More detail on the Owl Barf Bucket I posted on April 27:
Seattle.
Little clumps of feathers draw me towards the wooded edge of a meadow between a picnic area and a playground near the forest. One clump just under foot, a bigger one beyond, more scattered across the grass. The late owner of the feathers is lying in a bloody heap under a big Douglas Fir. A female Mallard - perhaps a meal abandoned by one of the Bald Eagles nesting close by.
I trace the trail of feathers back to the main path and almost step on a little heap of grey fur and bones. Stoop down to pick it up and find another, and yet another partially hidden in the grass.
Owl pellet containing Deer Mouse bones.
Over the years I've found it very difficult to find an owl roost tree when you're actively looking for one. You stumble onto them when you're looking at something else. Find yourself unexpectedly focused on a heap of fur and bones. Some are dark like the forest duff, half covered in the fallen stuff that rains incessantly and almost imperceptibly from the canopy. Some are pale, the bones bright white against the ground. Some are caught in the bark of the tree you are standing under. Sometimes if you look up you'll see an owl.
There are at least a dozen of these heaps under the tree, the undigestible remains of the owl's past meals. I gently tease apart a fresh one. The delicate bones of at least three Deer Mice (Peromyscus maniculatis): one complete skull, one right mandible, six pair of pelvic bones, three tibia.
And a second. The skull of a Creeping Vole (Microtus oregoni). Two femur. Radius. Ulna. Ribs. Spine. Sturdier than the mouse, built for tunneling.
Owl pellet containing Creeping Vole skull and bones.
A third is weathered and heavy with thick rat bones. These may represent the mainstay of the owls' winter diet.
More > > >
A couple of years ago I spent many hours hunkered over a dissecting microscope in the bowels of our local natural history museum, pulling apart owl pellets, comparing the bones I found with those of sample skeletons. My goal was to complete a list of the native mammals that were still living in this lakeside urban forest. The big guys - Coyote, Raccoon, Mountain Beaver, Douglas Squirrel, Townsend's Chipmunk, Beaver, River Otter, Muskrat - we knew they were there because we'd seen them going about their business. The little guys, those who spend their lives creeping under the duff, those were the mystery.
My notion was that the little guys were probably prey for the local Barred Owls, and if so, their remains would show up in the pellets the owls coughed up after a good meal or two. So I went hunting for owl pellets. Over two years I found 33 pellets. Some because I'd stumbled across a roost tree. The rest were found by accident, one here, one there.
The mammal curator at the museum generously offered me a space to work. Access to a dissecting microscope. Sample skeletons. A place to store my bones. Answers to many questions. Time.
The results may not seem like much, but here they are. Is it a complete list? I don't know, but my hope is that this information might be useful to some future grad student or another curious civilian.
Remains of native species found in owl pellets in the forest, 2007-2009, listed in order of the number found:
Townsend's Vole (Microtus townsendii)
Deer Mouse (Peromyscus maniculatis)
Trowbridge Shrew (Sorex trowbridgii)
Creeping Vole (Microtus oregoni)
Townsend's Mole (Scapanus townsendii)
In addition, remains of the following introduced species were present, again listed in the number found:
Black Rat (Rattus rattus)
Norwegian Rat Rattus (Rattus norvegicus)
House Mouse (Mus musculus)
The limiting factor in this study was the number of pellets I could find. 33 pellets is a pretty small sample. Still, I worked throughout the seasons whenever I had pellets to take apart. In doing so, a pattern began to appear: the pellets I found in the winter seemed to contain more rat bones than those I found in the spring and summer.
It makes sense that the owls would take more rats in the winter. Their native prey tend to tunnel then, but the rats continue to hang out in the open, especially along the shoreline where people gather and garbage accumulates. Rats could be the easier prey in winter, and easy is good. Unfortunately, I had no way of dating the pellets. Expiration bar codes would be nice, but I was limited to guessing their age, especially those found in middens under a just discovered roost tree.
The question of seasonal variation in the owls' diet was intriguing, but I wasn't able to follow up on it. Towards the end of the second summer the supply of pellets simply dried up. The owls abandoned the reliable roosts and found others that have remained hidden to me. Until now. There are over a dozen pellets in the vicinity of this tree. Curious, I checked the other two roost trees that I'd found in the past, and indeed, found a midden of pellets under one of them and a few under the other. I've removed all of these pellets and will continue to do so as the summer progresses. If I'm lucky the owls will use these roosts into the winter. Then I'll start picking through owl puke again, and maybe find an answer.
Updated by bwren at Sat Apr 30, 2011 at 12:21 PM PDT
Enhydra Lutris has today's bucket up now, Daily Bucket - Towhees and such. Head over there to post your news.