ies_

Some 56 species of animals that regularly prey on small vertebrates live
on the Reservation. Many of the larger kinds may take woodrats
occasionally. Because of size, habitat preferences and the time and
manner of hunting, five species stand out as the more formidable
enemies--the horned owl (_Bubo virginianus_), prairie spotted skunk
(_Spilogale putorius_), long-tailed weasel (_Mustela frenata_), pilot
black snake (_Elaphe obsoleta_) and timber rattlesnake (_Crotalus
horridus_).

Throughout the study horned owls were common on the area, but their
numbers were highest in 1948. Samples of pellet collections have shown
that the cottontail is the staple food, being represented in almost
every pellet. Various rodents also are important in the diet, the cotton
rat, prairie vole, or white-footed mouse being most prominent according
to the time and place of collection. The woodrat is approximately
optimum size for prey, and it constitutes one of the most preferred food
sources. Remains of only two woodrats were found in the pellets
examined, but at times when the pellets were collected woodrats were so
scarce that they constituted only an insignificant percentage of the
biomass of potential prey. On several occasions woodrats in live-traps
were attacked by horned owls, as shown by the overturned and displaced
trap and quantities of fine down adhering to them and to nearby objects.
The horned owl lives in the same habitat as does the woodrat. In other
regions woodrats are known to figure prominently in the diet of the
horned owl. At the San Joaquin Experimental Range in California, for
instance, _N. fuscipes_ was found 240 times, more frequently than any
other kind of prey, in 654 pellets of the horned owl, and this owl was
shown to be the one most important natural enemy of the rat, although
many kinds of carnivores, raptors and snakes also took toll from its
populations. On the Reservation the population of horned owls has been
fairly stable from year to year, with roughly one pair to 100 acres of
woodland. Some territories have been maintained continuously throughout
the eight-year period of observation, though changing to some extent in
size, shape and area included. In 1948, when livestock grazed on the
area, and the ground cover of herbaceous vegetation was relatively
sparse, cottontails were much less abundant than they were later when
the vegetation was protected. Small rodents including voles, cottonrats,
and d