Bird Songs

Even when we can’t see birds, we can usually hear them. This article explores the structure and functions of the vocal sounds of birds.

DATE:
September 19, 2020
CATEGORIES:
Author
Ivan Phillipsen

Bird Sounds

Birds are never very far from us, no matter where we are on Earth. Even when we can’t see them, we can usually hear them. If we slow down occasionally and use our ears to survey which birds are singing nearby, we can often be surprised by the diversity of species and of bird sounds that are all around us.

Birds are noisy. Compared to most mammals and other animals that we encounter daily, birds make a lot of racket. Besides having feathers and the ability to fly, singing is one thing that really stands out about birds.

We’re delighted and fascinated by the voices of birds. Humans, too, are a noisy bunch, so it’s not surprising that we’re interested in these creatures that also use sound extensively.

Birds make sound in a variety of ways, not just with their voices. Some species make sounds with their feathers. For example, the Common Snipe makes a winnowing sound with its tail feathers as it makes an aerial display dive.

A male Ruffed Grouse beats its wings rapidly to make a low-frequency, drum-like sound while perched on his favorite log in the forest. This low-frequency sound travels easily through dense forest vegetation.

And you’re probably familiar with the actual drumming sounds made by woodpeckers.

In this article, we’re going to focus just on the vocal sounds of birds and leave those other sounds for another time.

How do birds sing?

Humans make sound waves with the larynx, our so-called voice box. Air pushed out from the lungs vibrates the vocal cords in the larynx, and this makes a sound. Pitch and volume are changed by movements of muscles around the larynx. Further modifications of the sound result from movements of the tongue and lips. What comes out is the melodious noise of the human voice.

Bird voices are produced in a similar way. But there are a couple big differences.

For starters, birds don’t make sound with their larynx. They have a larynx, but it doesn’t serve that function. Instead, they have an anatomical structure unique to birds called a syrinx.

Compared to the human larynx, the syrinx of a bird is located further down in the respiratory pathway, closer to the lungs. It sits above a bird’s heart. This is where the airway, the single tube of the trachea, forks into two tubes, called bronchi, which connect to the lungs. The syrinx lives at that fork.

Thin membranes in the syrinx vibrate as exhaled air rushes past them. This makes a sound. Muscles in the syrinx contract and relax to manipulate the sound—the bird’s voice—to change its pitch and amplitude.

There are a few distinct, anatomical forms of the syrinx, found in different bird groups. Of these, the syrinx of songbirds is arguably the most sophisticated.

Another key difference between the voices of birds and mammals like us is that many birds can produce two independent vocal sounds simultaneously with their syrinx. These birds can control the two sides of their syrinx independently.

This is kind of like being able to sing a two-part harmony of “Bohemian Rhapsody” with yourself.

As a bird’s voice moves from the syrinx and out into the world for all to hear, it can be modified by things like the length and diameter of the trachea. The shape and size of a bird’s bill also can change the sound, as can the way a bird moves its bill while it sings.

As for the trachea, some types of birds—including swans and cranes—have long ones that increase the resonance of their voices.

For example, the Trumpet Manucode from the island of New Guinea. This member of the Bird of Paradise family has a super long trachea, which is coiled up like a garden hose just under the skin of its chest. The bird is only about 12 inches (31 centimeters) long, but it’s trachea is 30 inches (75 centimeters) long.

Taxonomic Variation

What exactly is a “songbird?” I used that word above, and it’s a common one that we all know.

Isn’t any bird that sings a song bird? Well, in the poetic sense, yes. But technically, scientifically, no.

In ornithology, songbird is the common word we use for members of a particular evolutionary lineage known as the oscine passerines. We can also just call them oscines. This is a subset of the order passeriformes—otherwise known as the perching birds.

Oscine comes from the Latin word oscen, which translated one way means, “bird which gives omens by its cry.”

Besides being great at giving omens, the oscine passerines have some of the world’s most amazing vocal abilities.

About half of the planet’s bird species, about 5,000 of them, are songbirds. So, in the evolutionary sense, this is a wildly successful group.

These are the wrens, thrushes, finches, starlings, warblers, larks, chickadees… and there’s no way I’m going to list them all here. Suffice it to say there are many bird families that belong to the oscine passerine group. These birds belt out many of the world’s most impressive songs.

But not all songbirds sing so nicely. They don’t all sing like Beyoncé or Ella Fitzgerald.

Common Ravens, for example, are actually big, fat songbirds. Meaning they are oscine passerines. But they have relatively grating voices. So ravens sing more like the avian equivalent of Bob Dylan.

