The Science of Birds - The Avian Digestive System

Episode Date: April 10, 2024

This episode—which is Number 94— is all about the Avian Digestive System. That’s right, kids, we’re looking at bird guts today!If you’ve ever wondered what happens to a fish swallowed by a kingfisher or what happened to that piece of bacon swiped off your lunch plate by a cheeky Ring-billed Gull... Well, you're about to find out.I’ll do my best to describe in words the anatomical features we encounter today. But I’ve also drawn a diagram for you. You can check that out on the show notes for this episode on the Science of Birds website. ~~ Leave me a review using Podchaser ~~Link to this episode on the Science of Birds website Support the show

Hello and welcome. This is the Science of Birds. I am your host, Ivan Philipson. The Science of Birds podcast is a light-hearted exploration of bird biology for lifelong learners. This episode, which is number 94, is all about the avian digestive system. That's right, kids, we're looking at bird guts today. Yay!

So this is another podcast episode in the anatomy category. Topics we've covered in other episodes include the avian skeleton, bird eyes, feather structure, and bird beaks. Today's episode should be fun. If you've ever wondered what happens to a fish swallowed by a kingfisher or what happened to that piece of bacon swiped off your lunch plate by a cheeky ring-billed gull? Well, you're about to find out.

I'll do my best to describe in words the anatomical features we encounter today. But I've also drawn a diagram for you. You can check that out in the show notes for this episode on the Science of Birds website, scienceofbirds.com. There should also be a link to that page in the show notes on your podcast app.

All right, it's time to do. dive into the avian digestive system. For birds, food is kind of a big deal. Birds spend most of their time every day looking for food and eating food. Birds are driven by a powerful instinct to eat. Now let's briefly ask a very basic question. Why do birds, or humans or any other animals, have this instinct? Well, food provides an animal with two essential things.

One, it provides energy, then two, it provides the body with raw material for growth and maintenance. As a source of energy, food fuels the trillions of chemical reactions taking place inside the cells at every moment. Those biochemical processes allow an animal to do basically everything, and they're what keeps the animal alive. Beyond energy production, food also supplies the basic building blocks of the body. We're talking proteins, carbohydrates, fats, vitamins, and minerals. These nutrients play crucial roles in various processes, including tissue growth and repair, hormone regulation, and the immune system. This all falls under the banner of metabolism.

Metabolism is the name for the collective activity of all the chemical reactions happening inside the body, the body of a finch or a swan or an octopus or a human. When a bird is just resting, just chilling, perhaps dozing while perched on a branch, it burns a certain amount of food energy every second, enough to maintain body temperature and to keep countless chemical reactions humming along inside the cells. That level of energy expenditure is what scientists call the bird's basal metabolic rate. That's B-A-S-A-L, not B-A-S-I-L, like the tasty

herb that we grind up with some garlic, olive oil, pine nuts, and cheese to make it the pesto. Mmm, pesto. Maybe that's what I'll do to get my energy and building blocks of nutrients tonight. I'll make some pesto. And by make, I mean, put a pre-made pesto-based meal in the microwave. But getting back on track, on average, birds run hot compared to us mammals. In other words, the basal metabolic rates of birds are higher.

They burn more calories per minute and they have higher body temperatures. You can probably guess why that might be. The main reason is because birds fly. Defying gravity by flying takes a boatload of energy. Even those bird species that are flightless today evolved from ancestors that did fly, ancestors that had supercharged metabolisms. All this is to say that birds not only need to eat like every other type of animal, they need to eat a lot and they need to eat often.

But maybe you see the dilemma here. The energy demands of flight are high, which motivates birds to stuff their little bellies with lots of food. But a belly full of food makes the body heavier and even harder to get off the ground for flight. It's a vicious circle, right? A vicious circle like when I hit the snooze button a few times in the morning, hoping to get a bit more sleep. And to keep dreaming some nonsense like I'm in Alaska driving a school bus full of blue and yellow parakeets to a death metal concert. So naturally I hit the snooze button because, of course, it's very important that I find out how

that story ends. But then when it's time for bed that night, I'm wide awake. because I slept in. The following morning, I'm even more tired, so I slam that snooze button, and it's basically a downward spiral from there. But somehow I'm still alive, so that's something. Anyway, birds need to eat a lot to get enough energy to fly. But by eating a lot, a bird gets heavier, which means it needs even more energy to fly, so it has to eat even more, and so on. Birds have solved this dilemma in a couple of ways. First, they generally choose to eat food that has the biggest bang for the buck, food that's densely packed with caloric energy. Imagine a peanut and a piece of celery that both weigh the same. If a bird was forced to choose only one of these food

items, it would probably scarf down the peanut because it has proteins and fats that offer way more calories, ounce for ounce, than the celery. Second, the digestive systems of birds are short compared to those of mammals, and they're highly efficient. Food doesn't stay long in the guts of birds. It gets processed quickly. But the digestive systems of birds are not all the same. They show some variation depending on what they eat.

