William R. Edwards

Parasitism of greater prairie-chicken nests by ring-necked pheasants

We studied nest parasitism of greater prairie-chickens (Tympanuchus cupido pinnatus) by ring-necked pheasants (Phasianus colchicus) as a possible contributing factor in the decline of an isolated population of prairie-chickens in Jasper County, Illinois. Both species nested in small, scattered grasslands maintained on prairie-chicken sanctuaries. Incidence of parasitic laying by pheasant hens in prairie-chicken nests increased from 2 to 43% between 1970 and 1983 and remained high through 1987. Nest success (≥1 host-egg hatching) did not differ (P = 0.33) between 60 unmanaged parasitized nests (43%) and 602 unparasitized nests (51%). However, success of 14 parasitized prairie-chicken nests managed by removal of pheasant eggs (86%) was greater (P = 0.02) than for 24 unmanaged parasitized nests (46%) during 1983 and 1985-87. Hatchability of fertile prairie-chicken eggs was less (P < 0.01) in parasitized nests (77%, conservatively) than in unparasitized nests (94%), because of earlier hatching of pheasant eggs, increased embryo mortality of prairie-chickens, or increased nest abandonment. Large clutches of prairie-chicken eggs typical of early nests were more likely (P < 0.001) parasitized than small clutches laid later. Factors correlated with rate of nest parasitism included numbers of pheasant cocks (P = 0.01) and numbers of pheasant nests (P < 0.001) found each year. Although pheasant control apparently eliminated nest parasitism during 1988-94, prairie-chicken numbers continued to decline. Without management intervention to control pheasants on sanctuaries, the survival of this isolated, remnant flock of prairie-chickens may be in greater jeopardy.

ESTIMATING SQUIRREL ABUNDANCE FROM LIVETRAPPING DATA

Estimates of squirrel (Sciurus carolinensis and S. niger) abundance were derived from several methods of population estimation applied to data obtained by livetrapping squirrels on the Waterloo Wildlife Experiment Station in southeastern Ohio, 1962 and 1963. The frequency of capture of marked squirrels suggests that the probability of capture is not the same for all squirrels; as a result, a trapped sample typically contains a disproportionately high number of recaptures. Thus, the multiple census methods of Schnabel and of Schumacher produced estimates lower than the number of animals con- sidered to comprise the population. Frequency of capture approximated the geometric distribution. The simplified equation for maximum likelihood estimation (MLE) for the geometric distribution, presented in 1967 by Edwards and Eberhardt, appeared useful for estimating squirrel abundance from livetrap- ping data, although estimates tended to be somewhat high. The intercept of a line fitted to a logarith- mic plot of data on the frequency of capture, using linear regression techniques, gave what appeared to be adequate approximations of the numbers of squirrels in the zero (uncaptured) class. Although esti- mates derived from MLE for the geometric distribution and from linear regression are based on assump- tions not strictly fulfilled by the data, these methods should prove useful until better techniques are developed. MLE for the Poisson distribution appeared to underestimate the zero class. Similarities in results of evaluations of techniques of population estimation for squirrels and rabbits suggest that fur- ther research on population estimation may provide findings applicable to a variety of species. Abstract: Estimates of squirrel (Sciurus carolinensis and S. niger) abundance were derived from several methods of population estimation applied to data obtained by livetrapping squirrels on the Waterloo Wildlife Experiment Station in southeastern Ohio, 1962 and 1963. The frequency of capture of marked squirrels suggests that the probability of capture is not the same for all squirrels; as a result, a trapped sample typically contains a disproportionately high number of recaptures. Thus, the multiple census methods of Schnabel and of Schumacher produced estimates lower than the number of animals con- sidered to comprise the population. Frequency of capture approximated the geometric distribution. The simplified equation for maximum likelihood estimation (MLE) for the geometric distribution, presented in 1967 by Edwards and Eberhardt, appeared useful for estimating squirrel abundance from livetrap- ping data, although estimates tended to be somewhat high. The intercept of a line fitted to a logarith- mic plot of data on the frequency of capture, using linear regression techniques, gave what appeared to be adequate approximations of the numbers of squirrels in the zero (uncaptured) class. Although esti- mates derived from MLE for the geometric distribution and from linear regression are based on assump- tions not strictly fulfilled by the data, these methods should prove useful until better techniques are developed. MLE for the Poisson distribution appeared to underestimate the zero class. Similarities in results of evaluations of techniques of population estimation for squirrels and rabbits suggest that fur- ther research on population estimation may provide findings applicable to a variety of species.

