Evaluating Line Transects to Monitor Gray Squirrel Populations

Evaluating Line Transects to Monitor Gray Squirrel PopulationsAuthor(s): William M. Healy and Christopher J. E. WelshSource: Wildlife Society Bulletin, Vol. 20, No. 1 (Spring, 1992), pp. 83-90Published by: Wiley on behalf of the Wildlife SocietyStable URL: http://www.jstor.org/stable/3782764 .

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MARTEN HABITAT MODEL * Schulz and Joyce 83

McLELLAN, B. N., AND D. M. SHACKLETON. 1988. Grizzly bears and resource-extraction industries: effects of roads on behaviour, habitat use and de- mography. J. Appl. Ecol. 25:451-460.

MINNESOTA STATE PLANNING AGENCY. 1987. EPPL7 user's guide. St. Paul. Unnumbered.

O'NEILL, R. V., J. R. KRUMMEL, R. H. GARDNER, G. SUGIHARA, B. JACKSON, D. L. DEANGELIS, B. T. MILNE, M. G. TURNER, B. ZYGMUNT, S. W. CHRIS- TENSEN, V. H. DALE, AND R. L. GRAHAM. 1988. Indices of landscape pattern. Landscape Ecol. 1:153-162.

REYNOLDS, H. G. 1966. Use of openings in spruce-fir forests of Arizona by elk, deer, and cattle. U.S. For. Serv. Res. Note RM-66. 4pp.

ROMME, W. H., AND D. H. KNIGHT. 1981. Fire fre- quency and subalpine forest succession along a topographic gradient in Wyoming. Ecology 62: 319-326.

SOKAL, R. R., AND F. J. ROHLF. 1981. Biometry. Sec- ond ed. W. H. Freeman and Co., New York. 859pp.

THOMAS, J. W., technical editor. 1979. Wildlife hab- itats in managed forests. U.S. For. Serv. Agric. Handb. 553. 512pp.

TURNER, M. G. 1990. Spatial and temporal analysis of landscape pattern. Landscape Ecol. 4:21-30.

U.S. FOREST SERVICE. 1990. HABCAP - Region 2 habitat capability program, HABCAP program documentation and users guide. U.S. For. Serv., Rocky Mt. Reg., Range, Wildl., Fish. and Ecol. and Wildl. and Fish Ecol. Unit, Denver, Colo. 25pp. + appendix.

U.S. GEOLOGICAL SURVEY. 1987. USGeoData digital elevation models, data users guide. Natl. Mapping Program, Technical Instructions, Data Users Guide 5. Reston, Va. 38pp.

WIENS, J. A. 1989. Spatial scaling in ecology. Func- tional Ecol. 3:385-397.

, J. T. ROTENBERRY, AND B. VAN HORNE. 1987. Habitat occupancy patterns of North American shrubsteppe birds: the effects of spatial scale. Oikos 48:132-147.

Received 4 March 1991. Accepted 15 August 1991. Associate Editor: Stauffer.

Wildi. Soc. Bull. 20:83-90, 1992

EVALUATING LINE TRANSECTS TO MONITOR GRAY SQUIRREL POPULATIONS

WILLIAM M. HEALY, U.S. Forest Service, Northeastern Forest Experiment Station, Amherst, MA 01003

CHRISTOPHER J. E. WELSH, U.S. Forest Service, Northeastern Forest Experiment Station, Amherst, MA 01003

Interest in estimating gray squirrel (Sciurus carolinensis) abundance arises from their val- ue as indicators of specific habitat conditions and as game animals. Forest managers are required to select and monitor indicator species on National Forest lands (National Forest Man- agement Act 1976, 90 Stat. 2949; 16 U.S.C. 1600); gray squirrels have been selected as the indicator species for mature and old-growth oak communities in 7 of 15 eastern region Na- tional Forests (Illinois: Shawnee; Wisconsin: Ni- colet; Michigan: Hiawatha, Huron-Manistee;

West Virginia: Monongahela; Vermont: Green Mountain; and New Hampshire: White Moun- tain). In the Green Mountain National Forest, gray squirrels are habitat representatives for species such as white-tailed deer (Odocoileus virginianus), wild turkey (Meleagris gallo- pavo), and blue jay (Cyanocitta cristata), for which mast is an important habitat component (L. K. Klein, U.S. For. Serv., Laconia, N.H., unpubl. manuscript, 1982).

