transcript

Speaker 1:
[00:01] Many years ago, I was doing fieldwork for my PhD research in Olympic National Park on the west coast of Washington state. I spent a glorious summer backpacking around in the park's backcountry wilderness, from the low elevation temperate rainforest to the subalpine zone high in the mountains. And I spent a lot of time hiking and exploring along the banks of streams and rivers. Now, I wasn't a diehard birder back then, so I was kind of surprised one day when I was chilling on the bank of a fast-flowing mountain stream, and I saw a small group of ducks floating along. I didn't think that such rushing cold water would make for good duck habitat. But after consulting my field guide, I figured out that these birds were harlequin ducks, and they were perfectly at home in this environment. Now, if you've never seen a harlequin duck, you have my condolences. I'm so sorry, because this is a beautiful creature, and I gotta tell you, you've been missing out. The male harlequin duck has a striking slate-blue body with bold white stripes and spots, and those are outlined in black. It also has bright reddish-brown sides and a colorful streak of that same color on top of its head. Harlequin ducks breed in upland streams and rivers. They're excellent swimmers that can motor around easily in the rapids. And these are diving ducks, so they dive down to the river bottom to snatch up aquatic insects from among the cobbles and boulders. So, I first fell in love with this species along the rivers of the Olympic Mountains, and I've seen them in streams here and there in North America and beyond. For example, I had a great time watching a female harlequin duck with her little brood in a beautiful Icelandic river. She was teaching her fuzzy ducklings how to navigate some small waterfalls. The cute little buggers were like wind up bath toys bobbing along with their mom. The harlequin duck is a specialist species that is well adapted for life in a fast flowing stream or river, at least during the breeding season. In winter, these ducks use a different habitat. They hang out on rocky coastlines. Many other bird species in the world are closely linked to flowing water habitats. Some, like the harlequin duck, are highly specialized for life among the rapids and swirling pools. And an even larger group of bird species depend on the lushly vegetated habitats that extend beyond the water's edge. Hello and welcome. This is The Science of Birds. I am your host, Ivan Phillipsen. The Science of Birds podcast is a lighthearted exploration of bird biology for lifelong learners. This episode, which is number 132, is all about the ways birds use rivers and streams as habitat. So far on the podcast, I've done episodes on habitats like wetlands, the coast, grasslands, and deserts. To choose the habitat for this episode, I ran a poll for my Patreon community. I gave them two options, mountain habitats or river and stream habitats. Mountains got 45% of the vote and rivers got the rest, 55%. So here we are. And I'm super excited about this topic. Streams are really special to me. First of all, I just think they're beautiful and fascinating environments. I can spend all day poking around in a mountain stream, looking for small critters and just splashing in the water. And second, a lot of my research back in the day when I was a biologist was focused on stream-dwelling animals, like frogs and aquatic insects. Besides beautiful, one word that describes rivers and streams is lotic, L-O-T-I-C. Lotic habitats are flowing water habitats. And a related word is lentic, L-E-N-T-I-C. And that one applies to the alternative category of freshwater habitats, the ones that don't move. Standing still waters like lakes, ponds, and swamps are lentic. Lakes, ponds, and swamps are super interesting in their own right, and we'll talk about them in another episode, at some point. Today, however, it's all about swiftly flowing waterways. So are you ready? Let's do this. We'll start off with some basic concepts about the ecology of rivers and streams. You know, just to make sure we have a good foundation, and that we're all on the same page. And it's just interesting stuff to know. A river or stream is simply a body of water that flows in a channel downhill until it reaches a larger body of water, like the ocean, a lake, or another river. The water we see in streams and rivers ultimately comes from a never-ending process known as the water cycle. This cycle moves moisture through the environment. You can imagine the cycle beginning as water falls from the sky as precipitation, such as rain or snow. A lot of precipitation falls on elevated ground, like mountains. From the mountains or highlands, the water moves across the land as surface runoff, or it sinks into the ground to become ground water. Ground water can come back to the surface again later through natural springs. As all this water gathers and flows downhill, it first forms small channels called headwaters. Headwaters then join together to create larger streams. And those streams converge to become broader rivers. And at each step, we're moving downhill to lower elevations. Finally, the big rivers complete their part of the cycle by transporting water across the landscape until it reaches its destination, draining into a lake or the ocean. Evaporation from all of these water bodies, rivers, lakes, and the ocean, ends up in the atmosphere. The cycle comes full circle when moisture in the atmosphere condenses and comes back down as rain or snow. So, that's the water cycle. And as I'm sure you know, it's kind of important. Not just for birds, but pretty much all life on earth. At least in a poetic sense, we can think of the water cycle as the circulatory system of our planet. Rivers and streams then are sort of like