Now remember that songbirds are a subset of the perching birds, which is what we call birds in the order passeriformes.

The other major subdivision of that order is the suboscine passerines. There are about 1000 species of these suboscines. The best-known examples from this group are the tyrant flycatchers of the Americas. They sing too, but their voices are often more mechanical sounding and less musical than those of true songbirds. We’ll come back to the suboscines in a bit.

Besides all these perching birds—that is, birds of the order passeriformes, composed of the songbirds and suboscines—beyond those guys, many other birds vocalize. Most birds do, actually. Think about chickens, geese, eagles, owls, parrots, and so on. We know their voices really well. They just don’t sing so pretty.

Learned vs Instinctual

One of the biggest scientific questions about bird songs is: Are they learned or are they instinctual?

This is a ‘nature vs nurture’ question—the type of research question that scientists tackle all the time, especially with human behaviors.

Like, is artistic ability something you learn from your parents or are you just born with it? Or is it a combo of these?

And what about Leporiphobia, that all too common fear of rabbits that makes us really nervous when Easter rolls around? Is that learned or did I inherit it?

Bird song is one of most well-studied behaviors in non-human animals. Many scientific studies over decades have given us an answer to whether bird song is learned or instinctual.

The answer is ‘yes.’

Because it depends on the bird you’re talking about.

It turns out that some types of birds learn their songs, while others are hard-wired by their DNA to sing a particular song.

More than half of the world’s bird species learn their songs. That we know of, there are four groups (i.e. evolutionary lineages) that do so. Songbirds constitute one of these groups. But parrots and hummingbirds also learn their vocalizations. And the fourth group is just one species, the Three-wattled Bellbird that lives in Central America.

Most of the research on vocal learning has focused on songbirds, which you may recall are multitudinous. Studies have revealed some interesting parallels between the way humans and songbirds learn to communicate with sound.

You know that young human children are great at learning languages, but then as we age and become beaten down by the harsh realities of life our brains become less good at soaking up languages—we don’t learn them as easily.

Similarly, many songbirds are better at learning when they’re young. These birds have a sensitive period, also called a critical learning period, when they’re able to memorize song details from an adult bird. It’s generally within the first year of life that these birds develop their song repertoires.

However, some types of birds are open-ended learners that can keep adding to their song repertoires throughout their lives. For example, mimics like the Northern Mockingbird are open-ended learners.

When I lived in California, there were a few evenings where I heard the beeping sound of my car alarm being turned off. I thought someone was unlocking my car, maybe trying to steal it. It was confusing. But I was relieved when I figured out that the sound was only a cheeky mockingbird in my neighborhood, engaging in some open-ended learning.

A cheeky Northern Mockingbird.

Often, it seems, there is also a genetic, instinctual component to a songbird’s learning. Many of the species studied so far are apparently born with what we call an auditory template. This is a filter or framework that allows the bird to learn the correct song, from members of its own species, rather than some other random bird in the neighborhood.

Another similarity with humans is that young birds, fledglings and up to a year or so old, will often sort of babble to themselves as they’re figuring out how to sing properly. These adorable practice vocalizations are called subsongs. This is much like a human toddler prattling on nonsensically about who knows what. Subsong seems to be a necessary step in the process for birds. First is the learning phase, then we have this practice phase.

I want to point out that there’s a lot of variation to how birds learn their songs. So some of what we’re talking about here is generalized. Just keep that in mind.

Now let’s consider those birds that don’t learn their songs. These birds are genetically programmed to sing a particular song or set of songs.

Remember that we have that division within the perching birds known as the suboscines. These are the flycatchers, cotingas, woodcreepers, and antbirds—as well as birds from a few other families.

In the eastern US, there are two small flycatchers that look, to us humans, pretty much identical. The Willow Flycatcher and Alder Flycatcher. The only way you can tell these little buggers apart when you’re out in the field is by their voices, their songs.

Willow Flycatcher
A Willow Flycatcher. Or is it an Alder Flycatcher? It's hard to say without hearing this bird's song.

In a 1984 study of these two flycatcher species by Donald Kroodsma, 10-day-old nestlings were taken from their nests and into the lab. These baby birds were played recorded songs of the opposite species, to see if they would learn the wrong song. But, nope, they ended up singing precise renditions of their own species’ song. This was a controlled experiment, and it showed that the songs of these birds are baked in from birth. They are instinctual, not learned.