Most methods used to estimate gray squirrel abundance are labor intensive. Bouffard and



84 Wildl. Soc. Bull. 20(1) 1992

Hein (1978) evaluated 6 counting methods and concluded none were satisfactory for extensive application in squirrel management. Mark-recapture has been the most common technique used to estimate squirrel populations (Nixon et al. 1967, Barkalow et al. 1970, Bouffard and Hein 1978, Brown and Batzli 1985). Mark- recapture provides reliable population estimates and demographic data that cannot be obtained any other way, but the technique is not practical for surveying large areas. Pack et al. (1971) used leaf nest counts, activity indices, hunter observations, and hunter success rates to determine gray squirrel population trends on a 9,500-ha area in West Virginia. They concluded that hunter success rates were reliable indicators of gray squirrel populations and the most efficient monitoring method for their area.

Leaf nest counts were recommended for monitoring squirrels in the Green Mountain National Forest (L. K. Klein, U.S. For. Serv., Laconia, N.H., unpubl. manuscript, 1982). In England, Don (1985) noted a close relationship between leaf nest density and squirrel density. However, leaf nests may be retained from year to year (Pack et al. 1971), leaf nests may not be uniformly distributed among forest types (Sanderson et al. 1975, Don 1985), squirrels may be using tree dens (Don 1985, Gurnell 1987), the number of leaf nests per squirrel may vary (Gurnell 1987), and short-term fluctuations in squirrel populations may not be accompanied by concurrent changes in leaf nest densities (Don 1985). Because of these drawbacks, leaf nest counts may be useful in evaluating relative abundance and habitat use within some forest types, but not population size (Don 1985, Gurnell 1987).

Line-transect sampling has been applied to a variety of biological populations (Burnham et al. 1980) but not, to our knowledge, to gray squirrel populations. Line transects are an efficient, economical means of monitoring many animal populations, and gray squirrels, though not conspicuous, are readily detectable by a

slow-moving observer in forest environments. Our objective was to evaluate the feasibility of using line-transect surveys to monitor gray squirrel populations. We present the results of 6 years of line-transect sampling of an un- hunted gray squirrel population in western Massachusetts and give recommendations for using the technique.

STUDY AREA AND METHODS

This study was conducted on the Prescott Block of the Quabbin Reservation, a municipal watershed located approximately 15 km east of Amherst in western Massachusetts. The Prescott Block is a 5,022-ha forested peninsula dominated by oaks (Quercus rubra, Q. vel- utina, Q. coccinea, and Q. alba). Roughly half of the forest is in sawlog stage (>25 cm dbh), and 42% of the sawlog areas are in forest types dominated by oak. White pine (Pinus strobus), red pine (P. resinosa), and northern hardwoods are the other major forest types. Most stands are open with an understory dominated by ferns and Vaccinium spp. Public access to the peninsula is restricted, and recreational use, including hunting, is prohibited.

We marked 6, 3-km transects running generally north-south with paint and flagging in fall 1983, and added a seventh 3-km line in fall 1985 (Fig. 1). Transect bearings and locations were chosen systematically from the area base map so transects would be >600 m apart and would provide relatively uniform coverage of the peninsula. Starting points were located a random distance < 100 m from the appropriate road or compart- ment boundary along the preselected bearing. Transect locations were chosen without regard for habitat or topographic conditions, and we assumed transect place- ment was random with respect to habitat type. Stations were identified with numbered flags at 100-m intervals, and lines were cleared each fall of fallen debris and eye-level branch stubs to allow observers to concentrate on detecting squirrels.

