanks to a study published in the Philosophical Transactions of the Royal Society B. Flocked birds often travel in groups consisting of a single species, in which individuals are almost indistinguishable from each other, as the proverbial saying goes. But something strange is happening in flocks of two or more Southeast Asian species. Even when the species in the herd are far apart, they always seem to converge on the same appearance, as if trying to fit in.
“They all share random traits, like crests or yellow bellies, which makes them nearly identical. You can’t really tell them apart without looking at their markings,” said study co-author Scott Robinson, a senior Ordway researcher at the Florida Museum of Natural History.
According to Robinson, this plumage similarity is likely a type of mimicry, which in itself is not uncommon among birds. Alfred Russel Wallace, the co-discoverer of natural selection, was the first to suggest that certain bird species engage in mimicry when noting the similarities between orioles and friarbirds in Australia. Birds can mimic each other to reduce the aggressiveness of a dominant species; look like a more formidable adversary to predators; and, in at least one case, to pass themselves off as toxic.
But resemblance in multi-species herds is something different, said lead author Rebecca Kimball, a professor of biology at the University of Florida.
“In mimicry, you often want to look like something because there’s an advantage to being that something else. You want the species to think you’re poisonous or unprofitable prey,” she said. “In flocks of birds, one idea is that it has more to do with a predator’s ability to isolate a target. When there is a flock of birds moving around, it may be easier for predators to identify an individual that has a distinct color pattern.
The Red-backed Sibia is a separate genus from the Himalayan Cutia (shown above). Photo by Wenyi Zhou
Safety in collective darkness
This idea that unrelated birds find safety in collective darkness was first proposed in the 1960s for flocks along the Andes. But follow-up studies have failed to show conclusive evidence of mimicry in Andean multi-species herds, and the theory has been largely abandoned.
“The idea lay dormant for a long time,” Robinson said. But in 2010, Robinson began working with a Chinese colleague in Yunnan province and observed what appeared to be the same phenomenon that had been described decades earlier.
Robinson and his colleagues spent the next few years documenting the similarities in China’s multi-species herds, finding the same pattern over and over again. While some of the similarities between the species are subtle, the authors point to several visually remarkable examples.
In Western Asia, the Himalayan Cutias (Nipalensis box) they look like they’re dressed in mismatched coats, with a black feather mask, brown wings, and white zebra-striped chests. This pattern may seem impervious to emulation, but the Red-backed Sibias (Leioptila annectens) they flock to make a remarkable good impression. Both species have similar behavior, foraging patterns, and markings, except for stripes, which sibias lack.
Some birds also seem able to mimic more than one species as they grow. White Hooded Juvenile Bulletin Boards (Gampsorhynchus rufulus) have rusty head feathers, brown wings, and creamy bellies, similar to the parrots they congregate with. Adults look like an entirely different species, with white heads and dark brown wings that resemble white-crested laughing thrushes (Garrulax leucolophus), all of which are part of the same herd.
Somewhat counterintuitively, this conformity within multispecies herds may contribute to diversity in the region. Not only can birds mimic more than one species at different stages of development, but their appearance can also vary across their range. In eastern China, great tits accompany birds with prominent crest feathers, which they mimic. In the Himalayas and Hengduan Mountains further west, the same species lacks a crest and congregate with other crestless birds.
If these differences persist long enough, Robinson said, it could eventually result in one species becoming two. “The possible role that this type of mimicry plays in speciation is the most interesting idea from our point of view. Many of these birds have wide ranges, and there can be a lot of differentiation in those traits involved in grouping within a species.
Why herd mimicry is important in China
There seem to be two main ingredients needed to create this kind of mimicry in herds, both of which could help explain why this pattern seems to be so prominent in China but absent elsewhere.
First, Robinson said, a herd should consist of just a few species, with some being more common than others. “When you have a herd with a really dominant and abundant species, there is a model worth emulating. If other birds look like this pattern, they get the same protection, they have access to the same resources, and they can travel with a compatible group.
In other parts of the world, many herds have more of an open-door policy, weakening the selective forces that contribute to mimicry. Mating pairs of several species come together in disparate groups, often relying on the warning calls of sentries to avoid predators rather than their ability to blend into the background.
The second ingredient is the winnowing fan of predation. For small and medium-sized birds, the greatest source of danger comes from above, in the form of raptors, and the skies above Southeast Asia are particularly cloudy. The region covers only 3% of the Earth’s land area, but it is home to nearly 30% of all raptor species. This puts enormous pressure on herds, Robinson said, which can promote mimicry.
To determine whether the flock species similarity is the result of mimicry, the authors say they will need to perform large-scale genetic analyzes to rule out other potential causes.
“The extent to which mimicry is prevalent among birds is something that has only recently been appreciated,” Robinson said. “Taxonomy work is based on appearance, with the assumption that birds are closely related if they look alike. Now that we can study DNA, we realize they often look alike because they live together.
Thanks to the Florida Museum of Natural History for providing this news.
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