From the Mouths of Owls

I have fond memories of many lessons in elementary school, but one of the shining highlights was the day us kiddos were each given an owl pellet to dissect. The ugly little grey-brown lumps, the size and shape of stuffed grape leaves, looked a whole lot like any number of small critter scat one might discover on the forest floor, and while they were indeed evidence of wild digestive systems at work, poop they were not. Along with our pellet, we were given tweezers and microscopes, and it didn't take much prodding before squeals traveled the room, for each and every nugget contained a terrific assortment of tiny, nearly perfectly clean, rodent bones. Like archeologists at an ancient burial site, we gingerly tweezed through the furry fibers, lifting out everything from skulls to the most minute metatarsals. I don't know if every kid in the room felt this way, but for me, it was a bit like Christmas... and I've been dying for the opportunity to paw through an owl pellet ever since. To my rescue this spring: the Great Mount Auburn...

Mount Auburn Cemetery in Cambridge, MA became a bit of an avian circus last spring when the rather conspicuous nest of a resident pair of Great Horned Owls was discovered. The proud birds produced 2 handsome chicks and raised them successfully in front of eager crowds. Sadly, the female of that mated pair, Roxanne, died tragically this spring - I'm told an autopsy revealed she was "with eggs" as well... such a shame. Luckily for us, her baby-daddy, Alexander the Great (I didn't name them) is still very much alive, and on one early visit to the Dell last month, it occurred to me that I might possibly be able to sleuth a pellet or two from under his favorite towering pine roost...

Oh happy day - not only did I find a pellet, I found heaps.


Owls tend to select favorite trees (most often coniferous ones) - right down to the branch - so combing the ground beneath a known roosting spot is a bit like crawling around underneath the kitchen table of a toddler - each day, a predictable selection of tidbits hits the same spot on the floor... In this case, substitute Cheerios and frozen peas with mouse femurs and shrew pelvic bones, and what you get is nothing short of a bonanaza for a naturalist.

If you missed out on aforementioned owl pellet dissection day in elementary school, you may be wondering: So if it isn't poop, what is an owl pellet anyway? Well, here's how it works:

Owls, like all birds, must swallow their food relatively whole; they may pull it apart with their beak and claws, but without teeth, they can't masticate (chew) their food. In creatures with teeth, mastication is actually the first step of digestion, but birds skip most of that and just send their meals down the hatch. Since owls are birds of prey, and meals consist of small animals, an owl's stomach is faced with an assortment of rather challenging-to-digest fare; some of it, like fur, feathers, bone, teeth, and claws, can not be digested at all... instead, it must be "ejected" via the same route it went in.

When an owl swallows a meal, it enters the mouth, passes down the esophagus and then heads through the first of a 2-part stomach (all birds have a 2-part tummy). Stomach #1 is called the proventriculus, or "glandular stomach", and it produces enzymes and acids - just like your own stomach - that begin the process of digestion. Next, the partially-digested owl snack moves on to stomach #2: the ventriculus, or "muscular stomach". The ventriculus, aka the gizzard, has no digestive enzymes, but uses muscular contractions to continue the process of digestion and move soft and soluble food bits on to the rest of the system (intestines, liver, pancreas, etc) where nutrients will be absorbed by the body. At this point, the ventriculus also serves as a block for all of that INdigestible material that the body can't process. Within hours after eating, all that fur, bone, and tooth (and sometimes inorganic items like leg-bands from songbirds!) is compressed in the gizzard into a pellet, which travels back up from the ventriculus and into the proventriculous, where it will sit for up to 10 hours before being regurgitated.
 

An owl pellet in the proventriculus thoroughly blocks the digestive system while it waits to be ejected, so pellet regurgitation must be quite regular in a healthy bird or it will starve. When the moment comes, like a cat getting ready to yerf a hairball, the owl will hunch up, appear momentarily uncomfortable, then extend it's neck up and out, open wide, and quite silently spit out a pellet (I've found 2 videos of this event - one featuring a young Barn Owl *I'd mute the commentary*, and one seriously impressive Snowy Owl).

*For more reading on owl digestion and the pellet process, check out this great overview - and much more info about owls of all types - on The Owl Pages website.

Predictably, pellets vary in size and contents based on species of owl and diet. Additionally, many other types of birds produce pellets besides owls: herons, gulls, crows and jays, are just a few. Falconers refer to the pellets their hawks produce as "castings" and monitor them closely, since regurgitation serves the additional purpose of scouring and cleansing the crop (the crop is a small pouch in the throat for food storage - owls do not have crops - and if it is not cleaned regularly through regurgitation, a raptor will quickly become ill from bacteria build-up). Of note, hawks have the ability to digest bone matter, so castings contain mostly compacted fur or feathers...

My morning in the cemetery was a fruitful one and I was able to collect many pellets, as well as many clean bones which had already been separated from their fuzzy fur-balls by the elements. A few of the photos above show the "raw" state in which some treasures sat waiting for me, though more fun were the lumps I got to dissect. Below is a record of some of my haul, laid out on a bone identification chart I made and printed out.


To my surprise, I discovered that there is an entire marketplace online for the owl pellet experience. Since it is such an entertaining activity for school children, there are businesses that source and sell pellets in bulk - pre-sanitized - for entire classrooms of eager young biologists to enjoy. There are many and varied bone ID charts (see very bottom of linked page) available for download, and even virtual pellets to dissect online (can't say as that I got anything out of that experience besides the novelty factor, but apparently - we have the technology!). Smart phone users can also download owl pellet ID apps, for quick reference in the field... 

Since my unusually critical attitude towards most "creative" science aides prevents me from being satisfied with most charts and illustrations I come across, I was compelled to create my own bone ID chart, based on reference I found online. I have included my chart here for download (just click on the image), should you be similarly irritated by poorly drawn anatomy, horribly compressed jpegs, and embarrassing composition and typographic labeling. And with that I'll just say - by way of the backyard, or a box in the mail, I hope everyone who loves science and wildlife as much as I do can find an opportunity to discover the wonder of an owl pellet. 

Comments

[Alexia]

Wow, thank you for the wonderful one chart! I was just quizzing people on vole teeth today, scavenged from owl pellets of course. Vole teeth have a zigzag pattern in the enamel.

Chirp,
Alexia

[Natalya]

Glad the post came in handy, Alexia! I didn't know about that unique feature of vole teeth until I started ID'ing the bones I was finding in the pellets - I also learned that some shrews have RED teeth! (again, after finding a jaw in a pellet). You can just barely see the shrew jaw, and a hint of red-tipped teeth, at the top of the row of 3 little jaws in my photo above (clicking the image brings up a larger photo).

If you want a little more info on that phenomenon - I found a pellet-related shrew-teeth post here!: http://bit.ly/L6RPgU

Cheers!

[Christina Rodriguez]

I LOVE your chart! Wish I had this around when I was a kid and we searched through owl pellets. Instead, we used Elmer's to glue them to black pieces of construction paper, then placed them on a table to dry. I then leaned against this table when playing Heads Up 7 Up and inadvertently got a mess of mice bones on my butt. Ah, the things that remind us of childhood...

[Natalya]

That is a fantastic story, Christina! I love the memories that spring up from random things we encounter as adults...