We conducted surveys in late October-early No- vember for 3 days in 1983; 6 days in 1984, 1986, and 1987; 7 days in 1985; and 12 days in 1988. Depending on the number of observers available, 5 to 7 transects were surveyed each day. All observers were either U.S. Forest Service employees, Massachusetts state biolo- gists, or wildlife students at the University of Massa- chusetts. We trained observers in the use of rangefinders and collection of transect data prior to data collection.

No surveys were conducted during periods when rain or snow or winds >15 km/hour inhibited observations and squirrel activity. All surveys began within one-half hour of official sunrise. Observers walked transects at a rate of 1 km/hour (100 m/6 min), recording station numbers and time as they passed each station. We measured perpendicular distance (PD) or sighting distance (SD) and azimuth to every gray squirrel seen



SQUIRREL LINE TRANSECTS * Healy and Welsh 85

I 0 1 2 3

KiLOETERS G-17 6 e 2

3

5

9

Fig. 1. Arrangement of the 7 survey lines on the Prescott Block of the Quabbin Reservation in western Massachusetts, 1983-1988. The shaded area is Quabbin Reservoir.

with a 15- to 180-m rangefinder (or a 0- to 30-m rangefinder for close observations) and a handheld compass. We computed sighting angles (SA) from azimuths. Time of observation was also recorded. If we saw - 2 squirrels together, we noted the number but recorded only the distance and angle to the first squirrel seen in the group.

Data Analysis

We calculated the number of gray squirrels seen per km of transect as an index to squirrel population levels. Because observers were occasionally unable to determine the location of a squirrel, some observations did not include measured distances. We included these observations in calculations of number of squirrels detected per km but excluded them from the samples used to estimate densities.

We estimated densities from grouped perpendicular distance data (10-m intervals) truncated at 80 m using the Fourier Series estimator of program TRANSECT (Laake et al. 1979). We adjusted these estimates by average group size to get estimates of number of squirrels/km2. Chi-square tests indicated a reasonable fit of the data to the model each year (Table 1). Because of small sample sizes on some days (<10 observations/ day), we pooled data among days and transects within years, resulting in 1 density estimate per year.

We analyzed the effect of time of day on squirrel observations (all years combined) using a chi-square test. We divided survey periods into 30-minute intervals ranging from one-half hour before sunrise to >3.5 hours after sunrise. We used a weight based on the proportion of all lines surveyed for at least 15 minutes during each interval to calculate expected values (interval weight x total observations for year = expected observations for interval). Weighting was necessary to compensate for varying sampling intensities per interval because all observers did not start and stop at the same time.

RESULTS AND DISCUSSION

Population Estimates

The estimated density of gray squirrels on the Prescott Peninsula ranged from 2.2 squirrels/100 ha in fall 1984 to 32.6/100 ha in fall 1985 (Table 1). Density in fall 1983 was prob-

Table 1. Estimates of fall density (squirrels/100 ha), index of abundance (squirrels/km), and percent coefficient of variation (CV) for gray squirrels observed from transects on the Prescott Peninsula, western Massachusetts, 1983-1988.

Index of Number of Year Km of transects abundance Density Cv (%) parameters Pb

1983c 51 <0.1 1984 94 0.5 2.2 48 1 0.19 1985 132 1.7 32.6 17 4 0.60 1986 126 1.4 26.2 22 3 0.72 1987 114 1.4 22.2 12 3 0.49 1988 249 0.2 4.5 22 2 0.45

Number of parameters in the Fourier Series model used to estimate density. Chi-square goodness-of-fit test of the Fourier Series model to the observed data, 7 df. P = 1.0 indicates a perfect fit.

'There were too few observations (1 squirrel) in 1983 to compute a density estimate.



86 Wildl. Soc. Bull. 20(1) 1992

Table 2. Mean perpendicular and sighting distances (m), sighting angles, and group sizes for gray squirrels observed in autumn from transects on the Prescott Peninsula, western Massachusetts, 1983-1988.

