The Science of Birds - Bird Party: Mixed-Species Flocks

Episode Date: December 12, 2020

Sometimes birds of many feathers flock together. Mixed-species foraging flocks are seen in forests all across the world. Different combinations of species are found in each region.First, we talk about what mixed-species flocks look and act like. Then we get into the major scientific hypotheses for why these flocks exist at all.Link to this episode on the Science of Birds websiteSupport the show

It's December in Ohio. You're out doing a little birdwatching in a deciduous forest. Your boots crunch in the snow as you stroll between the trunks of leafless maple and elm trees. So far, it's been a quiet day. The only birds you've seen in the last hour were a couple of crows and an American robin. But suddenly you hear some Carolina chickadees calling among the branches of a large maple in front of you. You focus on one in your binoculars. Now you hear a tufted titmouse, too.

Lowering your binoculars to widen your field of view, you see that the tree is positively taming with little birds. Besides several chickadees and a couple titmice, there's a white-breasted nut hatch scrambling around on the tree's trunk. And there's a brown creeper close by. Oh, and now there's a pair of tiny golden-crown kinglets, too. You also spot a downy woodpecker on a large branch, pecking at a patch of rotten wood. What is happening?

Your heart is beating fast and you've got a big stupid grin on your face because this is such a wonderful sight. A moment ago, the white woods were silent and still. Now you're enjoying the antics of an entire flock of charming birds. You pat yourself on the back for being such a keen-eyed and excellent birder. Never mind that you didn't find the birds so much as they just showed up where you happened to be. After only a few minutes, the little beasts flit away and disappear into the winter forest. Congratulations! You have just witnessed what some call a bird wave or a bird party. An experience like this can be exhilarating.

Ornithologists have a more technical name for this phenomenon. It's called a mixed species foraging flock. Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners. This episode is all about mixed species foraging flocks. For those of us who love birds, encountering one of these flocks can be the highlight of a

walk in the forest. But really, who wouldn't want to see a bunch of cool birds of many species all bouncing around in the same tree? This phenomenon is also really interesting from a scientific perspective. You're here for the science, I presume, so let's go ahead and dive in. Many types of animals gather in groups. We have a lot of animals gather in groups. We have a have names for them, a herd of elephants, a school of fish, a swarm of bees, a dump of squirrels, 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, like the one I described in the intro, are hetero-specific. These are comprised of two, three, or perhaps many different species. You know, I sometimes catch myself when I use the phrase, species in that way, because that's sort of redundant. It's like saying unexpected surprise or new innovation or adorable kitten. I'm fairly certain that all kittens are, by definition, adorable. Similarly, a species is always different from every

other 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. But at least birds don't have to fight over important things like who gets to ride shotgun on the way to the liquor store.

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 mud flat. 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 bird feeder. Or if you don't have a backyard bird feeder think instead about your life choices and ask yourself where you went wrong. If you do have a feeder, you might sometimes have three to ten 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 bird seed. 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 episode, the mixed species foraging flock, differs greatly from what we see at backyard feeders. Let's look at why that is. What are the characteristics of a true mixed species flock?

First off, the birds in such a flock travel around together. For example, a flock with six bird species might land in an oak tree. They spend ten 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 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 birds 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 broad leaf 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, mas-o-menos, but some can have more than 100 birds. And usually no single species within such a flock is represented by more than a few individual birds.

So, for example, a 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. Or, like it or not, they might attract an entourage by virtue of their raw charisma alone. Podcast hosts know all too well about this situation. 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 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 fisherman. wery 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, Parodye, such as tits and chickadees. These little buggers call to each other frequently. Here's a willow tit calling in northern Europe

Many leader species are vigilant and make loud alarm calls when predators like hawks show up on the scene They conserve their flock by acting as sentinels warning of danger Drongos, birds of the family Dikruradi, often show this behavior. Here's a black drongo calling in southern India. 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. So leaders get the bird party started and the party don't stop.