arteries and veins. Now, you might be wondering, is there any real technical difference between a stream, a creek, and a river? Well, not really, but sort of. The words stream and creek are pretty much the same thing. They're synonymous. We tend to call smaller watercourses streams or creeks, and large watercourses rivers. I mean, there are some scientific ways of separating streams and rivers, categorizing these things by size. But honestly, we ain't got time for all that today. For our purposes in this episode, I'll use the word stream to generally mean headwaters or small upland watercourses with high flow. They tend to have relatively clear, cool, well-oxygenated water. Upland streams also often have rocky beds. And when I say river, I generally mean a large, relatively slow-flowing water course, found at lower elevations. Rivers tend to carry more suspended sediment, so their water is murky or turbid. The river bottom is often muddy or sandy. And rivers are usually warmer than their tributary streams. Usually. Now, given these differences, you can imagine how upland streams and lowland rivers offer very different kinds of habitat for birds. Things like water flow, temperature, light, suspended matter, and bedrock substrate are what scientists call abiotic factors. Abiotic, as in not biotic. Not alive and not originating from living things. Compared to many other habitat types, abiotic factors can have a lot more importance than biotic factors in rivers and streams. Okay, let's consider the Mississippi River. The entire network of streams and smaller rivers that feed into the mighty Mississippi spreads across a land area that's over one million square miles, or almost three million square kilometers. This is the largest watershed in all of North America. Looking at our map of North America, let's zoom way in to just one tiny section of the Mississippi watershed. Now we're in the Great Smoky Mountains National Park in Tennessee. We see a small headwater stream flowing through a forested ravine. The water is clear, cold, and it splashes along at a swift pace. The stream is flowing under the shade of pine, hemlock, oak, and birch trees. This stream joins with another headwater stream to form a larger one. As we follow the water downhill, every confluence creates a larger stream. The temperature increases, there's more sediment in the water, and the streamside vegetation changes. Streams become small rivers, and these empty into the Tennessee River. The Tennessee River spills into the Ohio River, and that is a major tributary of the Mississippi itself. The water flowing at the confluence of the Ohio and Mississippi rivers, two large lowland rivers, is muddy and relatively warm, and it moves more slowly. The environment along the banks is very different as well. The bottomland forest here is dominated by bald cypress and swamp cottonwood. There are birds living along every stretch, of course, from the bubbling headwater streams in the Great Smoky Mountains to the powerful rivers of the lowlands. And although there is plenty of overlap, the mixture of bird species you find around the headwater streams is different from what you find in the lowland rivers. And this pattern is true for birds in the network of streams that feed into each of the world's major rivers, the Amazon, the Nile, the Danube, the Brahmaputra, and so on. In fact, because some rivers, like the Amazon and Brahmaputra, have a much greater elevational difference between the headwaters and the lowlands, the differences in their bird communities and cross elevations are more dramatic than what we find for the Mississippi. Okay, cool. Now let's take a closer look at the habitat along a stream. We're going to look at a cross section of a typical stream. Imagine we've sliced across the drainage perpendicular to the direction of water flow. In the middle, at the lowest point, is the channel. This is where water flows most of the time. The channel might have a bedrock bottom, mixed with boulders and cobbles, or it might have layers of gravel, sand, or silt. Moving left and right away from the channel, onto the banks, the ground slopes upward. The riparian zone begins here at the water's edge, and it extends away from the water for some distance. A riparian zone is the interface between the stream and the land. It's a transition zone that's heavily influenced by the presence of water, with distinct soil and a distinct plant community. In one stream, the riparian zone might be, for example, 50 feet wide on either side, which is about 15 meters. But in another stream, it could be narrower or much wider. During heavy rain or spring snowmelt, more water flows through the channel, and flooding occurs. The floodplain is the area along the stream margin that ends up underwater during a flood. This is a subsection of the riparian zone. The width of the floodplain also varies from one stream to the next. The plants growing in the path of destruction and drowning in the floodplain are often a different mix of species than what grows further up slope in the riparian zone, or in the rest of the riparian zone. Another feature of the riparian zone is that it has multiple layers of vegetation. There are understory grasses and herbaceous plants, mid-story shrubs and small trees, and large overstory trees that form the canopy. Moving still further away from the stream and higher in elevation, we leave the riparian zone. At this point, the stream's water has very little influence. We have entered the twilight zone. Oh, wait, nope, nope, my bad, not the Twilight Zone. I meant the Upland Zone, which is slightly less spooky. The mixture of plant species growing in the Upland Zone is very different from what you find in the Riparian Zone. So, if we're thinking about birds living along a stream, what