Perpendicular distance Sighting distance Sighting angle Group size

Year x SE x SE x SE SE

1983a 7.2 0.0 52.0 0.0 8.0 0.0 1.0 .0.0 1984 25.0 4.7 50.3 5.8 40.7 8.8 1.0 0.0 1985 14.9 1.2 28.9 1.4 34.9 2.8 1.2 <0.1 1986 15.2 1.2 29.3 1.6 28.5 2.9 1.1 <0.1 1987 17.9 1.7 32.8 1.9 37.4 2.4 1.1 <0.1 1988 15.6 2.3 30.1 2.5 39.7 4.6 1.0 <0.1

Only 1 squirrel observed.

ably <1/100 ha, but too few squirrels were seen to calculate a density estimate.

There was little change in the amount of squirrel habitat during this study. Between January 1983 and January 1988, 59 ha of red pine plantation were cleared and converted to fields. Thinnings were conducted in 10 ha of red pine and 113 ha of hardwood forest, but the area of oak pole and sawtimber forest remained the same.

We think density of squirrels in autumn re- flects the size of the mast crop the previous fall. This relationship has been demonstrated (Nixon et al. 1975), and it was supported by our evaluation of local mast crops and measurements of the abundance of other mast con- sumers. Our measurements of red oak acorn production at 1 site indicated good production in 1986 and crop failures in 1987 and 1988. Squirrel populations declined sharply between 1987 and 1988.

Mean perpendicular distances to squirrels were similar among years, and average group sizes ranged from 1.0 to 1.2 (Table 2). Mean sighting distances and sighting angles were similar in 1985, 1986, 1987, and 1988; but sighting distances tended to be longer in fall of 1983 and 1984 (Table 2).

The number of squirrels detected per km of transect followed the same trend as the density estimates (Table 1). Few squirrels were seen in 1983 and 1984, but numbers peaked in 1985 and declined after that. Relative measures of abundance (% detections per km and % rela-

tive density) differed by <7.4% within years and were highly correlated (r = 0.971). Bren- nan and Block (1986) suggested that similarity between relative abundance measures based on detections/km and density estimates, although not independent, provided a check for spurious density estimates.

Survey Effort

One of the advantages of line-transect sampling over other counting techniques is its relative efficiency. Our method required 5.3 person-hours/km to lay out, mark, and clear transect lines the first year. Thereafter, it took approximately 2.7 person-hours/km to clear the lines. Squirrel surveys averaged 1 person- hour/km. With this effort, it would initially take about 237 person-hours to mark, clear, and survey 7, 3-km lines 6 times, but in sub- sequent years it would only require about 183. For safety reasons, we cleared lines each year and always had at least 2 people preparing a line. Not clearing lines every year or having only 1 person clear each line could result in substantial time savings.

Time of Day

The probability of detecting a squirrel varied during the morning (X2 = 91.6, 8 df, P < 0.005). We observed more squirrels than expected in the first half hour after sunrise and generally fewer than expected for periods >2 hours after sunrise. Squirrel activity tends to



SQUIRREL LINE TRANSECTS * Healy and Welsh 87

peak in the first few hours of the day (Tonkin 1983, Reynolds 1985, Gurnell 1987), so squirrel counts should be conducted in early morning hours. We rarely started lines before sunrise, but activity may begin in the first hour before sunrise. We detected relatively few squirrels >3 hours after sunrise.

Line-transect Assumptions

Burnham et al. (1980) and Verner (1985) discussed necessary assumptions for line-transect surveys and biases resulting from failure to meet those assumptions.

Assumption 1.-Squirrels exactly on the line are never missed. For squirrels on the ground, we are confident that none were missed. We probably detected nearly all squirrels on the lower trunks of trees as well. However, squirrels higher in trees could easily have escaped detection by hiding on the opposite side of a tree or by remaining motionless, and squirrels in leaf nests or cavities were certainly missed. Gray squirrels spend most of their time for- aging on the ground (Kenward and Tonkin 1986), particularly in late fall when acorns have fallen. We think violations of this assumption were minimal because of good visibility (leaves were down, understory was sparse) and the fact that most of the squirrels that we saw were on the ground. We suggest densities estimated from transects be considered minimal estimates.