The author of that study, Donald Kroodsma, is one of the world’s experts on bird song. He’s written several books, including the recently published Birdsong for the Curious Naturalist: Your Guide to Listening. It’s a lovely book and you might want to check it out if this topic really interests you.

Let’s move on to talk about some characteristics of songs.

Characteristics of Songs

I’ve been throwing the word “song” around as though it’s something that we can clearly define with respect to bird vocalizations. Well, we can’t.

Birdwatchers and field guides talk about bird songs as something separate from bird calls. Many of us might have a sense of the difference between calls and songs. But, scientifically, there isn’t a simple way to define these as distinct forms of vocalization.

The sounds that we humans think are pretty, or at least complex, more lengthy, and interesting—we tend to call those songs. The label sounds that are shorter and simpler as calls.

For the moment, let’s talk about characteristics of the longer, more complex quote-unquote songs.

Each of these is some combination of notes, which are of a certain frequency or pitch. Notes strung together form syllables, and syllables form phrases. Phrases are discrete chunks of a bird’s song that might be repeated. A song contains one or more phrases.

Different parts of the song can vary in loudness, in amplitude. And there’s also variation in the timbre, the tonal quality of a song.

For example, the opening note of a White-crowned Sparrow song is a relatively pure tone, or whistle. The notes that follow have more overtones or harmonics. The latter sound more buzzy.

Pure tones or whistles are less distorted by vegetation or other objects, so many forest birds use more pure tones in their songs. Open country birds tend to have buzzier songs.

White-crowned Sparrow.
White-crowned Sparrow.

Like musicians, birds have repertoires. Some sing only one song. These are the “one-hit wonders.” At the other end of the spectrum, we have birds with hundreds, or even thousands, of unique songs that they can cycle through.

The grand prize winner in this category is the Brown Thrasher, a North American bird in the Mimidae family, which means it’s a cousin of the Northern Mockingbird.

Research on the vocal abilities of the Brown Thrasher has shown that an individual of this species can have over a thousand different songs in its repertoire. Over two thousand, by some accounts.

Another champion songster is the Winter Wren, an eentsy weentsy brown ball of a bird that has an amazingly complex song. A male Winter Wren can really rock out for such a tiny bird. One researcher noted that, if you account for the size difference, a Winter Wren has 10 times the vocal power of a crowing rooster.

A male of this species has only a few songs in his repertoire. But each of his songs is very complex, lasts 5-10 seconds, and is repeated up to 40 times before the next one.

Functions of Bird Calls and Songs

We’ve been talking about the “how” and the “who” of bird songs. We haven’t really talked about the “why,” the purpose of all these vocal sounds.

Birds spend a lot of time and energy singing, when they might otherwise forage for food, tend to their nests, or write the next Great American Novel. So these songs must be pretty dang important.

They are important, of course, and I suspect you already know what the two primary functions of songs are: (1) Mate attraction and (2) territorial defense.

Males sing to attract females, so they can pass on their genes to the next generation. Many females also sing, but we know much more about the function of male songs, so let’s focus on males for the moment.

Across many studies, it’s become clear that females are attracted by the songs of males of their own species. What exactly are these females cueing in on? It’s not always clear. Many birds seem to be able to discriminate subtleties and details in sound that elude us humans.

Although this surely varies across species, there is evidence that the number of songs that a male sings and/or his performance quality is what attracts females.

Do these or any other characteristics of a male’s song accurately reflect his health, vigor, and genetic quality as a father? Scientists might frame the question this way: “Is song an honest indicator of a male’s health or fitness?”

Research on this has had mixed results. There are cases where there isn’t any apparent correlation between a male’s song and his vigor.

So, maybe birds—at least some of them—are ‘blowing their own trumpets’ when they sing, exaggerating their fitness. An impressive song may not always be a sign of good health or genes. The song itself, with all of its various properties, might be what attracts females.

Many male birds set up and defend breeding territories. Song is a tool for marking the borders of a territory and broadcasting this information. When a male trespasses the territory of another male of the same species, he might be in for serious trouble. But songs can also help minimize aggression and violence. At least when the birds respect each other’s space.

Males often recognize the songs of the other males in their immediate neighborhood. Neighbors usually tolerate each other. The same can’t be said for some shady drifter that just rode into town. Any new bird on the block might be looking to establish his own territory and this might be seen as a threat—one that must be dealt with.

Now, remember that besides those lovely and complex songs, we also have calls. Some species have a dozen or more different sounds they make, besides their so-called song. These calls can have their own distinct functions.