resources do they need? They need food, of course. They need water, but luckily, there's no shortage of that, right? They also need shelter and for some species, they need good nest sites. The types of food, shelter, and nest sites differ from the stream channel to the Riparian Zone to the Upland Zone. For example, food in the water and on the immediate banks of the stream is going to be almost entirely small animals, like fish, insects, other arthropods, and amphibians. Birds in this habitat rarely eat plants. Healthy headwater streams and other small streams often have lots of aquatic insects and other invertebrates like snails. Many of the aquatic insects are actually larvae, so they're the early life stages of critters like stoneflies, mayflies, caddisflies, blackflies, dragonflies, and so on. They cling to rocks and to plants and pieces of wood under water. But other kinds of insects, spiders, and other small critters, the ones that need to breathe air, they're hanging out at the water's edge, on exposed rocks, gravel, or plants. All of these invertebrates are rich in protein and fat, and so they're a vital food source for many river-dwelling birds. In the riparian zone, there are many other small prey animals to eat, as well as some plant material, like seeds and berries. And how about nesting? A bird that nests close to the water, basically in the channel, might make a nest among rocks on the shore, or in a cavity dug into the bank, or in the roots of a fallen tree that's hanging over the water. Birds in the riparian zone have more nesting options. For example, many species in this zone build their nests among the branches of tall trees. The riparian zone, with its high diversity of food, shelter, and nesting resources, is often able to support a higher diversity of bird species than either the channel or the upland zone. So at this point, we've highlighted the big differences in habitat between headwater streams and lowland rivers. And by zooming in to look at the cross section of just one stream, we see there are also major habitat differences between the channel, the riparian zone, and the upland zone. But I know you're here to learn about birds. I appreciate your patience so far as I've geeked out a little on all this stream ecology stuff. So now let's actually focus on the birds themselves, starting off with specialists. In any major habitat type on Earth, there are some bird species that are specially evolved to live in that habitat. So there are species we think of as desert birds, grassland birds, sea birds, high elevation birds, and so on. And there are indeed some bird species that are adapted for a life among the flowing waters. Of the 11,000 or so bird species alive today, only a small percentage are these, let's call them, stream specialists. And so, what is the actual number of stream specialist species? Well, when trying to answer that question, I was happy when I dug up a scientific paper published in 2002 titled Global Patterns of Diversity Among the Specialist Birds of Riverine Landscapes. It was published in the journal Freshwater Biology. That paper identified 60 obligate stream specialists worldwide. But just FYI, they used the term riverine specialist in the paper, not stream specialist. But how about that word obligate? We've talked about it before on the podcast. In this case, the term obligate stream specialist means these 60 species must have river or stream habitat to survive and or reproduce. They don't have a choice. Obligate stream specialists feed exclusively, or almost exclusively, on things produced in the water, in the stream itself. Fish, aquatic amphibians, aquatic insects, snails, and other invertebrates. Or plants, of course. The 60 or so stream specialist species are scattered across the avian tree of life. There are, for example, stream specialist ducks, herons, shorebirds, owls, kingfishers, flycatchers, dippers, and warblers. All told, 16 bird families include at least one stream specialist species. And two of those families contain nothing but stream specialist species. The first of those is the family of the dippers, synchlidae, spelled C-I-N-C-L-I-D-A-E. There are five dipper species in the world, and all of them are highly adapted to living in streams. We have just one dipper species in North America, and that's the American dipper. Another example is the white-throated dipper, which is common in Europe and Central Asia. The other stream specialist bird family is Ibidyrhynchidae. Ibidyrhynchidae, spelled I-b-i-d-o-r-h-y-n-c-h-i-d-a-e. You got that? That's a mouthful. Well, this is a monotypic family, meaning it has just one species in it, and that is the ibis bill. It's not an ibis, but it has a long, down-curved bill, sort of like an ibis. Otherwise, it looks like a medium-sized shorebird. The ibis bill is an Asian species found in mountain rivers with lots of pebbles, cobbles, and boulders. An honorable mention here goes to the forktails. These birds are in the large and diverse family Muscacapiti, which is the Old World flycatcher family. But the eight forktail species are all stream specialists in Asia, and they make up the genus Anacurus. Forktails are really beautiful, charismatic birds. All of them have bold plumage patterns in black and white, and they've got a long black tail with white bars. And the tail is, of course, forked. Now again, the forktails are in the family Muscacapiti. This family has 13 stream specialist species. The eight forktails plus five others, which is way more than any other avian family. And that's pretty interesting. I'll give you examples of species across different families in a moment. But first, let's look at some major geographic patterns among stream specialists. These are some of the key findings in that 2002 paper that I mentioned. Geographically, Asia