Groups of Animals
Many types of animals gather in groups. We have names for them: a herd of elephants, a school of fish, a swarm of bees, there are countless examples. And of course we have a flock of birds.
But not all animals that hang out together do so for the same reasons. And there are different kinds of groups. They don’t all have the same membership requirements.
The words herd, swarm, and flock conjure images of many individuals of the same species. They look more or less the same, like someone just copied and pasted one animal over and over.
When every bird in a flock is of the same species, biologists would say the flock is a monospecific group. But some flocks are heterospecific. These comprise two, three, or perhaps many different species.
There are advantages and disadvantages to living in solitude versus living in a group. Similarly, there are pros and cons to being a member of a monospecific versus a heterospecific group.
For example, one challenge for a bird when hanging out with members of its own species is that it and all its buddies are going to compete for the same limited resources. These birds all eat the same food, seek the same habitat for shelter, and desire the same high quality mates. Competition for these things can make life a bit stressful in a monospecific group.
Now, a further distinction is that not all heterospecific groups of animals are created equal. Only in some of these groups are individuals benefiting from the presence of the other species. In other words, just because you see animals of several species milling around in the same place at the same time doesn’t mean their actions are coordinated for their mutual benefit. It’s possible that each animal is just attracted to a resource in that place, so they all end up there.
Watering holes in Africa are one example. A watering hole can be crawling with giraffes, zebras, antelopes, and many other animals. Another example is when many shorebird species are poking around on a mudflat. In these cases, the critters aren’t really working together or exchanging useful information. And they aren’t in those places because of social attraction. The zebra would go to the watering hole even if there were no other animals around. And the sandpiper would forage on the mudflat even if it was all by itself.
Think about your backyard birdfeeder. If you do have a feeder, you might sometimes have 3 to 10 bird species all flitting and scrambling around it at once. There's no question that you'd be looking at a heterospecific group.
But is each species benefiting from the behavior of the other species? Or are they there only because they’re all attracted to the same food? Usually, it’s the latter. The different species you’re seeing aren’t there because they expect to benefit from the presence of the other species. We’d call this a feeding aggregation, rather than a mixed-species flock. Each bird is simply there to stuff its face with your expensive birdseed. And each of them would probably love to be the only bird on the scene, having those seeds all to itself. The subject of this blog—the mixed-species foraging flock—differs greatly from what we see at backyard feeders. Let’s look at why that is.
Mixed Species Flocks - Characteristics
First off, the birds in such a flock travel around together. For example, a flock with 6 bird species might land in an oak tree. They spend 10 minutes devouring as many insects and spiders as they can find, then take off as a cohesive group to find another tree.
While this mixed flock is foraging, each species is usually targeting a different prey item. So that's another key characteristic: each species in the flock often eats something different. One might eat spiders, one might eat caterpillars, and one might eat insect eggs. This means there's probably less competition among individual birds than what we observe in a single-species flock.
Mixed-species flocks are almost always composed of birds that eat arthropods—that is, insects, spiders, and the like. So these are insectivores. We don’t see this phenomenon as much with birds that eat other things, such as fruit or seeds or small vertebrates.
Even when more than one species in the flock prefers the same food, they often search for prey in different microhabitats. For example, imagine a flock that lands in a large fig tree. While one species is scooching around on the tree’s trunk, probing into bark crevices, another species is peering into curled up clusters of dead leaves. A third species is meticulously gleaning bugs from green twigs.
So the habitat in this example might be the canopy of a broadleaf evergreen forest, but there can be many microhabitats within the canopy. Or multiple microhabitats in a single tree.
While we’re talking about trees, I should point out that mixed-species foraging flocks are most common in forests. They don’t occur as often in more open ecosystems, like grasslands or wetlands.
These flocks are not as large as many single-species flocks that you might be more familiar with. Mixed-species flocks usually include only between 10 and 40 individuals. But some can have more than a hundred birds.
And, usually, no single species within such a flock is represented by more than a few individual birds. So, for example, your flock might have 10 individuals from 7 species. Or maybe 25 individuals from 18 species. You get the idea.
Another important characteristic of mixed-species flocks is that their members play one of two roles: leader or follower. One or a few species in the group will act as the leaders and the rest will be their followers.
Leader species, often called nuclear species, are the birds that initiate the formation of the flock. They sometimes attract other species intentionally, by making certain calls, for example. Leader birds tend to be the ones flying out in front when the flock moves to a new location. So they take the initiative to dictate the movement patterns of their flock.
Many unrelated species around the world fill this role—the role of the nuclear or leader species. Depending on where you are in the world, the leader of a mixed flock might be a chickadee, a tanager, a babbler, or a fairy wren… to give just a few examples.
As wildly diverse as these species are, they share several traits that apparently make them great leaders. First off, they tend to be noisy, chatty birds. Some call often and loudly to communicate with members of their own species. They do this whether or not they’re in a mixed flock. This is the case for members of the family Paridae, such as tits and chickadees. These little buggers call to each other frequently.