Assumption 2.-Squirrels do not move in response to the observer. Movement is not a serious problem if it is random with respect to the observer and slow relative to the speed of the observer (Burnham et al. 1980). Squirrels were not attracted by the observer's presence. We did see squirrels respond to an observer's approach, but evasive action in response to the observer usually covered short distances, i.e., to the nearest tree trunk. Because observers moved cautiously, it was often possible to identify the location from which a squirrel was disturbed. Movement away from the line re-

All years combined 200

50

0

0-10 10>29 20-0 90A 450- 504W 0 0-70 7W0-

Right angle distance (m)

Fig. 2. Histogram of 444 perpendicular distances to gray squirrels observed during fall surveys f rom 1983 to 1988 on the Quabbin Reservation, western Massa- chusetts.

sults in increased distance measures and a con- sequent underestimate of density. Progress of observers along the line was fast (1 km/hour) relative to the undisturbed movement of squirrels. If squirrel movement away from the line was substantial, more observations should have occurred further from the line and sighting angles should have been skewed toward higher values. Histograms of perpendicular distances (Fig. 2) and sighting angles did not indicate substantial movement. Although examination of plots of PD and SA does not rule out movement, we believe any movement that did occur was small.

Assumption 3.-No squirrels are counted more than once. Because we moved along the lines relatively quickly compared to undisturbed squirrel movement and did not notice any squirrels following observers, we are confident that no squirrels were counted more than once on any day. Repeated sampling of the same transects probably resulted in observations of some of the same squirrels, but although the observations may not have been independent, they allowed a more precise estimate of abundance than 1 sample could provide.

Assumption 4. RThere are no errors in mea- surement of distances or azimuths. This as-



88 Wildl. Soc. Bull. 20(1) 1992

sumption is impossible to meet, but we at- tempted to limit the extent of violations by measuring distances with a rangefinder rather than estimating them. Rangefinders were cal- ibrated each day against targets placed at known distances. Observers took at least 2 readings for each sighting to ensure consistent measurements.

Assumption 5.-Sightings are independent events. Most observations were of single squirrels. Average group size ranged from 1.0 in 1983 to 1.2 in 1985. Occasionally, vocalizations of 1 squirrel may have elicited a response from another, but whether the answering squirrel would have been detected if it had not re- sponded is not known. Violation of this assumption affects the sampling variance of the density estimate, but not the density estimate itself (Burnham et al. 1980:132-133).

RECOMMENDATIONS

We recommend the line-transect method for monitoring gray squirrel populations on extensive areas. Theory and methods of analysis are better developed for line-transect (Burn- ham et al. 1980) and mark-recapture methods (White et al. 1982) than for leaf nest counts, activity indices, or harvest statistics. Line transect compares favorably with the other methods in terms of sampling effort, but we could not make direct comparisons from the literature. Pack et al. (1971) used 137 km of transects to estimate leaf nest density on 9,500 ha. It took 2-4 assistants to check 200 live-traps daily on an 81-ha plot in the Piedmont of North Carolina (Barkalow and Soots 1965). Total sampling effort for 2, 81-ha plots varied from 3,000 to 11,305 trap nights among 13 samples (Barkalow et al. 1970). Population estimates could not be obtained for 2 samples because of interference by bad weather and a bumper mast crop (Barkalow and Soots 1965). Line- transect sampling requires less equipment than mark-recapture and is clearly easier to conduct

in roadless areas. None of these techniques have been tested on squirrel populations of known density, so the relative accuracy of the density estimates is unknown.

Based on experience hunting and sampling squirrels and leaf nests in West Virginia, we are confident that line-transect methods will work with hunted-squirrel populations. Move- ment along the line at a rate of 1 km/hour approximates still hunting. In rugged terrain or areas where squirrels are unusually wary, it may be necessary to traverse lines at a slower pace.

There seems to be no upper size limit to the area that could be sampled using line-transect methods. We do not know how small an area could be sampled efficiently, but it would depend on squirrel density. Obtaining an ade- quate sample (60-80, Burnham et al. 1980:35) of distances to squirrels is essential for density estimates using transects. Higher squirrel densities than we observed have been reported for other areas and habitat types, and presumably line-transect samples could be designed for small areas with high-density populations.