holds the highest species richness with about 28 stream specialist species. One reason Asia wins this prize is that a major global hotspot for stream specialist birds is the Himalayan mountains. Up to 13 of these species have overlapping ranges in the Himalayas. For example, I remember being in the Himalayan foothills in the country of Bhutan and seeing four stream specialist bird species all in one short section of a river, just all hanging out together. There was white-throated dipper, brown dipper, white-capped redstart, and plumbius redstart. And that was just super cool to see. Around the world, species diversity for stream specialists peaks at mid-elevations, around 4,500 feet or 1,350 meters. Diversity is also strongly tied to surface structure. Mountainous regions with high relief and complex river channels support significantly more species than flat plains. So looking at all of that, it makes sense that the Himalayan mountains would be a hotspot for these kinds of birds. Most of these specialized bird species prefer small, fast-flowing streams with coarse bottoms like pebbles, cobbles, and boulders. So, yeah, that is why I went with calling them stream specialists rather than river or riverine specialists. The warmer a stream and the more primary plant productivity it has, the more aquatic invertebrate biomass it can support. In general, it's those aquatic invertebrates that are the primary food resource for most stream specialist birds. So, the overall pattern is that the most diverse assemblages of stream specialist birds are found in warm, productive, and mountainous tropical or subtropical regions where high-energy stream habitats are common. The mid-elevation slopes of the Himalayas in Asia and the Andes in South America tick all these boxes. Okay, as promised, I'll highlight a handful of stream specialists. We'll go in taxonomic order, climbing through the branches of the avian tree of life, starting with the most ancient lineages and progressing through more recently evolved groups. There are about eight stream specialist duck species in the family Anatidae. The harlequin duck is one. If I had to pick just one other duck to talk about, I'd probably go with the torrent duck from South America. But I already talked about that bird at some length because it's one of my favorite bird species. I talked about that in a previous episode. So for today, let's go instead with the African black duck, Anas sparsa. This is a large, chunky, very dark-colored bird. More of a dark brown than truly black. But I guess African black duck sounds cooler than African dark brown duck. It's also got some white spots on the back. African black duck populations are scattered across Sub-Saharan Africa, mostly in the East and South. This species prefers secluded, shallow, fast-flowing mountain streams and rivers in wooded or open country, though it can also be found in some slower-flowing rivers. This is one of the rare stream specialists that actually eats plants, either dabbling on aquatic plants or plucking weeds along the shore. But African black ducks also eat some insects and small fish. They build their nests on the ground among driftwood, or in the grass on the bank, or in reed beds. Okay, next we have Blackiston's fish owl. The largest owl species in the world! This beast lives in a few specific regions of Northeast Asia, the Russian Far East, Northern China, and Northern Japan. Blackiston's fish owl is a habitat specialist that depends almost entirely on river and stream environments, particularly riparian forests. For a habitat to be suitable for this owl, it must have two key features, large old growth trees for nesting, and access to water that stays open and ice-free all year. As its name suggests, Blackiston's fish owl was named after somebody named Blackiston. But also as its name suggests, and more interestingly, it is a fish eater. It hunts by watching the water from a nearby branch or standing in the shallows to snatch prey with those powerful sharp claws. While its favorite meals are freshwater fish like salmon, trout, and ayu, it also eats other river-dwelling critters like crayfish and frogs. I'm actually leading a birding tour in Japan next year, and there's a chance, probably just a slight chance, but we might be able to see a Blackiston's fish owl. Fingers crossed, because that would be really amazing. Okay, our next featured stream specialist is the sharp-tailed stream creeper, Lacmaeus nematura. And I love that name, stream creeper. This is the only bird that has stream creeper in its common name. So, I mean, I suppose we could just call it the stream creeper. Like, if this bird had to choose a handle for its Instagram account, it could just be at stream creeper. But what if some jerk already took that one? Well, in that case, how about at the stream creeper? Or at the real stream creeper? And then the bird could get its account verified with one of those little blue checkmarky things. So that way, we know that it's not just some imposter trying to get rich by pretending to be a celebrity like the sharp-tailed stream creeper. In any case, this is a passerine bird, and it's in the large neotropical family Fernoriidae. It's about the size of a house sparrow. The sharp-tailed stream creeper is mostly reddish-brown, but it has some lovely white spots on its underparts and a white supercilium. In other words, a white eyebrow line. And the beak is longish, sharp, and slightly down-curved. The sharp-tailed stream creeper is found throughout much of South America and parts of Central America. It's almost always found near rocky streams and small rivers with thick, dense vegetation. The stream creeper depends on this kind of habitat for both protection and nesting. It often builds its ball-shaped nest inside a long tunnel