Birds that eat mostly protein like meat have relatively short and simple diet. digestive systems. But those that eat complex carbohydrates like seeds have more complex digestive systems to match. For example, ornithologists can look at just the gizzard and intestine anatomy of passerine songbirds to tell which of these birds are insect eaters, fruit eaters, or seed eaters. So, digestive system anatomy varies among birds with different diets and among different avian families. The efficiency of the system also varies by diet. Research has shown that raptors use up to 66 to 88% of the meat they eat.

In other words, this is the percentage of the food calories and nutrients that get assimilated into their bodies. That's why you often hear an eagle saying this after it catches a rabbit. You will be assimilated. resistance is futile. Food that isn't assimilated becomes waste. Birds that eat plant material have a harder time with assimilation. A bird that eats soft young plant material might assimilate 60 to 70% of it.

But a bird that eats mature plant material that's harder to digest can only assimilate maybe 30 to 40%. An example of the latter is the spruce grouse, Canakites Canadensis. This species in the pheasant family lives mostly in the far northern latitudes of North America. In winter, the spruce grouse has a special diet. It eats almost exclusively the needles of coniferous trees, like pine, fur, spruce, and larch. This kind of plant material isn't very nutritious, so a spruce grouse is able to assemble. only about 30% of the food it eats in winter.

The size and shape of the digestive system differ not only among groups of birds with different diets. These can change dramatically from one season to the next in the same individual bird. This happens in many types of birds. Digestive organs can grow or shrink by 10% up to 100% between winter and summer. and some of these changes can occur in less than a week. Using the spruce grouse, again as an example, parts of the stomach and intestines in this bird increase in mass by about 75% during the winter.

It seems a bigger gut is needed to process all those pine and spruce needles. And in many birds that migrate long distances without stopping, like shorebirds, large parts of their digestive systems sort of shrivel up before they depart. They atrophy. Gut tissue takes a lot of energy just to maintain. So this seasonal atrophy of the digestive system helps a migrating bird to conserve energy and to reduce some weight. Okay, before we get into the details of each component, let's take a look at the avian digestive system as a whole. Like the digestive systems of all vertebrates, that of birds is essentially a long

tube. It starts at the mouth and terminates at the anus. Or in the case of birds, it ends at the cloaca. The tube takes lots of fun twists and turns along the way, like a slide at the water park. But, you know, with less water and more slime. And come to think of it, if you find yourself sliding down the tube of a bird's digestive system, because maybe you're a fish or a mouse or something, you're probably having slightly less fun than you would at a water park. In any case, the avian digestive system has several specialized sections. Seen all together, like in a diagram, the whole thing is rather convoluted, looking like some kind of fantastical, musical instrument, like something Dr. Seuss would dream up. I like to imagine.

that if a bird's digestive system was an instrument, like a horn, it would sound like this. Or maybe it would really sound something more like this. The tube is, more technically, called the digestive tract or the elementary canal. I'll use these terms interchangeably. In humans, the digestive tract is about 30 feet or 9 meters long. You've got a very long tube coiled up inside your abdomen right now. In birds, it makes more sense to talk about the length of the tube relative to the bird's overall body length, since birds come in such a wide variety of sizes.

On the short end of the spectrum, birds like the common swift have a digestive tract that's only about three times body length. At the other extreme, the common ostrich has a tract that's about 20. times its body length. In general, the digestive tract is longer in birds that eat food that's more difficult to digest. All right. Now it's time to take a magical journey, a fantastic voyage, if you will. Join me as we venture into the dark, into the strange and slimy tunnel that is the avian digestive tract. We begin by entering the mouth. Since birds don't have teeth, they don't chew up their food the way most mammals do.

