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THE EASTERN COTTONTAIL RABBIT
(SYLVILAGUS FLORIDANUS)
IN PENNSYLVANIA
Shane Hershman
and
Peter Burkhardt
May 14, 1980
ACKNOWLEDGMENT:
We thank the Pennsylvania State Game
Commission for issuing the permit to conduct this study
and for the literature which they sent concerning the
cottontail rabbit. We also acknowledge the literature
received from Mr. Edwards of the Illinois Natural
History Survey. We also express our appreciation to our
advisor on this project, Dr. Frank T. Kuserk, Department
of Biology, Moravian College.
Introduction:
During the past few years there has
been quite a discussion concerning the decline in the
population of the cottontail rabbit (Sylvilagus
floridanus) in Pennsylvania. Through our experiences we
did not believe that the decline was as drastic as many
people were claiming. Through our years of hunting, we
have found just as many rabbits now as in the past.
During the past hunting season (1979), the Pennsylvania
Game Commission shortened the rabbit season from 43 days
to 29 days to aid the rabbit population. Dale E. Sheffer
states that "the rabbit population is not as great
now as it was two to three decades ago." 1
We can believe this in terms of total rabbit population
across the state, and we shall investigate some of the
factors responsible for this decline, but some areas are
still supporting high concentrations of rabbits as we
have proved with our study. It was our contention that
with the proper food and cover a fairly high rabbit
population could be maintained and this was the basis of
our work.
Some of the factors placing pressure
on the rabbit population in general are: changes in
logging practices; increase in population levels of
crows, hawks, and owls; roaming domestic cats and dogs;
loss of habitat to building and succession of growth;
changes in farming practices; changes in fire fighting
practices; and acid rain. All of these are factors which
may have an influence on the total rabbit population of
the state. Most of the data used to determine the
state's rabbit population comes from correspondence with
hunters, some of which after an unsuccessful day in the
field claim there are no rabbits. Also, how many of
these hunters years ago hunted with a dog, for a whole
day and in heavy cover, and, today, for one reason or
another, do not use a dog, hunt for a few hours and in
open areas.
We undertook this project to study
the habits of the cottontail rabbit and to show that
with the proper food and cover a fairly high rabbit
population could be supported. We also wanted to study
the movements and interactions of rabbits with other
animals. There has been a paper published by Wiggins and
Rothenbacher 2 which correlates the parasite
burden on the cottontail rabbit with the decrease in the
rabbit population size. We had planned to run fecal
flotations, but the rabbits rarely deficated in the
traps. We, therefore, obtained no valuable data from
this part of the study.
Methods:
Wooden box traps, measuring 911 x
6" x 1211, baited with apples were used to capture
the rabbits. The trapping was begun on February 13, 1980
and continued through April 25, 1980. The traps were set
on Wednesdays and checked on Thursdays and Fridays. The
rabbits that were captured were weighed, sexed and the
length of the hind foot and ear were measured. A metal
tag bearing a number 1 - 18 was affixed to their right
ear and the rabbit then released. The trap location,
trap number and date were recorded along with the tag
number and other data (Table 1). The trapping area
consisted of approximately 12 acres, with the primary
cover being 9 - 10 acres of heavy green briars and two
acres of Blackberry brambles and a portion of an
overgrown field. The primary food source was sassafras
trees, briars, sumac bark and, in the spring, some fine
grasses were noted. The maps included with our data
indicate the trapping areas (Fig. 1). 16 traps were
employed in the study. As seen on the maps, the overall
area was divided into three distinct trapping areas. The
first and largest area being the heavy briars, the
second area being a more open wooded area. The third
area was that of a small overgrown area at the edge of
an abandoned field.
We also chose to place two traps on
the opposite side of the road by the house to see if
there was any migration across the road, since we never
saw any evidence of dead rabbits along the road. The
wooded and field areas were chosen to support the theory
that the proper food and cover is needed to maintain a
high rabbit population. During the study we encountered
quite a bit of trouble with mice removing bait from our
traps. Traps for the mice were placed in the areas where
the problem of the bait being taken was the greatest.
Data:
(Table 1, 2 & 3) Follows text
Discussion:
Considering the data of the
individual rabbits (Table 1-2) we see that the sex ratio
was 10 males to 7 females or a ratio of 1.43:1. Previous
studies indicate that the embryonic sex ratio of males
to females ranges from .65:1 to 1.33:1, 10 with an
average of 1.2:1 from three studies. The study which
obtained the sex ratio of 1.33:1 was done using a sample
size of 205 rabbits. Returning to the present study and
table 1-2, we see that the ear length ranged from
2-1/2" - 3-1/8", and the hind foot length
ranged from 3-1/4" - 3-3/411.