that it digs into the banks along the river's edge. This bird's diet is a wide variety of arthropods, like insects and spiders, that it finds along the water. It forages by hopping along the ground, searching through fallen leaves and the moss that covers riverside rocks. That pointy beak is used to toss leaves aside or probe into shallow stream beds to probe for hidden prey, like aquatic insects. Let's give a listen to the song of a sharp-tailed stream creeper, recorded in eastern Brazil. Next up, we have the Torrent tyrannulite, Serpa Fega cenaria. This is a specialized flycatcher in the family Tyrannidae. The Torrent flycatcher has a wide distribution across Central and South America. This is a small bird with a light gray body overall, but with a black tail and wings and some black on the head. It's sort of pot-bellied and shaped like a little puffball, a borb, if you will. It's super cute. This bird is typically found at middle elevations in the mountains, so between 5,000 and 10,000 feet, which is about 1,500 to 3,000 meters. The Torrent tyrannulite lives along rocky mountain streams and rivers. And just so you know, this is one of four bird species in the world with the word torrent in its common name. Torrent as in a strong, fast-flowing stream, right? Another species I mentioned earlier is the torrent duck. Anyway, our little tyrannulite thrives in habitats with clean, high-energy water. It perches on boulders or low branches that sit directly in or hang over the rushing current. It uses exposed rocks in the middle of the stream as hunting platforms. And it builds its mossy cup-shaped nest in vegetation that overhangs the water. The diet of the torrent tyranulite is mostly small insects and other invertebrates found near the water. It is particularly good at aerial hawking, a technique where it flies out from a rock to snatch a flying insect directly out of the air before returning to its perch. So it's catching things like adult dragonflies, mayflies, and stoneflies. This fly-catching or aerial hawking behavior isn't too surprised if we remember that this is a member of the tyrant fly-catcher family tyranidae. I'll never forget seeing my first torrent tyranulite. It was in the Urubamba River, not far from the famous archaeological site of Machu Picchu in Peru. Now let's listen to a recording of a torrent tyranulite. This one is in Ecuador, and we're hearing its call. Moving along, we have a bird that is loved by many bird enthusiasts in North America, and that is the American Dipper, Sinkless Mexicanus. Now I know many of my listeners love dippers, and they'd be happy for me to carry on for the next three to twelve hours yapping about nothing but dippers. And hey, I get it, I love them too, it's one of my favorite birds. So, what I want to do is give them the spotlight with an entire episode on the dipper family, Sinklody. Again, there are five species in the family, and full episodes could and should be devoted to individual species like the American Dipper and White-throated Dipper. But for today, we're focusing on this broader topic of river and stream habitats, so we'll just briefly talk about the Dipper. The American Dipper is another Borb. It's a stocky, medium-sized bird that's slaty-gray or grayish-brown. At least on the head, it's got a little bit of a brown tone. It lives in the mountainous regions of western North America, ranging from Alaska and western Canada, down through the western United States, and into Central America as far as Panama. For its habitat, the American Dipper requires fast-flowing, clear, and unpolluted mountain or coastal streams. It relies on mid-stream boulders for perching and steep cliffs for nesting. The Dipper's dome-shaped nest is often built in a precarious spot right over the water. Sometimes, the nest is actually built behind a waterfall. And in that crazy situation, the parent birds have to fly directly through the falling water to reach their young. How wild is that? The primary diet of the American Dipper is aquatic insects and their larvae, like caddisflies, mayflies, and stoneflies. Dippers also occasionally eat small fish, fish eggs, or tadpoles. To find its food, the Dipper has evolved an amazing approach to hunting. It dives into the rushing current and walks along the river bed, using its wings to push against the flow and keep itself submerged while it flips over stones to uncover hidden prey. This behavior, which is shared with the other Dipper species, makes the American Dipper the only truly aquatic songbird in North America. And this bird has a lovely complex song. Here is one singing in Colorado. Okay, I've still got a few more stream specialists I'd like to tell you about, but I'm gonna move through them a little more quickly. The next three species are all in the family Muscacapidae, which again is the Old World Flycatcher family. First, we've got the Plumbeus redstart, Phenicurus fuliginosa. You'll also see it called by the common name Plumbeus water redstart. This is a small gray bird with a rusty red tail. And remember that Plumbeus refers to that gray color, because it comes from a Latin word that gives us the roots for lead, right? So plumbum, plumber, plumbing, because they used to use lead pipes. So that's where Plumbeus comes from. The Plumbeus redstart is widely distributed across Asia, ranging from the mountains of Afghanistan and Pakistan, through the Himalayas to China, Taiwan, and parts of Southeast Asia. It loves rocky mountain streams and rivers, with plenty of boulders and shady banks. The Plumbeus redstart typically lives at high elevations during the breeding season, and then it drops down to lower ice-free river levels during the winter. Its diet consists primarily of insects, particularly aquatic varieties like mayflies, that it catches by flying