Many leader species are vigilant and make loud alarm calls when predators like hawks show up on the scene. They can serve their flock by acting as sentinels, warning of danger. Drongos, birds of the family Dicruridae, often show this behavior.
Leaders use their calls not only to initiate a mixed species flock, but also to keep the flock together as it moves from place to place. Another feature common to a number of leader species is that they’re cooperative breeders. Cooperative breeding is where chicks are raised by not only their parents, but by several helper birds. Helpers are usually older siblings or other close relatives.
The social cues and warning calls used by cooperative breeders might predispose them to be leaders in mixed flocks. Babblers in the genus Turdoides are good examples of cooperative breeders who often assume the leader role in mixed flocks.
Follower species, too, share some features. Most followers—also called attendants—are small, tree-dwelling birds that eat insects or other arthropods.
Leaders and their followers often move around in the forest together in a predictable daily pattern. As they travel, some individual birds will join the group while others leave. For example, a bird might join the flock as it moves through her territory. Then, as the flock reaches the edge of her territory, she drops out and lets the other birds move on without her.
So the size of a flock and its mix of species changes throughout the day and from one day to the next. Flock size and composition might also change over the course of a season. In any case, there will always be some leaders present in the core of the flock.
Not surprisingly, the species compositions of flocks also vary from region to region. In the temperate latitudes of North America and Eurasia mixed flocks are mostly a winter phenomenon. So, this happens in the non-breeding season. Birds that flock together in these northern regions are tits and chickadees, nuthatches, treecreepers, kinglets, and woodpeckers. In North America, wood warblers from the family Parulidae also join bird parties.
In the New World and Old World Tropics, mixed flocks occur year round. In the Neotropics of South America, the most common participants are various cardinals, warblers, sparrows, and tanagers. Different combos of species form flocks in tropical Asia, Africa, Madagascar, and Australia.
Now we have a sense of what a mixed species foraging flock is. But we can’t just leave it at that, can we? As curious naturalists, our next step is to ask, “Why?” Why do so many birds all over the world band together with other species?
Hypotheses for Why
Ornithologists have long pondered the remarkable phenomenon of mixed-species flocks. They’ve been trying to figure out why birds do this for at least a hundred years.
There are two main hypotheses. The first hypothesis is that birds have better chances of finding food when they flock together with other species than when they forage alone. Or they at least improve their efficiency in finding food.
One way this might work is by flushing. A hyperactive gang of small birds flitting around in a tree canopy will inevitably stir up some insects, causing these prey animals to get spooked and reveal themselves. As the prey are flushed from their hiding places, the birds pounce on them.
You might think, “Okay, but wouldn’t that also be true for monospecific flocks… flocks where all the birds are the same species?” Indeed, that’s true. But let’s remember that the species in our mixed flock generally aren’t eating the same types of prey. When they flush prey, they aren’t all fighting over the same insect or other invertebrate. That is what happens in a single-species flock, since every bird is homing in on the same prey.
The second hypothesis for why a bird joins a mixed species flock is that the bird is reducing its risk of being eaten by a predator. There are several ways birds could gain this tremendous benefit. First, the more eyes you have in a group, the more likely a raptor or other predator trying to swoop in will get noticed. Biologists call this the ‘Many Eyes Effect.’
On top of that, some species are more vigilant than others. So it would be smart to hang out with those dudes if you want to get a ‘head’s up’ about any danger while you’re having lunch. A bird may also benefit by being less likely to be singled out by a predator. That's because a predator has many other birds to choose from in the flock. This is called the ‘Dilution Effect.’
But, you could argue, a bird doesn't need to join a mixed-species flock to gain the benefit from the many eyes effect or the dilution effect. Mixed-species foraging flocks do exist, and this phenomenon is widespread across the world. So there must be some strong evolutionary or ecological forces that cause birds to show this behavior. There must be advantages for these birds to join up with other species. Otherwise, they’d just go it alone. Or they’d hang out with only members of their own species. Even if there are some downsides, some costs, to being a member of a mixed flock, the benefits must generally outweigh those costs.
So we have these two big hypotheses: Birds join mixed species flocks because (A) they improve their feeding efficiency or (B) they reduce their risk of being swallowed by a predator.
But this isn’t really an A or B situation. These hypotheses aren’t mutually exclusive. So it’s actually A and/or B. Ornithologists have been working hard for many years to figure out which hypothesis best explains why mixed-species flocks exist. Countless experiments and observations by these scientists have added a lot to our understanding of how mixed flocks operate and why they form in the first place.
Much of this research has involved hardcore field work. Imagine running through a dense jungle in Brazil. You’re following a restless mixed-species flock of about 30 birds. You try not to stumble as you push through the undergrowth. You wipe sweat and maybe a few spiders from your brow, then crane your neck to aim your binoculars straight up into the canopy. The tiny, colorful birds are scampering around in the branches. Many of them are just silhouettes against the glaring tropical sky, making it a challenge for you to identify which species they belong to. You’re calling out species names and numbers of individuals to your colleague, who is frantically recording the data. You barely finish counting the birds—much less catch your breath—before the flock shoots off again in a random direction. You run to catch up. Field work like that can be exhilarating and lots of fun, but also totally exhausting and sometimes frustrating.