So the beak and mouth of a bird are more for catching food, for gathering food from the environment. Digestion in general has a series of steps, beginning with ingestion, which is where we are now, and ending with defecation. In the middle, the food items are broken down into smaller and smaller pieces by a combination of mechanical digestion and chemical digestion. Mammals like us start right off with mechanical digestion. We use our teeth and strong jaws to mash our food into smaller bits. Then, when the chewed-up Oreo cookies or tater tots or whatever get down to the stomach, chemical digestion kicks into high gear. Well, this process is somewhat reversed in birds. Birds start with chemical digestion, then they proceed with mechanical digestion.

And we'll get into more detail about that very soon. But for the moment, we're still in the mouth. This is where a bird grabs a food item and just horks it down its gullet as fast as possible. It could be an acorn, a berry, a grasshopper, a fish, or a small rodent. Whatever it is, it's going down the tube. Let's not forget the tongue, though. Bird tongues come in an interesting variety of shapes and sizes. Some are tiny and simple.

Others are super long and relatively complex. Many woodpeckers, for example, have long tongues with sticky or barbed tips that help with pulling grubs or other insects out of holes. One thing bird tongues don't seem to be all that important for is taste. Birds have fewer taste buds than mammals, and most of the taste buds in birds are either on the roof of the mouth or further down in the gullet. So there's no point for a bird in trying to savor the flavor of that wiggling fish. Just swallow the thing whole and let the digestive system do its thing. Now as we, like the fish, go down the hatch, we enter the esophagus. This is a long

section of tube that connects the mouth to the stomach. Glans in the esophagus secrete mucus to lubricate the passage so that food slides down smoothly. Waves of muscle contractions in the esophagus actively propel the food further along. Besides swallowing food, the esophagus has been modified in some birds for some other functions. Now I know I was joking earlier when I said the avian digestive system looks like a musical instrument, but some birds really do use their esophagus to make sound. For example, birds like busters, umbrella birds, and sage grouse have what are called esophageal air sacs. These are balloon-like expansions of the esophagus that amplify sound. Displaying birds inflate their esophageal airs to make louder, more impressive

songs or calls. Even the humble rock pigeon that you see strutting around on the sidewalk has these air sacs. A male rock pigeon inflates his air sac while he's cooing as he attempts to win the heart of a female. Next we have the crop. This is a specialized, permanently expanded section of the lower esophagus. It's a pouch-like holding chamber for food. A foraging bird can quickly fill its crop with food, sort of like a person frantically filling a shopping cart on the TV show Supermarket Sweep. Later, when the bird feels safe and it has some downtime, it swallows the contents of its crop leisurely, bit by bit. Many bird species, though certainly not all, have a crop. For example, we find this structure in pigeons and doves, pheasants, quail, and other birds in the order

Galiformis, cormorants, vultures, hawks and eagles, parrots, and some passerines. The shape of the crop varies among types of birds. Shapes include the relatively simple and narrow spindle to the unilateral sack, to the relatively elaborate double sack. And of course, you've probably heard of crop. circles. But I'm pretty sure crop circles have more to do with aliens and flying saucers than they do with birds. Well, that's what the birds want us to think anyway. The crop has evolved a unique function in one particular bird species. In the Watsine, the crop is enormous and it's

teeming with bacteria and other single-celled microbes. Watsines live in the lowland rainforests of South America, east of the Andes. They eat leaves and buds. Plant material like this is difficult to digest because it's made of mostly cellulose and other complex carbohydrates. The microbes in a Watsine's large crop secrete enzymes that break down and ferment cellulose and all that other plant stuff. So the Watsine depends on symbiotic microbes, very similar to how cows and other ruminant mammals depend on microbes. The Watzine is a strange and wonderful bird, and I'll definitely make an entire podcast episode about it at some point. Another interesting thing related to the crop is crop milk. I've talked

about this before on the podcast. When raising chicks, some types of birds secrete a special substance called crop milk. This fat and protein-rich slurry comes from the lining of the crop. A parent bird regurgitates crop milk into the hungry mouths of its chicks. Crop milk is produced by pigeons and doves, by flamingos, and some penguins. Now it's time to enter the stomach. I guess if the English language made any sense, we'd pronounce it stomatch. But English is wacky, right? So stomach it is. All birds have a two-chambered stomach.

Food first reaches the chamber called the proventriculus. In most birds, this is the smaller of the two chambers. It's sort of spindle-shaped, usually with a more bulbous end that connects to the second chamber. Now, if the first chamber is the pro-ventriculus, you might think that logically the second one should be called the amateur ventriculus. But it's not. It's just called the plain old ventriculus. But most of us know it by its other name, the gizzard. Gizzard is much more fun to say than ventriculus. And hey, it rhymes with wizard. In terms of shape, the gizzard is more spherical or disc shape. Each of the two chambers of the stomach has a different function.