vertically over the water or snatching from the surface of wet stones. So, the Plumbeus redstart looks and behaves a lot like the torrent tyranulite. And the two species have similar habitats, even though they aren't close relatives and they live on opposite sides of the planet. So what do we call this? That's right, we call it convergent evolution, where different species that are not closely related evolve to have similar features or similar lifestyles because they live in a similar environment. Okay, now let's listen to the happy little song of a plumbius redstart, recorded in the mountains of Taiwan. And here's another recording of the same species, this time from Sichuan China at the edge of the Tibetan Plateau. This song variation is a bit less complex. The next species, the white capped red start, is also in the genus Phenicurus. So that means it's a close relative of the plumbius red start. The white capped red start, you'll also see it called the white capped water red start, is also found across Central and Southeast Asia. It's easy to recognize with its bold white cap, which stands out against its glossy black top half of its body. And the rest of the body and tail are a bright orange red, as is the tail and there's a black band at the tip of the tail. Like its cousin, the plumbius red start, this bird lives almost exclusively near fast-flowing mountain streams and waterfalls, often perching on boulders right in the middle of rushing water. And the white capped red start also eats invertebrates. Now even though the two species can be found in the same river drainage, they forage a bit differently, and they prefer slightly different habitats. So, that's the way that they can avoid direct competition with each other most of the time, the way that they can partition their niches. In the following recording, this is a white capped red start, listen to its shrill, upslurred call. This recording was also from Sichuan, China. Our next bird is the Bornean Forktail. This is one of those eight forktail species, all of which are in the genus Anacurus, and they're all stream specialists. The Bornean Forktail is a small bird found exclusively in the mountainous upland regions of the island of Borneo. The Bornean Forktail has a black head, back, and breast that contrast with its white forehead and its white wing bands and white lower body and rump. True to its name, the bird has an elegantly long black tail that is deeply split into a V-shaped and it's marked with white patches. It frequently flicks its tail and fans it out as it balances on wet stones. The Bornean Forktail lives along fast flowing rocky rivers and smaller streams that run through high altitude forests. It forages for food in the water, along the damp edges of the stream, and on wet rocks. On rare occasions, it even submerges itself completely, sort of like a dipper. Its exact diet, however, isn't fully understood. But it most likely eats insects and other small invertebrates, since that's what other Forktail species do. Here's a recording of a couple of Bornean Forktails calling to each other. Next up, we have a North American bird, the Louisiana water thrush Parquesia moticilla. This is a member of the New World warbler family, Perulidae. So yes, despite its name, this is not a thrush, it's a specialized stream-dwelling warbler. The Louisiana water thrush is a small, energetic bird with a brown or olive brown back and white underparts marked by dark brown streaks. It's got a bold white supercilium, which again is the eyebrow stripe, and that widens behind the eye. These warblers breed across much of the Eastern United States and Southern Canada. Then they spend the winter in the Caribbean, Mexico, Central America, or Northwestern South America. The Louisiana water thrush requires clear fast-flowing streams and rivers in mature forests. It builds its nest in the roots of trees overhanging the water or in steep dirt banks. Its diet consists primarily of aquatic insects and their larvae, such as mayflies, which it finds by flipping over wet leaves or probing into the stream. Here is the song of a Louisiana water thrush recorded in Mississippi, not far from the border with Louisiana. And here is another Louisiana water thrush recording. Perhaps, the Louisiana water thrush's most famous trait is its constant tail bobbing behavior, where it teeters its little butt up and down as it walks along the ground. A habit so distinct that the specific epithet of its scientific name, motosilla, motosilla actually means tail wagger or tail shaker. The last and certainly not least stream specialist bird I want to tell you about is also a tail wagger. And it's also one of the four species with the word torrent in its name. I'm talking about the torrent lark, Grolina bruigenii. No, it's not a true lark, but it's a species in the family Monarchidae. And that's the monarch flycatcher family, which is restricted to the old world. The torrent lark found only on the island of New Guinea is a medium-sized bird with bold black and white plumage. There are clear differences. Some dichromatism between the sexes. The male has a black face and breast, while the female is recognized with her white eyebrow and her white breast. And like the northern water thrush, the torrent lark frequently bobs its body and wags its tail while perching on stones in the middle of a stream. Its habitat is fast-flowing, rocky mountain streams and rivers surrounded by dense forest. Torrent larks build their cup-shaped nest out of mud, and they place it on branches or rock ledges directly overhanging the rushing water. The bird's diet consists primarily of insects and other small invertebrates found along the water's edge or on submerged rocks, which it captures by darting through the air or diving into the water. Let's