Out of the Mouths of Owls: Casting Pellets

Wednesday, January 29, 2020
If you've ever had the privilege of viewing an owl for a period of time, you may have seen them perform various behaviours like preening, flying, sleeping, and maybe - if you're really lucky - hunting and consuming their prey. There's a chance you may have also witnessed an interesting behaviour where the owl stretched its neck, held its beak agape, hunched over (seemingly gagging), and perhaps shook its head furiously, dropping a dark object from its mouth. This action is called casting, and the dark object that projected from the owl's beak is called a pellet.

Snowy Owl (Bubo scandiacus) casting a pellet. Photo by Brenda Hartley-Foubert

Like most birds of prey, owls swallow their food whole, or nearly, depending on the size of the prey. Many bird species, such as Mourning Doves and Blue Jays, have a sac called a 'crop' in the throat that serves as a food storage compartment - the reason they're able to empty your feeders so quickly. Owls lack a crop, so the prey they swallow is passed straight to the digestive system which begins in their two-part stomach. The first part is called the glandular stomach, this section produces the acids and enzymes needed to begin the process of digestion. The second part is the muscular stomach (or gizzard), which acts as a filter to catch insoluble matter including fur, feathers, teeth, and bone. The soluble pieces of food pass through with ease and are broken down by contractions in the stomach, and make their way through the rest of the digestive tract. These soluble items are turned in to waste which is held in the cloaca, and excreted through the vent. 

So what happens to the insoluble matter held back in the gizzard? This is where that dark projectile known as a pellet, is formed. The indigestible food items (fur, feathers, bone, teeth, etc) are compressed in the gizzard for several hours, then move back up into the glandular stomach where it will stay for up to 10 hours before being regurgitated. Once the pellet is cast, the digestive process is finished and the owl is able to feed again. Not all pellets are created equal, some will be larger than others depending on the amount of prey consumed within a few hours of each other. For example, if the owl were to eat three meadow voles in a short period of time, the pellet would contain the contents of those three voles rather than forming one pellet per vole.

Long-eared Owl (Asio Otus)
Pellets cast by a Long-eared Owl. Contents include bones and fur from mice & voles.
Because owls are creatures of habit, finding a pellet can be a good indicator of an area the bird frequents, particularly if there are more than one pellet and an abundance of whitewash (poop). I personally have been able to find several owls of various species by paying close attention to the ground below the trees, especially in evergreen stands. Winter is often the best time of year to find fresh pellets, as the contrast against the snow will help to spot them. While the thought of touching one of these regurgitated balls of animal parts may repulse some (okay, most) people, dissecting pellets can help provide important information regarding the bird's digestive health, what it eats, and what other animals live in that particular habitat. Researchers have even found bird bands that were attached to a smaller bird when it was consumed. If you happen to find an owl pellet, you can try this fun and educational activity at home, just be sure to follow the necessary health & safety precautions.

Black-billed Cuckoo (Coccyzus erythropthalmus)
Owls aren't the only birds who cast pellets. Any bird who consumes food that has indigestible matter such as bones, teeth, fur, feathers, insect exoskeletons, beaks, and some plant matter, also casts pellets as part of its digestive process. Birds besides owls who cast pellets include hawks & falcons, eagles, herons, cormorants, grebes, kingfishers, shrikes, swallows, and more. One bird that has a particularly odd way of producing a pellet, is the Black-billed Cuckoo. This species is specialized in eating spiny caterpillars such as Fall Webworm, but cannot digest the prickly hairs so they wind up sticking to the bird's stomach lining. To get rid of the spines, the Black-billed Cuckoo will shed its entire stomach lining, coughing up one giant, spiny pellet.

Whether you find them fascinating, or repulsive, pellets play a vital role in an owl's digestive system, and are an invaluable source of information for scientists, researchers, and conservationists. I will leave you to witness the enchanting act for yourself, performed by an adorable Barred Owl chick on a forest floor.

    Barred Owl Chick Regurgitating Pellets video                        by Justin Hoffman Outdoors                                                                                                                                                                                                                                    

Happy trails!
- Shayna