What makes owls regurgitate their food? by Stefan Pociask
Answer by Stefan Pociask:
This’ll be a lovely article just before breakfast. I’ll try to make interesting, the subject of things that come back up… specifically, in birds. It’s not as bad as it sounds, and I’ll be gentle. You may find it interesting because I’ll try to clear up a lot of confusion about a subject that’s pretty complex. Complex, because there is a wide variety of ways this happens, depending on the species. I’ll simplify best that I can.
There are different terms involved with this, and we’ll find out the difference. They are “regurgitate”, “cough-up”, and “vomit” (don’t worry… it’ll be okay).
There are three more terms we have to learn: “crop”, “glandular stomach”, and “gizzard”. The latter two, together comprise the stomach. Basically, its 1st stomach and 2nd stomach. The 1st stomach (glandular) is where the acids and enzymes are. It softens the food. The 2nd (gizzard) is muscular, and crushes & grinds the food. And the crop… well, that’s just a sort of storage area to handle the overflow before it gets to the stomach. And to confuse things just one tiny bit more… only certain birds have crops. Hawks have crops. Owls do NOT have crops.
We’ll start with the owl, and answer the main question, upfront. “What makes owls regurgitate their food?”. Okay, so… this question is inherently flawed for two reasons, because “technically” the owl neither “regurgitates”, nor is what comes up actually “food”. I’m assuming the question refers to the owl pellet.
“Regurgitation” is part of a process by which an adult bird catches food, stores it temporarily, softens it up a little in its glandular stomach, transports it back to its young in the nest, and then feeds it to them in a form that is easier for them to eat. Oh… and on top of all that, it is also transferring some of its gut bacteria to the baby, to boost their ability to digest, in a sense… seeding the baby’s gut with good healthy digestive bacteria.
An owl does none of that, when feeding its young. An adult owl will bring back an entire animal, to the nest, in its talons. At the nest, it them proceeds to pinch off tiny pieces of meat, and feed it to each owlette. The bigger the baby, the bigger the pieces, until the babies can bite off pieces of their own. Here I am, feeding very young baby Barred Owls little bits of meat, just like mama would.
So… what is an owl pellet? To begin with, owletts don’t produce pellets until they are on their own and catching their own food. As a baby, it is fed just meat, which easily digests. But as adults… when an owl catches something like a mouse or chipmunk for itself, it will try, if at all possible, to swallow it whole. Skin, bones, teeth… the whole shootin’ match. This fella is about to gulp down the mouse I just brought him.
They have pretty huge mouths, when compared to other raptors, like hawks. BUT… compared to those other raptors, owls have very weak stomach acids. The result is that the bones, skin and teeth of what they ate does not digest. Only the meat digests. The extra junk stays in the gizzard until the meat is gone, and what is left is the pellet, all compressed into a neat little bundle. This is then guided back up the esophagus, and easily slides out. This is called “coughing up” a pellet. These pellets are excellent tools for study. They are generally pretty dry, because they were squeezed really tight. But they can be examined by students or researchers and often an entire skeleton can be reconstructed from this pellet of fur and bone.
This process has nothing to do with feeding their young, and what comes up is not actually “food”… it’s everything BUT the food, because the food got digested. This is how regurgitation differs from coughing-up.
Hawks and eagles also cough-up pellets, but since their stomach acid is much stronger, the bones get digested, and the skin gets digested, and all that comes back up is the actual fur and maybe the teeth. Plus, since hawks don’t swallow entire animals in one gulp, the bones already are partially broken up by the beak, or… they just don’t swallow as many bones, but rather pick a carcass clean. Here is a beautiful Red Tailed Hawk, picking clean a meal I just fed her, and leaving the skin behind.
So which birds actually regurgitate? Sea birds… gulls & terns… practice true regurgitation. These birds can be 100 miles away from their young when hunting fish. They can be away for a couple days. It’s not worth it to catch one fish and fly all the way home with it, so they put many fish into their crop as a way to store them, and some of it gets sent to the stomach. When they get back home, they open their mouths and allow the chicks to eat the partially digested fish from their throats. If they are very young, the mama will regurgitate from her stomach… or, she can decide to give larger, less digested fish from her crop. They don’t use their feet and beaks to tear off pieces to the extent that raptors do. So by being partially digested, the chicks can just help themselves. This is true regurgitation.
Seed-eating birds do the same. They will first fill their crop, and the crop will meter out seeds to the stomach slowly. When a mama bird feeds her young, she can decide how much hard seed to give from her crop and how much partially digested seed to give from her stomach. Insect eaters like martins, or worm eaters like robins also store food in their crop, and can decide which form of food to hand out to chicks. So all these types of birds can regurgitate if they want, or crop feed if they want. Cropless birds like owls and geese don’t have that choice.
Having a crop can be an advantage, but also it can be dangerous. Since it is very close to the surface of the skin, it’s not protected like a stomach is. If it gets super-full, it runs the risk of bursting because the skin is stretched so tight.
If a sharp stick happens to scratch a too-full crop, the bird could have a big problem. I’ve seen burst crops at the Raptor Center. One hawk had a frog leg sticking out. That had to be sewn up quick.
Finally, we’ll get to vomiting. This is the infamous domain of vultures. Since vultures are masters of the super-high glide, way up in the sky… their wings are not great at getting them up off the ground very quickly. In order to not be vulnerable when on the ground, they have perfected the art of instantaneous projectile vomiting, to immediately lighten their load of whatever they just stuffed themselves with. If you startle them, they will spasm their stomach and their crop and jettison everything quickly, and get up off the ground… like those airplanes in movies, where they are throwing seats and luggage out the door to lighten the load. And… as raptor rehabilitators have found out, they will give you a bath in it whenever they feel like it, when you handle them. Depending on what they last ate, the yuk-factor can be considerable. So that’s the difference between regurgitating and vomiting. One is a controlled function in order to feed young’ins, and the other is an emergency massive dump of everything possible… even the most digested portions.
And now you know far more than you ever thought you wanted to know about birds “bringing things back up”. You’re welcome!
These are all healthy actions. Sick or poisoned birds will also throw-up, of course, just like you do when you’re sick, if need be. So I guess that’s the 4th category.
See… that wasn’t too bad, under the circumstances. Now, go enjoy that breakfast, and I’ll see ya next time.