listen to this recording, which is the buzzy call notes of a torrent lark. Alrighty, that completes our line up of stream specialist examples. But let's return for a moment to this whole tail wagging thing, the movements that look like the bird is bopping along to a dance beat in its head. It turns out that some other stream specialists show similar behavior. For example, both male and female plumbius redstarts fan their tails conspicuously. And, wag tails do exactly as their name suggests. They pump their long tails up and down as they walk around. Dippers display other visual signals. They bob their entire bodies up and down by pumping their legs. And their eyelids are covered in tiny white feathers so that their eyelids make a flash of white every time they blink. Now as far as I know, ornithologists aren't exactly sure why river birds have these visual displays. There are multiple hypotheses out there. But at least in some cases, it seems that tail wagging and whatnot is a way for the birds to communicate with other members of their own species. It allows them to be more conspicuous to each other in a river or stream environment, where all that moving water might otherwise make it hard to see each other. And the same kind of thinking could be applied to the calls and songs of stream specialist birds. Their vocal sounds tend to be high-pitched and loud. So maybe this is an adaptation so that the birds can actually hear each other above the background noise, all that splashing and roaring of the river. Okay, so let's finish off this section with the high-pitched call of one more stream specialist, the gray wag tail. And wag tails actually are in a genus called motocilla, which is the second part of Louisiana Water Thresh's scientific name. But in this case, it's actually a genus, which again means tail wagger or tail shaker. The gray wag tail is motocilla cinerea. This one was recorded in Kazakhstan. Okay, so far we've been talking a lot about stream specialists. You might have noticed some patterns there, that most of the species I just described live specifically in fast-flowing mountain streams. And their diet is primarily invertebrates found either in the water or at the water's edge, or maybe flying around just over the water. But as I mentioned earlier, rivers and streams of all kinds can support bird species that are not so specialized. Many other bird species make good use of these habitats, even though they aren't strictly confined to those habitats. Or they might use river or stream habitats only part of the year, or only under certain conditions. The number of river-dependent and generalist species that breed in or rely on the riparian zone is much higher than those 60 or so obligate stream specialists. For example, a large temperate region like North America or Europe can expect to have between 100 and 150 bird species that are significantly dependent on riparian or stream-side habitats for breeding and survival. And most of these species are more associated with the riparian zone than with the water in the channel. Unlike specialists that forage in the water or right at the water's edge, these more generalist birds typically spend their time in the lush vegetation away from the banks. They forage either on the massive production of insects like mayflies and stoneflies that emerge from the water, or on other insects that hang out in the dense foliage of the riparian zone. And aerial insectivores like swallows might focus more on the airspace above the river and riparian zone. Heck, the scientific name of the bank swallow is actually riparia riparia. Here in North America, there are dozens of species that are associated with riparian habitats to some extent. We're talking birds like the willow flycatcher, northern yellow warbler, yellow-breasted chat, and song sparrow. Species like these hop or fly around in the riparian zone without needing to get in or even near the flowing water. They can probably live happy and productive little lives without ever needing to dip their tiny toes in the stream. And yet, they thrive in habitats intimately connected to flowing water. Especially in the desert or other arid places, riparian habitats are oases of greenness that are great for nesting and finding lots of high quality food. So rivers flowing through arid regions can attract bird species of many kinds, including those we don't normally think of as being riparian or river dwellers. I lead a nature tour every year called Deserts of California, and we visit several streams that act like oases for birds. There's one stream I'm thinking of that's surrounded by sun-baked desert. But there it is, a beautiful flowing stream. And the riparian zone is very shady. It's dominated by native palm trees, sycamores, and cottonwoods. And there are all kinds of birds flopping around in there, like western bluebirds, yellow-rumped warblers, house finches, gambel's quail, scott's oriole. Even the American dipper shows up now and then. It's pretty cool. And finally, this oasis effect is one reason that rivers and their riparian zones can also provide migration corridors and stop oversights for long distance migrants. The lotic habitats of the world are essential to many birds, but also to countless other wildlife species. Fish, amphibians, reptiles, and mammals. And of course, rivers have been vital to humans throughout our evolution and our history. They've provided us with clean water to drink, food like fish, water for our crops, transportation, recreation, and now energy from hydroelectric dams. And in many cases, humans have overdone it. As is our nature. We've over-exploited rivers and streams. We suck them dry, alter their temperature, deplete their oxygen, reshape them, block their flow, and pollute them with chemicals and disease. And human-caused