The age of a rabbit can be determined
by the length of the hind foot and ear up until about 14
weeks old; after this time the only way of accurately
determining the age of the rabbit is by the weight of
the lens of the eye. 11 The weight range of
the rabbit captured during the study was 2-1/4 lbs. -
3.0 lbs. The rabbits all weighed approximately 2-1/2
lbs. throughout the first month of the study. The
females caught from about the 20th of March until the
end of the study were noticeably heavier probably due to
the carrying of young. Around this time, the males
tended to lose weight, going from approximately 2-1/2
lbs. in the middle of February to about 2-1/4 lbs. in
mid-March thru April 25th. Two possible explanations for
this weight loss could be increased activity on the part
of the males for breeding of the females or the quality
of the available food was not sufficient to maintain
their weight.
The location where the individual
rabbits were captured was plotted on maps of the area.
(Fig. 2 - 18). Considering fig. 2 rabbit #1 moved
approximately 300 ft. during the time he was first
captured on February 15 to the time of his recapture on
March 7. Rabbit #2 (fig. 3) was only captured once, but
we noticed a rabbit in that area on a number of
occasions. Rabbit #3 (fig. 4) was recaptured a week
later at her point of original capture. Rabbit #4 (fig.
5) was a little trap happy. We captured her a total of I
I times, in traps nos. 4, 5, 6, and 8. She maintained a
fairly constant territory, moving a distance of
approximately 700 ft. from the point where she was first
captured (trap #8) to the farthest point of her range
(trap #4). As shown by the map (f ig. 6) she never went
into trap #3 which is near U 10 f t.) trap #4. Taking
trap #6 as the center of her range, there were a total
of 8 traps in a circular area around Trap #6 although
she only was captured in 4 of the 8 traps. A possible
explanation for this pattern of capture is that rabbit
#4 was searching for a new food source, or new and/or
better nesting area, or some outside forces (man or
predators) caused her to move to thicker cover. If we
look at the data for rabbit #4 (Table 1-2) we see that
from March 20 to March 28 she gained a quarter of a
pound, and in the 2 weeks from April 4 to April 17 she
lost a half pound. Perhaps she had been bred but the
quality of the food present was not good enough and she
did not conceive; it is very hard to say what the reason
for this change in weight was caused by. Rabbit #5 (fig.
6) was captured three times over a period of 8 weeks in
the same trap. Rabbit #6 (fig. 7) was captured only once
but trap #1 was in an area of about 25 acres which could
have provided rabbit #6 with a range which was virtually
unchecked. Rabbit #7 (fig. 8) was recaptured in
approximately the same area as when he was first caught.
Rabbit 1/8 (fig. 9) was recaptured at the same location
approximately one month after her original capture.
Rabbit 1/10 (fig. 10) was captured only once during the
study period. Rabbit #11 (fig. 11) was also caught only
once. Rabbit #12 (fig. 12) was captured a total of three
times over a four week period at the same location each
time. Rabbit 1/13 (fig. 13) was captured once during the
study. Rabbit #14 (fig. 14) was captured a total of four
times during a 3-1/2 week period. All points of capture
were within 230 feet of one another. Upon release on one
occasion, the rabbit ran approximately 200 feet and
entered a hole in the ground, whether or not this hole
was a permanent home was not discovered. Rabbit #15
(fig. 15) along with rabbits nos. 16 (fig. 16), #17
(fig. 17), and #18 (fig. 18) was caught only once during
the entire study period. Rabbits #17 and 1/18 were
captured near the yard of a house on two consecutive
days near the end of the study period. It is possible
that they had moved out of the heavy cover of the briars
where the young grasses had not yet sprouted into the
area of the study to feed on the tender grass present in
the yard. One general trend which can be seen from a
composite map of all the rabbits captured more than once
(fig. 19) is that except for rabbit #4, the female
rabbits tended to maintain smaller range than the males
and even the range exhibited by the males was much
smaller than that shown by rabbit #4 which somewhat
supports the theory that rabbit #4 was forced to extend
her range over a period of 8 weeks.
