A Pleistocene Great Horned Owl’s (Bubo virginianus) Roost in Virginia

Clark’s Cave is a rare site where remains of Ice Age birds have been found.  Bird bones are fragile and less commonly preserved than mammal fossils.  We know little about the distribution of bird species during the Pleistocene, even though many species must have been at least as abundant as they are today.  Yet, Clark’s Cave provides the only known Pleistocene occurrence for some bird species.  Scientists excavated these remains in the 1970’s, and an 80 page paper about them was published in 1977 by the Carnegie Museum of Natural History.  The paleobiology database lists the species recovered from this site.  I studied this list and concluded the cave must have served as a great horned owl’s roost for decades. Perhaps generations of owls roosted here.

Bath County, highlighted in map of Virginia

Clark’s Cave is in Bath County, Virginia.  The remains of prey inside the cave suggest it was a great horned owl’s roost for generations, perhaps centuries.

Posted Image

Great horned owls prey on skunks so frequently, they often smell of skunk.  The spray doesn’t seem to bother them.

Great horned owls are a top predator of small animals wherever they occur, and they have been recorded taking a wide range of prey species.  I noticed the list of small species recovered from Clark’s Cave closely matched what one could find on a great horned owl’s menu.  This large owl carried striped skunks (Mephitis mephitis) and 5 kinds of weasels into the cave including pine marten (Martes americana), mink (Mustela vison), long-tailed (M. frenata), short-tailed (M. ermine), and least (M. nivalis).  Though some of these species may have established dens inside the cave, I think it’s highly unlikely every species that lived in the region would naturally inhabit and die in the cave.  This diverse assemblage could best be explained by the activities of a predator’s daily hunting.  Snowshoe hares (Lepus americana) and cottontail rabbits fell victim to the Pleistocene owls.  The paper identified the latter remains as a New England cottontail (Sylvilagus transitionalis), but this paper was written before the Appalachian cottontail (S. obscurus) was recognized as a distinct species.  I’m not sure if a distinction between the 2 cottontails can be made without a molecular study of the genes.  The owls brought 23 species of rodents into the cave.  Flying squirrels are active at night, so it’s no surprise both northern (Glaucomys sabrinus) and southern (G. volans) were eaten by nocturnal owls.  Even porcupines (Erithizon dorsatum)get killed by great horned owls.  Great horned owls probably also ate the smaller birds of prey found in the cave–sharp shinned hawk (Accipiter striatus), broad winged hawk (Buteo platyperus), kestrel (Falco sparverius), screech owl (Otus asio), short-eared owl (Asio flammeus), and saw whet owl (Agolus acadicus).  Great horned owls are notorious decimators of game birds such as turkey (Meleagris gallopavo), bobwhite (Colinus virginianus), and ruffed grouse (Bonasa umbellus).  Great horned owls destroy waterfowl–both ducks and wading birds.  The owls brought 5 species of ducks and 4 species of rails into the cave.  This is the best evidence the cave represents owl predation because neither duck nor rail would frequent cave habitat.  Some of the 29 other species of songbirds may have ended up in the cave, not as a victim of owls, but for some other reason.  Chimney swifts (Chaetura pelagica) are too fast for owls to catch but would nest inside a cave.  However, most of the species of birds found in the cave were likely killed by owls.  The great horned owls even caught slow-moving fish (eels, suckers, minnows, catfish, pickerel) that otherwise would have never been found in a cave environment.

Remains of the least chipmunk (Eutamias minimus) were the most surprising find at this site.  The least chipmunk no longer occurs this far east but is presently restricted to western Canada and the Rocky Mountains.

a photo of a least chipmunk

Remains of the least chipmunk were found in Clark’s Cave.  It no longer ranges this far east.

Distribution of Tamias minimus

Modern day range map of the least chipmunk.  Over half of its present day range was under glacial ice during the Ice Age.  The uninhabitable ice sheet forced it to occupy range south at least as far as Virginia where it co-occurred with the eastern chipmunk.

An uninhabitable glacier covered about half of their present day range during the late Pleistocene.  The range of this species shifted south where it co-occurred with the eastern chipmunk (Tamias striatus).  Remains of the least chipmunk have also been excavated from New Trout Cave, West Virginia–a site also dated to the late Pleistocene.

Not every species recovered from Clark’s Cave was a victim of owl predation.  Black bears (Ursus americanus) likely used the cave as a winter den and dire wolves (Canis dirus) raised pups here.  These carnivores dragged white tail deer (Odocoileus virginiana) and elk (Cervus canadensis) into the cave.  None of the familiar now extinct Pleistocene megafauna (aside from the dire wolf) left remains in the cave.  Was it chance or did the deposition of the cave occur after most of the Pleistocene megafauna became extinct?  The cave was studied before carbon dating was refined.  Researcher should re-date this material.

7 species of bats roosted in the cave.  Most were probably not owl prey but a few might be.

The composition of animal species recovered from the cave tells us what kinds of environments existed within a great horned owl’s home range here during the Pleistocene.  The presence of northern flying squirrel, red squirrel, and pine marten indicates red spruce forest at higher elevations.  A mix of red spruce and deciduous forest must have occurred at lower elevations because gray squirrel, southern flying squirrel, and chipmunk lived here.  Direct evidence of oak, hickory, hackberry, and tupelo was found inside the cave.  Elk and woodchuck are evidence of mountain meadows.  Muskrat, duck, and rail prove grassy marshes contributed to the diversity of habitats.  Young dense forest and shrub covered some areas preferred by rabbit, hare, and grouse.  Meadowlark and 13-lined ground squirrel needed treeless prairies.  A Paleo-Indian could have walked across all 6 of these habitats in a day.  Thanks to the great horned owl, we can imagine the kinds of environments a Paleo-Indian may have experienced on a day’s walk.

Reference:

Guilday, J.E.; P.W. Parmalee and H.W. Hamilton

“The Clark’s Cave Bone Deposit and the Late Pleistocene Paleoecology of the Central Appalachian Mountains of Virginia”

Bulletin of the Carnegie Museum of Natural History 1977

The above reference is available at the Carnegie Museum website for $13 plus shipping.  I never read the paper but the following link lists every species found inside the cave including 6 orders of insects.  That was enough info for this blog entry.

http://fossilworks.org/bridge.pl?a=collectionSearch&last_collection=94426&collection_name=Clark%27s+Cave

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