We recommend following the design guide- lines of Burnham et al. (1980). Sample size needed to estimate density at any level of pre- cision can be calculated if the number of observations/unit of transect can be obtained from a pilot study (Burnham et al. 1980:33-36). In this study, the number of squirrels observed/ km changed by 85 x from 1983 to 1985. No squirrels were seen during the pilot work in summer 1983. The estimated sample size required to achieve a density estimate with a coefficient of variation of 20% would be 75 observations, but because of variation in the number of observations per replicate (days in the present study), more observations may be needed to ensure reduction of the coefficient to an acceptable level. Squirrel populations resulting in an index of less than 0.5 squirrels/ km would require more than 150 km of transect to detect 75 squirrels. Such effort may be



SQUIRREL LINE TRANSECTS * Healy and Welsh. 89

cost prohibitive. Line-transect sampling did detect the presence of squirrels in 1983 when squirrel density was probably <1/100 ha. There may not be a technique that could have estimated density under those conditions.

Sampling during late October and early No- vember worked well in our area. Most leaves and mast had fallen. Squirrels were active on the ground, visibility was good, and the weather relatively mild. Sampling should be done in early morning when squirrels are most active and wind and humidity are usually favorable for moving quietly and observing squirrels.

We had planned to alternate the direction of travel on successive days, but adapted the convention of traveling with the sun to our back because it was impractical to work a rangefinder facing a rising (or setting) sun. In late October, the sun remained low in the southern horizon through the morning sampling period. Our transects ran within 30 degrees of north because the peninsula study site was oriented north-south.

Distance should be measured for all observations. Burnham et al. (1980) recommended taking both sighting distance and perpendicular distance. We usually measured only sighting distance and azimuth because forest veg- etation invariably obscured the reference point when we moved to measure the perpendicular distance. We calculated perpendicular distances and sighting angles (true angle from the transect) in the office.

Density estimates depend on horizontal distance measurements. When squirrels were on or within 2 m of the ground, we made no adjustment for deviation from the horizontal. We considered the error negligible because the topography was gentle and most measurements were within 10 degrees of horizontal. When squirrels were more than 2 m above the ground, we estimated horizontal distance by measuring to the tree trunk at eye level.

Observers could be trained in about 1 hour, but most required additional practice with

rangefinders to become proficient. We recommend frequent practice with rangefinders.

SUMMARY

Line-transect sampling is a useful technique for monitoring gray squirrel abundance on large areas. The theory and methods of analyses have been developed by Burnham et al. (1980). In our experience, the line-transect technique is easier to use than any of the other available techniques for estimating gray squirrel abundance. Density estimates can be obtained efficiently when sighting rates are >1 squirrel/km, and indices to abundance can be obtained when squirrels are seen less frequent- ly.

Monitoring squirrel populations on a regular basis will provide an indication of habitat differences among areas. Interpreting population response to habitat management on a partic- ular area is more difficult because acorn production may vary independently of management. On our study area, estimates of squirrel density varied by almost 15 fold during a 5-year period when the acreage of squirrel habitat did not change.

To identify the effects of management activities on gray squirrel abundance, agencies should consider using a repeated-measures ex- perimental design for monitoring. This strat- egy would involve monitoring both control areas and managed areas and should extend over a number of years (>5). Differences in abundance between control and managed areas over time would be the response variable. This would provide insights into whether observed changes were in response to management activities or were related to some other habitat variable, i.e., mast production.

The need to estimate squirrel abundance can arise for many reasons. Squirrels are an important component of oak forest ecosystems, they disperse seeds, and their populations respond to fluctuations in mast production.



90 Wildl. Soc. Bull. 20(1) 1992

Squirrels are easier to observe than most other small- and medium-sized mammals. Squirrel populations may serve as indicators of important ecosystem functions and the abundance of other less conspicuous species that depend on mast. Although we had no "known" population size to compare our results to, we believe line-transect sampling is suitable for estimating squirrel abundance.