The proventriculus is where chemical digestion really gets going. It's lined with glands that secrete digestive enzymes and hydrochloric acid. These chemicals begin to break the food down at the molecular level. The proventriculus is especially well developed in seabirds and other birds that eat whole fish. All those scales, spines, and pointy bones need to be digested with strong enzymes and acid. Some seabirds also have the superpower of spewing projectile vomit at their enemies, all thanks to that well-developed proventriculus. After food stews for a while in the acid bath of the proventriculus,

it's time to move into the gizzard. The gizzard has thick walls made of strong muscle tissue. This is the site of mechanical or physical digestion. Those strong muscles contract to squeeze and pulverize the food. The gizzard is sort of like a fist that squeezes and crushes whatever's in its grip. The gizzard is like, I will crush you! The inside surface of the gizzard has a leathery lining made of protein and carbohydrates. It's called the gastric cuticle or coilin, K-O-I-L-N.

This coylin membrane protects the gizzard from all those harsh chemicals seeping in from the proventriculus. I like the name coelin better than gastric cuticle. It has a nice ring to it. Maybe I'll name my first kid, Coilin. It's gender neutral, so that's nice. Coilin Phillipson. I'm sure little Coilin will be happy to learn that their name was inspired by the leathery lining of the avian gizzard. The coilin membrane also provides protection from the sharp bones of prey and from rocks. Yes, rocks. You may already know that many birds eat gravel or small stones. These end up in the gizzard, where they help to mash up food. Rocks in the avian gizzard work the way teeth do in the mouths of mammals. Eating rocks and dirt is called geophagy, G-E-O-P-H-A-G-Y. Rocks swallowed for the

purpose of digestion are called gastroliths. That translates as stomach stones. Paleontologists sometimes find gastroliths in the fossils of herbivorous dinosaurs, which is pretty cool. Gizzard shape and size in modern-day dinosaurs, that is, birds, varies somewhat depending on diet. For example, our friend the spruce grouse has an extra large gizzard. This helps to grind up all those pine needles and whatnot. In contrast, some songbirds that feed mostly on nectar have relatively small, simple gizzards. After all, nectar doesn't need to be mechanically digested

since it's just a liquid. The gizzard is also where pellets are formed. I talked and joked about pellet formation in owls when I did an episode about those birds. But hawks, grouse, swifts, night jars, and a bunch of songbirds also cough up pellets. A pellet is made of bits and pieces of prey that are especially difficult to digest. Bones, fur, teeth, earrings, bracelets, or the exoskeletons of invertebrates. These materials are compacted into a pellet by the gizzard, then hacked up a few hours after the prey was first eaten.

So that was the stomach. It has two parts, the proventriculus and the ventriculus, aka the gizzard. The gizzard has an exit hole that opens into the intestines. But some birds have what we might think of as a third chamber of the stomach, a small structure called the pyloric stomach. Grebes, for example, have this structure. Grieves have this seemingly weird behavior of and then swallowing some of their own feathers. For hundreds of years, this behavior was a mystery to scientists. Why do these birds do that? But what we now understand is that those feathers end up in the gizzard and the pyloric stomach. In the latter chamber, the feathers compact together into a sort of plug. This plug seems to act as a filter, keeping sharp fish bones and other

crunchy stuff from entering the delicate intestines. And then Grebes eventually regurgitate their pyloric plugs and repeat the process. Our journey through the tube continues. We have just entered the small intestine. It's called small because. of its diameter, not because of its length. The small intestines are a lot longer than the large intestines, especially in birds. And what exactly happens in the intestines? Well, a couple of things. First, there's another round of chemical digestion. Glans lining the walls of the small

intestines secrete chemicals that break down fats, sugars, and other nutrients. Some of these digestive chemicals also enter the intestine after being made in the liver or pancreas. Second, nutrients get absorbed through the walls of the intestines. They end up in the blood or in the lymphatic system. The length of the small intestine varies among birds, depending on, you guessed it, what they eat. Among passerine birds, for example, the ones that eat insects have relatively long small intestines, while the ones that eat nectar have short intestines. The inner surface of the intestinal tube is folded and wrinkled. This uneven lining increases the surface area available for absorption of all those nutrients.