Well, it don't stop until the birds get sleepy anyway, usually as soon as the sunsets. 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 Terdoides 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. As in the little vignette I gave you in the intro, birds that flock together in these northern regions are tits and chickadees,

nut hatches, tree creepers, kinglets, and woodpeckers. In North America, wood warblers from the family Parulidi 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. 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 for 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 of 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 heads 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. A predator, like a

pygmy owl might look at the flock and think, So many to choose from. Geez, I don't know. They all look delicious. I just can't decide. Meanwhile, each bird in the flock is thinking, Don't pick me, don't pick me, don't pick me. But once again, you could argue that a bird doesn't need to join a mixed species flock

to gain the benefit from the many-eyes effect or the dilution effect. Just a regular old flocky flock should do the. the trick, and that would be a valid argument. But 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. As I said, 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 are.

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. So what has all this hard work amounted to? How have ornithologists answered the question of why mixed species foraging flocks exist? Let's find out. But first, I want to take a quick break to chug some coffee. And I have a little favor to ask of you. If you're enjoying the Science of Birds podcast, which I hope you are, and you're getting some value from it,

it would be fantastic to get a review from you. If you're listening on Apple Podcasts, you can leave a review there. Otherwise, a great place to leave me a review is at podchaser.com. Podchaser is a great resource for people who love podcasts. Just search for the science of birds on that website and you can leave a review.

If you're new to Podchaser, you'll need to create an account first, but it's super easy and free. Reviews are so important. for helping me spread the word and grow this podcast. So thank you. Now let's get back to those birds. 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 Behavior, 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 I give meta-analysis two enthusiastic thumbs up. This 2009 study used a meta-analysis of data from those 191 earlier studies. There were some interesting conclusions. 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, right? 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 Downey Woodpecker, Pecoides pubescence. 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 its cute little head frequently to look around and listen for any predators. A Downey 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 Downey

woodpeckers around in a New Jersey forest, which sounds like a good time to me. She documented the bird's head cocking and feeding behaviors. She found that when Downey's 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 any trouble. 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, the way rock stars attract groupies. 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, Dichruous Paradisius. This glossy black bird lives in tropical forests in southern and southeastern Asia. Both males and females have a pair of long, spatula-shaped tail feathers that give the species its common name. Here's a greater racquet-tailed drongo singing. This bird frequently participates in mixed flocks. In fact, it actually mimics the calls of the other bird species to attract them. So the drongo is like, Mixed species flock, assemble!

Once the gangs altogether, the drongo also acts as a vigilant sentinel, sounding the alarm if it spots a predator. The greater racquet-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 racquet-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 flockmates 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 kleptoparasatism,

where one species steals a meal from another species. The drongo is like, oops, false alarm, mate, sorry. But wait, what's this? You seem to have dropped your grasshopper. Let me just help you with that. Oh no, it fell into my mouth. Purely by accident, I assure you.

So sorry. In case you didn't know, drongos have really bad Australian accents. 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, and I wouldn't be making this podcast episode, and our lives would be far less interesting. But there are trade-offs and some costs, too, like getting your lunch stolen by a low-down,

no-good, kleptoparasitic drongo. If you've never experienced a mixed species foraging flock, I highly recommend that you head out to your nearest forest and do some birding. Keep getting out there and don't forget your binoculars. If you live in North America, Europe, or Northern Asia, winter is the best time to look for mixed flocks. Sooner or later, one will cross your path and it will be glorious. Thanks so much for listening to this episode.

If this is your first time checking out the Science of Birds podcast, I hope you'll subscribe and check out more episodes. If you have any thoughts or feelings about the show, shoot an email to Ivan at Scienceofbirds.com. You can see the show notes for this episode, which is number 17, on the Science of Birds website, scienceofbirds.com. This is Ivan Philipson,

and I look forward to sharing more Bird Learning's with you next time. Cheers.