Acknowledgments.-B. A. Spencer and P. J. Lyons, Metropolitan District Commission (MDC), allowed access to MDC land and helped conduct surveys. Much of the field work was done by volunteers, too numerous to mention individually, from MDC, Massachusetts Divi- sion of Fisheries and Wildlife, and University of Massachusetts, Division of Forestry and Wildlife. J. C. Pack, H. R. Perry, Jr., H. R. Sanderson, G. W. Smith, D. F. Stauffer, and G. C. White provided helpful reviews.

LITERATURE CITED

BARKALOW, F. S., JR., AND R. F. SOOTS, JR. 1965. An analysis of the effect of artificial nest boxes on a gray squirrel population. N. Am. Wildl. and Nat. Resour. Conf. 30:349-360.

, R. B. HAMILTON, AND R. F. SOOTS, JR. 1970. The vital statistics of an unexploited gray squirrel population. J. Wildl. Manage. 34:489-500.

BOUFFARD, S. H., AND D. HEIN. 1978. Census methods for eastern gray squirrels. J. Wildl. Manage. 42: 550-557.

BRENNAN, L. A., AND W. M. BLOCK. 1986. Line transect estimates of mountain quail density. J. Wildl. Manage. 50:373-377.

BROWN, B. W., AND G. 0. BATZLI. 1985. Field ma- nipulations of fox and gray squirrel populations: how important is interspecific competition? Can. J. Zool. 63:2,134-2,140.

BURNHAM, K. P., D. R. ANDERSON, AND J. L. LAAKE. 1980. Estimation of density from line transect sampling of biological populations. Wildl. Monogr. 72. 202pp.

DON, B. A. C. 1985. The use of drey counts to estimate grey squirrel populations. J. Zool. 206:282-286.

GURNELL, J. 1987. The natural history of squirrels. Christopher Helm (Publishers) Ltd., Bromley, Kent, U.K. 201pp.

KENWARD, R. E., AND J. M. TONKIN. 1986. Red and grey squirrels: some behavioural and biometric differences. J. Zool. 209:279-281.

LAAKE, J. L., K. P. BURNHAM, AND D. R. ANDERSON. 1979. User's manual for program TRANSECT. Utah State Univ. Press, Logan. 26pp.

NIXON, C. M., W. R. EDWARDS, AND L. EBERHARDT. 1967. Estimating squirrel abundance from live- trapping data. J. Wildl. Manage. 31:96-101.

, M. W. MCCLAIN, AND R. W. DONOHOE. 1975. Effects of hunting and mast crops on a squirrel population. J. Wildl. Manage. 39:1-25.

PACK, J. C., H. R. SANDERSON, AND R. W. DONOHOE. 1971. Comparison of gray squirrel census techniques. Trans. Northeast Sect. Wildl. Soc. 28:145- 158.

REYNOLDS, J. C. 1985. Autumn-winter energetics of holarctic tree squirrels: a review. Mammal Rev. 15:137-150.

SANDERSON, H. R., W. M. HEALY, J. C. PACK, J. D. GILL, AND J. W. THOMAS. 1975. Gray squirrel habitat and nest-tree preference. Proc. Annu. Conf. South- east. Assoc. Game and Fish Comm. 29:609-616.

TONKIN, J. M. 1983. Activity patterns of the red squirrel (Sciurus vulgaris). Mammal Rev. 13:99-111.

VERNER, J. 1985. Assessment of counting techniques. Curr. Ornithol. 2:247-302.

WHITE, G. C., D. R. ANDERSON, K. P. BURNHAM, AND D. L. OTIS. 1982. Capture-recapture and re- moval methods for sampling closed populations. Los Alamos Natl. Lab. Publ. LA-8787-NERP, UC- 11. Los Alamos, N.M. 235pp.

Received 24 June 1991. Accepted 11 October 1991. Associate Editor: Stauffer.



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Evaluating Line Transects to Monitor Gray Squirrel Populations