Estimates of Some Attributes of a Population of Nuttall's Cottontails

Abstract

Density, natality, and mortality of a population of Nuttall's cottontails (Sylvilagus nuttallii) in shrub-juniper scabland of central Oregon were monitored from August 1972 to March 1974. Monthly censuses indicated population density ranged from 6.6 to 254.4/100 ha. Densities declined 89 and 85 percent from August to December in 1972 and 1973, respectively; within these periods, greatest losses occurred at times of unusually low minimum daily temperatures. In 1972, 4 litters were produced (April-July) of which 10, 22, 68, and 80 percent, respectively, survived to 30 August. In 1973, only 3 litters were produced (April-June) of which 25, 21, and 11 percent survived to 30 August. The decline in production and survival of juvenile cottontails in summer 1973 was believed to be related to reduced precipitation during the reproductive period; precipitation was 64 percent less in 1973 than in 1972. Differential survival of litter groups indicated that drought had the greatest impact on younger cohorts, perhaps through reduction in succulent forage for weanlings. J. WILDL. MANAGE. 42(1):159-168 This is a report on our attempts to estimate density, natality, and mortality throughout most of 2 annual cycles for a population of Nuttall's cottontails, and to show relationships between changes in these attributes and differences in ambient temperatures and amounts of precipitation. In studies of lagomorph population dynamics, semiannual estimates of density commonly are made prior to the hunting season in autumn and prior to or during the breeding season in spring (Lechleitner 1958, Lord 1959, Meslow and Keith 1968, Petrusewicz 1970, Gross et al. 1974). Occasionally, single annual estimates made in midsummer (Korschgen and Stuart 1972, Fitch 1947, Janson 1946), or in autumn (Edwards and Eberhardt 1967, Andersen 1957), or monthly indices to roadside activity (Lord 1963, Kline 1965) are used to establish trends in population density. Mortality (and its antithesis, survival) usually is calculated on an annual basis (Lord 1959, Meslow and Keith 1968, Eberhardt et al. 1963), although Lord (1961) attempted to calculate monthly mortality from lens weights. Reproduction in lagomorphs, particularly after Wight and Conaway (1962) reported synchronous breeding in eastern cottontails (S. floridanus) and Casteel (1966) verified immediate postpartum breeding in the same species, frequently is reported on the basis of mean number of young in each of the synchronous litters, and the number of litters produced during each breeding season (Meslow and Keith 1968, Powers and Verts 1971, Trethewey and Verts 1971, Hill 1972, Gross et al. 1974). However, to our knowledge, simultaneous estimates of density, natality, and mortality (survival) at short, regular intervals throughout the annual cycles for a population of wild lagomorphs have not been attempted. This study was supported, in part, by grants to the senior author from the Theodore Roosevelt Memorial Fund of the American Museum of Natural History and from the Grants-in-Aid Committee of the American Society of Mammalogists. H. Holechek generously supplied space and utilities for field headquarters. Numerous students in the Department of Fisheries and Wildlife, Oregon State University, aided in conduct1 This is Technical Paper No. 4057, Oregon Agricultural Experiment Station. 2 Present address: Woodward-Clyde, 4791 Business Park Blvd., Suite 1, Anchorage, Alaska 99503. J. Wildl. Manage. 42(1):1978 159 This content downloaded from 157.55.39.178 on Fri, 05 Aug 2016 05:15:24 UTC All use subject to http://about.jstor.org/terms 160 POPULATION STATISTICS OF NUTTALL'S COTTONTAILS McKay and Verts ing censuses. H. F. Horton, E. C. Meslow, and P. A. Vohs, Jr., provided critiques of the manuscript.