EVIDENCE FOR A NORMAL AGE COMPOSITION FOR COTTONTAILS

The juvenile : adult age composition of statewide cottontail (Sylvilagus floridanus) popula- tions in Ohio, Illinois, Wisconsin, Missouri, and Michigan showed similar characteristics of age structure which support Lotkas (1925) concept of the normal age distribution. Because of apparent similarities in the age composition of statewide populations, and because age ratios of cottontail populations on individual units of range differ significantly between years and areas, it is believed that (1) there is, in the midwest, a normal age structure for cottontails, and that (2) the age structure of the individual population varies about the normal, within limits, in response to factors in the physical environment. The normal or expected proportion of juvenile rabbits in fall populations in the midwest, with 95 percent confidence limits, is estimated as 0.834 + 0.005. This statistic is suggested as a possible norm for evaluating age structures of experimental populations.

Surgical Anesthesia for Cottontails

Intravenous injection of 30-50 mg/kg body weight of pentobarbital sodium into the medial auricular vein of the ear satisfactorily anesthetized cottontail rabbits (Sylvilagus floridanus) of various ages. Intraperitoneal injections of pentobarbital sodium were not satisfactory for inducing surgical anesthesia. Phencyclidine hydrochloride proved unsatisfactory as an anesthetizing agent, but was used successfully to immobilize cottontail rabbits. Ether was unsatisfactory, probably because adequate apparatus were not available for its efficient use. During a study of the reproductive behavior and the timing of ovulation in the cottontail, several rabbits were subjected to laparotomies so that their reproductive status could be determined without killing them. This paper reports on surgical anesthesia in the cottontail. It is a contribution from Illinois Federal Aid Project W-66-R, the Illinois Department of Conservation, the U. S. Bureau of Sport Fisheries and Wildlife, and the Illinois Natural History Survey, cooperating. Acknowledgment is gratefully made to Drs. G. C. Sanderson and P. J. Dziuk for technical advice, and to F. E. Eads, D.V.M., Parke, Davis and Company, for supplying Sernylan. Initial laparotomies were performed with pentobarbital sodium (Nembutal; Abbott, North Chicago) administered intraperitoneally at a rate of 143 mg/kg body weight as recommended by Taber and Cowan (Taber, R. D., and I. McT. Cowan. 1963. Capturing and marking wild animals. Pages 250-283. In H. S. Mosby (Editor), Wildlife investigational techniques. The Wildlife Society. xxiv + 419pp.). The results were not satisfactory. When the recommended dosage was used, or slightly exceeded, mortality was high (Table 1). At lower initial dosages, success was variable, and additional amounts of the drug were frequently necessary to attain satisfactory anesthesia. T 10to 15-minute delay between intraperitoneal administration of the drug and anesthesia of the animal was an added in-

Pecking as a Mortality Factor in Penned Ring-Necked Pheasant Chicks

Pecking of chicks by adult hen pheasants ( Phasisnus colchicus) accounted for a known mortality of 2 to 14 percent of chicks hatched in pens 0.82S to 40 acres in size in a series of studies on pheasant reproduction conducted in Ohio from 1951 through l954. Repeated observations of pecking as a cause of mortality of confined juvenile pheasants raise questions as to the possible occurrence of pecking in wild populations. The purpose of this paper is to report observations of pecking of juveniles by adult hens as a cause of mortality in confined populations of ring-necked pheasants. These observations resulted from four studies of the behavior and productivity of confined pheasants, conducted from 1951 to 1954 by graduate students of ie Ohio Cooperative Wildlife Research Unit, The Ohio State University, Columbus, in cooperation wii the Ohio Division of Wildlife. Results of this work are available only in theses. The observations on which this report is based were made by R. J. Smith, F. W. Kessler, W. L. Muhlbach, and V. D. Semones. I thank Helen C. Schultz and Dr. G. C. Sanderson for editorial assistance.