Estimating abundance of cottontail rabbits using live trapping and visual surveys

Density estimates for a cottontail rabbit (Sylvilagus floridanus) population on a 25-ha area at TVA's Land Between the Lakes, Stewart County, Tennessee, were calculated using mark-recapture models with live-trapping and visual surveys data. Field work was conducted from 8 March 1985 to 1 March 1986.

Rabbits were live-trapped with 50 box traps for 15 consecutive days during March, August, and December 1985 resulting in 2250 trapnights. A total of 114 rabbits were captured on 241 occasions resulting in a trap success of 10.7%. The number of individual rabbits captured during each trapping period was 40, 46, and 66 for March, August, and December, respectively. Sex ratios favored females during all 3 trapping periods but none differed significantly from 1:1. The overall sex ratio was 46 males to 68 females which was significantly different from 1:1 (P=0.019). Live-trapping data were used with Program CAPTURE to provided density estimates of 1.8, 2.4, and 2.4 rabbits per ha for March, August, and December, respectively. These estimates were lower with low probability of capture, individual heterogeneity in capture probability, and trapping design contributing to the negative biases.

A second census method was employed to provide estimates that were independent of trapping bias. Rabbits were marked with pelage dyes or reflective ear tags to allow identification of marked rabbits during visual surveys conducted during morning and evening hours subsequent to trapping periods. These marked-unmarked ratios were used with the Petersen-Lincoln Index to obtain estimates used in determining densities. Visual surveys conducted at night with aid of a spotlight (spotlight surveys) were the most productive yielding density estimates of 3.4, 3.3, and 3.6 rabbits per ha for March, August, and December, respectively.

Rabbits were most observable during morning surveys in June and July but observability decreased during fall and winter due to shifts in habitat use and decreased food. Rabbits remained observable throughout the year during spotlight surveys. Use of spotlight surveys enhanced density estimation efforts by providing independent samples of marked-unmarked ratios for use with Petersen-Lincoln Index and by providing a minimum number alive figure to evaluate the validity of mark-recapture estimates.

Reflective tags made detection of marked rabbits easy during spotlight surveys. Pelage dyes are recommended for morning surveys and reflective ear tags are recommended for spotlight surveys. Rabbit observability was enhanced by the presences of mowed strips 5 m wide and camp pads on the study area. Usefulness of visual surveys may be limited in areas which have few open areas to allow rabbits to be observed.