Best Explanations
Across the many scientific studies of mixed-species flocks, evidence has been found that supports both the ‘feeding efficiency’ hypothesis and the ‘reduced risk of predation’ hypothesis.
But in 2009 a study was published that tried to find a consensus among most of the previous studies of mixed species flocks. This large-scale study, published in the journal Animal Behaviour, used a meta-analysis to look at data from 191 earlier studies.
Meta-analysis is a really cool thing. This is where data from multiple scientific studies is pooled and analyzed all at once with a special suite of statistical tools. A meta-analysis gives us a sort of weighted average of all the included studies. It allows us to find the big trends or patterns in all that data. Such trends might be hard to detect if we just looked at each of the original studies on their own.
So there were some interesting conclusions from this 2009 study. Importantly, the authors determined that the biggest driving force behind mixed-species flocks is predation. A couple results in their study led to that conclusion.
First, it turned out that birds who join mixed flocks tend to be small insectivores that forage high up in trees. I told you about this result earlier. This meta-analysis study is what really solidified our understanding. Little birds that hunt bugs in the canopy are more vulnerable to predators than larger birds that forage closer to the ground.
Second, this study found that birds in mixed-species flocks spend more time foraging and less time being vigilant. That’s when compared to birds that forage on their own or in single-species flocks.
So, even though the threat of predators is the key driving force here, it seems we have support for both of our original hypotheses. Vulnerable birds have less risk of being torn to shreds by a predator if they join a mixed flock. But because they can relax a bit, they spend less time looking around nervously for predators. After all, there are several other species in the flock that are really good ‘watchdogs,’ and those guys will sound the alarm if there’s danger. So our small, vulnerable species can be more efficient in their never-ending quest for snacks. They benefit by having a lower risk of predation and greater efficiency in finding food.
A study published over two decades before, in 1984, illustrates this phenomenon in the Downy Woodpecker (Picoides pubescens). This is North America’s smallest and most widespread woodpecker species.
When foraging alone on the bark of a tree, this woodpecker stops and cocks it’s cute little head frequently to look around and listen for any predators. A Downy Woodpecker is like a delicious burrito for a hungry Sharp-shinned Hawk. So the woodpecker has to be vigilant if it wants to survive.
Kimberly Sullivan, the author of the 1984 study, followed Downy Woodpeckers around in a New Jersey forest. Sounds like a good time to me. She documented the birds’ head cocking and feeding behaviors. She found that when Downies joined other species in a mixed flock, they decreased their rates of head cocking and increased their rates of foraging. And these rates changed most dramatically when a Downy Woodpecker joined three or more other birds in a flock. So, in a mixed flock, a Downy is more relaxed. It focuses its attention on finding that next bug. The chickadees leading the flock will let the woodpecker know if there’s trouble.
And Such
Now this all sounds fine and dandy. But does every member of a mixed-species flock gain some benefit? What about the leaders? What’s in it for them?
This isn’t quite as clear. As I alluded to earlier, leaders may just attract followers. There doesn’t have to be a benefit to the leader species for this phenomenon to exist. Maybe some leaders would rather be alone, but like a teenager with a pesky kid brother, they just can’t shake their followers.
That said, it seems likely that leader species do get some benefit from the flushing of insects by the flock. And they might also be less at risk of predation because of the ‘many eyes’ effect and/or the ‘dilution effect.’
As an example, consider the Greater Racket-tailed Drongo (Dicrurus paradiseus). This glossy black bird lives in tropical forests in southern and southeastern Asia. Both sexes have a pair of long, spatula-shaped tail feathers that give the species its common name.
This bird regularly participates in mixed flocks. It plays the role of a nuclear or leader species. It actually mimics the calls of other bird species to attract them.
Once the gang's all together, the drongo also acts as a vigilant sentinel, sounding the alarm if it spots a predator.
The Greater Racket-tailed Drongo has been the subject of multiple studies in India, Sri Lanka, and Myanmar. At least one of these studies showed that drongos in mixed flocks do have increased rates of feeding, compared to when they’re on their own. They apparently benefit from the flushing of insects by the other birds. If they didn’t get some benefit, why would they use mimicry to assemble a flock?
Occasionally, Greater Racket-tailed Drongos can be jerks. While acting as a sentinel in a mixed flock, a drongo makes its alarm call. “Hit the deck everyone, there’s a predator about!” The drongo’s flock mates freeze or hide, and some drop whatever juicy bug they were about to eat.
But this was actually a false alarm. There was never a predator, and the drongo knew that. It just wanted a flockmate to freak out and drop its prey. Then the drongo could swoop over and get a free lunch. This is an example of kleptoparasitism, where one species steals a meal from another species.
Life in a mixed-species flock is complex. Most of the birds must benefit from their association with the other species. Otherwise, mixed flocks wouldn’t exist. But there are trade-offs and some costs too. Like getting your lunch stolen by a low-down, no-good, kleptoparasitic drongo.