Fats and waxes can be particularly tricky to digest. Some birds are better at this than others. Seabirds, for example, are great at digestive. the fats and waxes that are common in their diet. They have higher volumes of bile in their intestines to help with fat digestion, and they also move wads of food back and forth in the small intestine to repeatedly expose it to enzymes that break down fats and waxes. Other birds regularly eat some wax in their diet. Wax is impossible to digest for most animals, but these birds have evolved ways to break down this substance. Birds that digest wax include the yellow-rumped

warbler and the tree swallow in North America. During the winter especially, these little birds eat the fruits of bayberry plants, which contain lots of wax. And then there are the honey guides. These are the 16 species in the family indigatoridi. They specialize in eating the larvae of bees, honey, and yes, beeswax. Honey guides have a set of. of enzymes in their digestive systems that allow them to break down waxes. I mentioned the liver a moment ago. This is actually the single largest organ in the body of a bird. The liver has several functions.

It filters toxins and waste substances from the blood. It processes nutrients and it makes bile. Bile is an alkaline fluid that helps with the digestion of fats. A small tube or duct connects the liver. to the small intestine, where bile is released to help break down fats. Onward to the large intestine. This section of the digestive system is relatively short and straight in birds. This is where water and salts are reabsorbed, and where wastes are packaged for export.

One interesting feature of the large intestine in some birds is the presence of structures called Sika, that's spelled C-A-E-C-A. Sika are narrow side pouches that split off from the large intestine. Sika is the plural form, sikum is the singular. About half of the world's bird species have Sika, and naturally they come in different shapes and sizes. Small, water-soluble food particles make their way in and out of the Sika. These pouches are home to an assortment of microbes.

These tiny organisms help to further break down uric acid and carbohydrates that are in the food. At last, we're reaching the end of our long journey through the darkness, through the elementary canal. There's a light at the end of the tunnel. It's the cloaca! Birds, unlike mammals, have just a single opening at the back end. The cloaca is where feces and uric acid from the kidneys mix to form bird poop.

And this is where the eggs arrive from the ovary. All of these things exit through the cloaca. So the cloaca has a function in getting rid of wastes from both the digestive and urinary systems. And it plays a central role in the reproductive system. The cloaca is party central. There's a lot going on down there. And just like that, we emerge into the light of day. We've explored every nook and cranny of the avian digestive system.

As a review, the sequence goes like this. We have the mouth, the esophagus with its crop, the proventriculus and gizzard that together form the stomach, the small intestine, the large intestine with its sika, and finally the cloaca. Now, I know the cloaca isn't necessarily the most glamorous place to end today's episode. But hey, as scientists, as naturalists,

we just tell it like it is. we have to face reality no matter how icky. But if we want to be more poetic or superstitious about the whole thing, just remember that in many cultures it's considered good luck to get pooped on by a bird. What certainly is true is that because birds evolved the power of flight, they require enormous amounts of energy to fuel their supercharged metabolisms. They get that energy from food in the form of small prey animals or from plants. The avian digestive system displays a sophisticated suite of adaptations that allow

birds to squeeze as much energy as possible from their food, while still keeping their body weight to a minimum. It's quite an amazing thing. Thanks for joining me here for episode 94. Did you learn a few things? Maybe a couple of new bits of ornithological jargon, like Proventriculus or Coelan. In any case, I hope you enjoyed the episode. As always, I want to thank those of you who support the Science of Birds through Patreon. You are making it possible for me to keep making these episodes. My newest supporters slash members are Sheila Fabrizio, Gail Jindrick, Amy Borchart,

Rebecca Lagas, M, Barbara Obergfeld, Priscilla, Sarah Martinez, and Ryan Shostack. I'm sorry if I mispronounced any of your names. And wow, thank you guys so much for stepping in to support my work. I'm incredibly grateful for the help. If you have some interest in becoming a supporting member, you can check out my Patreon page over at patreon.com slash science of birds. You can also reach me by email if you have something you'd like to share,

Maybe your personal theory on the origin of crop circles, or your favorite recipe that uses crop milk, or pesto. In any case, my email address is Ivan at Scienceofbirds.com. Again, this is episode 94. You can check out the show notes for this episode, along with some curated photos of species I talked about today on the Science of Birds website, Scienceofbirds.com. I'm Ivan Philipson, and I wish you a joyful day. Cheers. Thank you.