climate change looms over all of this, compounding all of that damage. The degradation and destruction of freshwater habitats, including rivers and streams, is a worldwide environmental crisis. In the United States, for example, recent assessments show that less than a third of our river and stream miles are in good biological condition. One of the biggest problems is high levels of nutrients like phosphorus and nitrogen ending up in the water. These pollutants can lead to harmful algae blooms and lower the water's oxygen levels, and those are major threats to aquatic plants and animals. Dippers and some other stream specialist birds can act as bioindicators for the health of a river or stream, because the presence, health, and behavior of such birds directly reflects the overall cleanliness of the water course. These birds are habitat specialists that depend almost entirely on clear, fast-flowing water for their survival, so they're highly sensitive to changes in their environment. For example, species like the white-throated dipper, American dipper, and Louisiana water thrush rely on a rich supply of aquatic invertebrates, like larval caddisflies and mayflies. These insects can only thrive in well-oxygenated, unpolluted water. If a river becomes polluted or filled with silt because of a dam, these sensitive insects disappear, and that leaves the birds without food, and they're going to struggle. So scientists can monitor bird populations and their breeding success, and even the chemicals found in their eggs to get a sense of a river's health. In this sense, stream specialist birds aren't just inhabitants of these ecosystems. They're an important window into them. Now, I could go on for a few hours if we wanted to focus on the environmental crises faced by rivers and their birds. But rather than a bunch of doom and gloom, I'd like to leave you with a little bit of hope. And there is still hope, I think, for the future of river and stream habitats. Because large-scale restoration efforts are successfully bringing nature back to some of these habitats. Across the United States and beyond, government agencies and conservation groups have completed thousands of projects to fix damaged riparian habitat and remove barriers like old dams. Recent success stories show that when rivers are reconnected to their natural floodplains, allowed to flow freely, and replanted with native riparian trees, birds and other wildlife can return remarkably fast. For example, endangered salmon have returned to California rivers in record numbers following habitat repairs. And beavers are moving back in to restored areas. And the beavers themselves are engineers that naturally create the complex riparian habitats that many bird species need to thrive. And over in the UK, long-term monitoring research in English rivers shows that the populations of small insects that birds rely on for food have reached their healthiest levels in 30 years, because water quality has actually improved. And that's pretty amazing. So let's thank all the people around the world who have been working hard to save our rivers and streams. In many cases, they're unsung heroes. The earth's waterways are so, so important to people, to wildlife in general, and to all those wonderful river-dwelling birds. The dippers, ducks, stream creepers, fish owls, forktails, wagtails, redstarts, water rushes, and many more. We want to make sure they all have healthy habitats long into the future. Well, there you go. I hope you enjoyed today's episode. We can add another major habitat type to what we've covered on the podcast. Future episodes will cover mountains, tropical rainforests, temperate rainforests, tundra, and so on. Each of those should be really interesting and fun. Thank you to all my amazing supporters on Patreon. You guys rock. And a big welcome to the newest members, Dorothy Kopchick, Nancy Anderson, and Patti Reichsinger. Thank you so much. If you, dear listener, have some interest in supporting this podcast, you can check out my Patreon page at patreon.com/science of birds. And there's a link somewhere in the show notes in your podcast app. You can also shoot me an email if you have something you'd like to share with me. My address is Ivan at scienceofbirds.com. Let me know what you think about the podcast. Or you could let me know what your common name would be if you were a stream specialist. I think mine would be something like scruffy faced boulder hopper, or maybe smart mouthed water monkey. I don't know. In any case, this is episode 132. 132. You can check out the show notes for the episode along with some curated photos of the birds I talked about today over on the Science of Birds website, scienceofbirds.com. Don't forget to check out Bird Merch, my online store over at birdmerch.com. And if you're interested in the birding tours I offer, check out Wild Latitudes over at wildlatitudes.com. For example, I still have space available on my Bhutan tour in January 2027. It's going to be amazing. We'll be in the Himalayan mountains, so we should see a lot of these stream specialist birds. Dippers, forktails, redstarts, and all of that. I cannot wait. Anyway, I'm Ivan Phillipsen, and true fact about me, I am fascinated by liminal spaces. Liminal spaces are transitional or in between places, like empty hallways, airport terminals, or deserted malls. They're kind of eerie, surreal, and also nostalgic because of the weird emptiness and lack of context. The word liminal is derived from the Latin limen, which means threshold. And these liminal spaces evoke unease by disrupting our familiar environments. They have sort of an uncanny valley effect that feels both unsettling, and surprisingly, sometimes comforting. And that's all I've got for today. I wish you all the best and all the birds.