For example, many birds have contact calls, which mated pairs or flock mates use to keep track of each other as they forage. There are calls used only in flight, also perhaps to keep tabs on each other. Males sometimes have specific calls to show aggression towards their rivals. There are also alarm calls that birds use to warn each other about predators. Some birds even have different calls for different predator types, to distinguish between flying vs terrestrial predators, for example.

So calls are a sort of language, and they can serve different purposes.

Other Levels of Variation

One of the themes that pops up again and again at The Science of Birds is variation. Nature is ripe with variation, and we can’t escape it despite all our efforts to categorize everything into tidy little boxes.

There is a lot of variation in bird songs. At several levels. We already reviewed some variation among major taxonomic bird groups.

There’s variation among species, obviously. But there’s also important variation within many species.

This shows up as geographically distinct dialects. This is particularly true for birds that learn their songs. Birds that grow up in a certain area often end up sounding similar, because they learn from their parents or neighbors. This is a form of culture, just like human dialects.

White-crowned Sparrows have been a model for the study of bird dialects. Each male of this species has pretty much one, simple song. But the structure of the song varies across North America.

White-crowned Sparrows in Manitoba sound different from those in California, for example. Song dialects from these two regions both have a pure-toned introductory note, but the rest of the song structure is different.

The parts of a species’ song that get preserved across space and time might be those that are the most attractive and important to females. So the other elements—the ones females don’t care as much about—would be under less evolutionary pressure to stay the same. They’d be free to change from region to region, from generation to generation.

White-crowned Sparrow dialects often differ over short distances. As in, tens of meters. So this species has many local dialects. Some of these seem fairly stable over time, lasting for decades, while others evolve more rapidly.

For example, a recent study of White-throated Sparrows in Canada describes how a particular dialect has spread from British Columbia all the way to Ontario in about 20 years. That’s almost 2,000 miles or 3,000 kilometers. This is the fastest spread of a song dialect that has ever been recorded. And the researchers don’t know what made this song go viral among White-throated Sparrows.

Now, how do we know if two dialects reflect two separate species or at least subspecies? This is a good question, because sometimes two species or subspecies that look identical, to us anyway, are actually genetically distinct—and their songs can reveal this cryptic diversity.

Sometimes dialects are matched by an underlying genetic difference between geographic areas. In those cases, the songs might reflect some hidden species or subspecies diversity.

At small, local scales, dialects seem to be more cultural and passed on as memes rather than through genes. In other words, these are learned behaviors rather than inherited. In these cases, there wouldn’t be any corresponding genetic differences.

But sometimes we find two or three dialects at big geographic scales, like on two halves of a continent. These are what we call regional dialects, and they’re more likely to represent some actual genetic differences that have accumulated over thousands of years. Perhaps when two populations of one species were separated by huge glaciers during the last Ice Age.

Some birds don’t have regional or local dialects.

The Black-capped Chickadee, for example, sings the same tune all across North America.

The Olive-sided Flycatcher also sings the same song across its range. It sounds like he’s yelling, “Quick! Three beers!”

The Olive-sided Flycatcher is not satisfied with only two beers.

Here’s the thing, though. The Black-capped Chickadee is a songbird, a member of the oscine passerine lineage. Songbirds learn their songs and they pass them on to each other the way humans share language and culture. This is why so many songbirds have local dialects. So why don’t these chickadees have dialects? We just don’t know. This is an unsolved, ornithological mystery.

What about that Olive-sided Flycatcher that just can’t wait to chug a few brewskies? Well, he’s a suboscine passerine. Remember that suboscines don’t learn their songs. They are pre-programmed by their DNA to sing their particular songs. So it’s no surprise that the Olive-sided Flycatcher has no dialects across its range.

Variation is also rampant at the individual level. Males of many species develop their own unique songs. These are adapted from the songs of their parents or, more often, their neighbors. There’s good evidence that many birds can identify each other individually by their songs.

Dawn Chorus

If you’re a morning person and/or an experienced birder, you’ve likely heard what we call a dawn chorus. Starting before dawn and continuing for a while after the sun rises, birds of multiple species often sing loudly together. This is the summation of individual birds starting their day with loud proclamations. Often, each species sings at a specific time of the morning. So the composition of the chorus changes as the hours pass.

In the UK, for example, the dawn chorus can start as early as 3 AM. The progression of birds, in order of who starts singing first, generally goes like this: Eurasian Blackbird, European Robin, Eurasian Wren, Tawny Owl, Chaffinch, and so on.