A total of 17 different rabbits were
trapped with 20 recaptures, 4 of these being multiple
recaptures. Using the linear regression 3
method as described by Edwards and Eberhardt in 1967 for
estimating population size, the data from table 4 and
applying linear regression analysis yields a population
size of 34. Considering this population estimate and the
size of the area being investigated, the density would
be 2.83 rabbits per acre. The State of Pennsylvania's
Game Commission states that "about one rabbit per
acre in the fall is a good density." 4
We have approximately three times this density in our
study area. This result supports our theory that with
the proper food and cover a high rabbit population can
be maintained. This study area is not in any special
area; hunting had taken place at the study site in the
fall of 1979 with approximately 17 rabbits being taken.
Had our study been done in the fall of the year rather
than midwinter and spring, our population estimate would
have been increased by 1/3 or 4.25 rabbits per acre. One
must remember that this is the estimate of the
population in the spring when the rabbit population
would be at its lowest naturally occurring point.
The primary cover on the study areas
are heavy green briars and blackberry brambles. The
sources of food for the rabbits include sassafras and
sumac bark, briars, and in the spring and summer grasses
are available. As one can see from the map (Fig. 1), the
trapping areas are separated. When there was snow on the
ground we did not see any evidence of tracks in the
wooded area and the traps in this area proved fruitless
and were removed so we could concentrate our efforts in
the other areas.
What are some of our views why the
population has declined in some areas? The primary
reason is probably the loss of habitat due to two
reasons; first, the wooded and forested areas are in
succession and there is not enough cover close to the
ground, and not enough food in the area for the growth
of the juvenile rabbits. When areas had been timbered
off the rabbit population has grown as a result of the
increase in new and heavy growth being utilized for food
and cover. There may be a bright side in the future for
the rabbits with the increased usage of wood burning
stoves - there will be more wood lots started which will
aid the rabbit population. Another factor affecting the
loss of habitat in rural areas is the loss due to
building. In 1976,1 million acres were lost in the U.S. 5
In Northampton County during the years 1972-1978, 5,097
acres of land was lost to houses. 6 With the
increase in new housing comes an increase in the number
of domestic dogs and cats which are allowed to roam in
rural areas at will, killing rabbits in the nest and
adults. The days of the huckleberry fires are also over,
when acres would be set on fire, so the people could
pick the huckleberries. By burning the woodland, 'the
area would be cleared of large growth thus providing
finer young growth for the rabbits.
Another factor that may be affecting
the size of the cottontail rabbit population in the
state is acid rain. Hill has shown as soil pH is lowered
litter size decreases. 7 This decrease in the
pH of the soil could be the result of acid rain. D. E.
Samuel has shown that juvenile rabbits survived and
showed good growth on red clover. if we correlate these
two factors, we see that clover grows better in a basic
soil and the female rabbits produce larger litters
probably due to the better quality of food the pregnant
females would receive during the gestation period. The
growth and development of the juvenile rabbits will also
be supported better in an area where red clover Is
present. 8
Increased predation plays an
important role in controlling the size of the rabbit
population. By allowing the natural predators to
increase their population size unchecked and allowing
predation from domestic pets an increased burden is
placed on the rabbit population. Some natural predators
of rabbits are hawks, owls, weasels, opossums, and
crows. "Rabbits may make up as much as 40% of the
diet of a horned owl. 9 Crows are also a
leading predator of young cottontail rabbits in the
nest. During our study we found many signs of predators
in our area. Considering predation in our area there is
probably a low attack rate due to the heavy cover that
the briars provide. The hawks and owls cannot dive on
the rabbits through the briars. Since it is not just one
small clump of briars, but a 9-10 acre area, the adult
rabbits can easily evade cats and dogs. Only in the
field did we find scats from hawks and owls, and remains
of pigeons and pheasants that were possibly killed by
hawks and owls which gave evidence to their presence in
the area. We caught three opossums in our traps during
the study. Fox manure was found in the briars. When
there was snow on the ground we found both dog and cat
tracks, and on a number of occasions we saw a cat in our
area and, on one particular occasion, a cat was seen
sitting on top of one of our traps.
There have also been changes in
farming practices that could have affected the total
population of the cottontail. Fence rows have been
removed in order to make farming more efficient. These
fence rows provided the rabbits with good covering the
form of holes in the rocks and dead falls. The bark of
the young trees Provided the rabbits with a source of
winter food and in the spring and summer food was easily
obtained from the nearby fields. In our research of the
topic, we could not find any information on the effects
which herbicides, pesticides, and the no-till method of
farming have on the cottontail rabbit population.
One can see through this small
consideration that rabbits have quite a bit of pressure
against them. These factors may have reduced the rabbit
population in some areas. The major consideration seems
to be the loss of habitat or cover and food. As we have
shown by our study with the proper food and cover a
fairly high population can be maintained even in the
presence of predators and hunting.
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