I recently recorded the dawn chorus from my backyard in Oregon, at about 5:30 AM in June. A few species in that chorus were Swainson’s Thrush, Western Tanager, Spotted Towhee, and Pacific Wren.

For many species, the dawn song has a distinct quality and/or more frenetic energy than songs used later in the day. Ornithologists aren’t entirely sure of the reason behind this. One hypothesis is that male birds are driven to aggressively re-establish their presence and their territorial boundaries every day, first thing in the morning. Another explanation is that the wee hours of the morning are best for singing because it’s too dark to forage for food. Singing later in the day is relatively wasteful from an energy standpoint, because that’s when a bird could be searching for food.

Depending on where you are in the world, the dawn chorus can be soothing and beautiful or it can wake you up way too early with the strident sounds of screeching, barking, and wailing birds. In any case, the dawn chorus is a fascinating aspect of bird behavior and a wonderful part of our world’s soundscape.

Female Song and Duetting

As I mentioned, it’s not just males that sing. Female birds of many species make sounds. Some are short, simple calls that closely match the calls that males make. But females of hundreds of species produce more complex songs.

This phenomenon is more common in tropical birds. And since a disproportionate amount of bird song research has focused on temperate species in Europe and America, it’s not surprising that many people think it’s only male birds that sing. This is just a geographic bias.

Female songs seem to serve the same functions that male songs do. Primarily mate attraction and territorial defense.

But there also seem to be some song functions unique to females. Some female songs seem to advertise reproductive state. For example, when she’s ready to breed, a female Alpine Accentor sings to alert males across her mountain meadow habitat. Females of some species also sing to keep in contact with their fledglings.

Females sometimes join their male partners in a duet. These can be simple call-and-response exchanges or they can be highly elaborate, tightly synchronized performances.

This behavior, duetting, is also more common among tropical species. Duets probably help reinforce the bond between a mated pair. Unlike many temperate species, tropical birds tend to have long-term monogamous bonds. So duetting might make more evolutionary sense for those birds of tropical lineages.

Mimicry

It’s estimated that about 20% of the world’s songbirds exhibit some level of vocal mimicry. That is, they do impressions. Of other birds, mostly, but sometimes they mimic the sounds of frogs, cows, chainsaws, or other things they hear in their environments.

It’s not always clear why birds do this. Sometimes, they seem to do it to trick other birds. In my backyard, for example, I often hear what sounds like a Red-tailed Hawk. I can sometimes tell that it’s actually a Steller’s Jay doing his best impression of a hawk. The crafty jay may be trying to spook other birds away from the bird feeder.

But some of the most impressive mimics don’t appear to be trying to trick other species. A male Northern Mockingbird can have a repertoire of over 150 songs that he cycles through. Many of these songs are borrowed from other species. It’s hypothesized that birds like this are mimicking to increase the diversity of their repertoires, because knowing lots of songs is impressive to the ladies.

Mockingbirds and thrashers are in the family mimidae, which has that “mime” root right there in the name. The starlings, which are also amazing mimics, are in the closely related family, Sturnidae.

Another champion of mimicry is the Superb Lyrebird in Australia. This bird mimics the songs of many other species in its environment. And its also famous for doing amazing impressions of chainsaws, cars, and other human noises.

Superb Lyrebird.
That's not a chainsaw—it's a Superb Lyrebird! Photo by John Manger.

Bioacoustics

Using bird sounds to identify species has long been invaluable for birdwatching and research in ornithology.

And now technology has advanced to where fancy computer algorithms and machine learning can help us ID birds automatically using just their sounds.

Mobile apps that use images to identify birds have been around for a few years now. Check out the Merlin app, for example.

But now there are apps that ID birds by their songs. These have come a long way. I’ve been playing with the BirdNET app and it has been really impressive. BirdNET uses simulated neural networks to analyze bird sounds rapidly. Do I know how simulated neural networks work? I’m not going to lie… I don’t. Maybe you do. In any case, this kind of computational magic can do some pretty cool stuff. This technology is revolutionizing some aspects of biological research, including ornithology.

Final Thoughts

I hope this article inspires you to pay some extra attention to the vocal sounds of birds.

If you’re a relatively new birdwatcher, I encourage you to learn the songs and calls of some common birds in your neighborhood. When you begin to recognize species by their sounds, it’s really exciting and rewarding.

You’ll be able to know which birds are around even when you can’t see them. This opens up a whole new way of experiencing nature.

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