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Effects of Time of Day and Activity Status on Bobcat (Lynx rufus) Cover-type Selection in Southwestern Georgia
Jordona D. Kirby, Jessica C. Rutledge, Ivy G. Jones, L. Mike Conner, and Robert J. Warren

Southeastern Naturalist, Volume 9, Issue 2 (2010): 317–326

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2010 SOUTHEASTERN NATURALIST 9(2):317–326 Effects of Time of Day and Activity Status on Bobcat (Lynx rufus) Cover-type Selection in Southwestern Georgia Jordona D. Kirby1,3, Jessica C. Rutledge1, Ivy G. Jones1,4, L. Mike Conner1,*, and Robert J. Warren2 Abstract - Lynx rufus (Bobcat) cover-type selection and activity patterns have been studied in a variety of landscapes in the southeastern United States, but effects of individual activity status (active or inactive) and time of day (day, night, crepuscular) on cover-type selection have not been investigated for this species. Therefore, we investigated Bobcat habitat use in a Pinus palustris (Longleaf Pine) forest in southwestern Georgia to determine whether activity status of individuals or time of day affected seasonal cover-type selection. We monitored 43 radiocollared Bobcats from 2001–2004 and determined habitat use at Johnson’s third order of selection (i.e., habitat selection within the home range) using Euclidean distance-based analysis. Bobcats selected (Λ = 0.017, P = 0.001) habitat within their home ranges; however, although Bobcats are typically classified as crepuscular, neither activity status (Λ = 0.990, P = 0.981) nor time-of-day (Λ = 0.972, P = 0.647) affected cover-type selection. Bobcats on our study site preferred agricultural areas and other early to midsuccessional habitats, probably because they produced abundant prey. Introduction Prey abundance and distribution are important determinants of Lynx rufus Schreber (Bobcat) habitat use throughout their distributional range (Anderson 1987, Pollack 1951, Rucker et al. 1989). Previous research illustrated the importance of prey availability on Bobcat cover-type selection in the Southeast (Godbois et al. 2004). Godbois et al. (2004) found that Bobcats were located closer to areas in which supplemental food had been provided for Colinus virginianus Catesby (Northern Bobwhite; hereafter quail) as a result of the supplemental food locally concentrating rodents that were attracted to the grains used to feed quail. Bobcat habitat use is also influenced by availability of resting and denning sites, protection from environmental extremes, dense cover for hunting and escape, and freedom from disturbance (Anderson 1987, Bailey 1974, Boyle and Fendley 1987, Kitchings and Story 1984, Pollack 1951, Young 1958). Although Bobcats are typically classified as nocturnal or crepuscular, factors potentially influential on cover-type selection, such as activity status (i.e., whether an animal is active or inactive) and time of day (i.e., whether 1Joseph W. Jones Ecological Research Center, Route 2, Box 2324, Newton, GA 39870. 2Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30606. 3Current Address - USDA, APHIS, Wildlife Services, 4708 Western Avenue, Suite A, Knoxville, TN 37921. 4Current Address - Kentucky Department of Fish and Wildlife Resources, 1 Sportsman’s Lane, Frankfort, KY 40601. *Corresponding author - mconner@jonesctr.org. 318 Southeastern Naturalist Vol. 9, No. 2 it is day, night, or a crepuscular time period) have not been studied. When monitoring wildlife using radiotelemetry, it is critical to obtain locations such that the data adequately represent the variation in behaviors for the species of interest (Samuel and Fuller 1994). Commonly, the ideal schedule for wildlife telemetry data collection includes both diurnal and nocturnal or 24-hour sampling (Beyer and Haufler 1994, Smith et al. 1981). When logistical difficulties necessitate deviation from this ideal schedule for data collection, research results may be questioned (Beier et al. 2006). Thus, we assessed whether Bobcat cover-type selection varied with activity status or time of day. We predicted that active Bobcats would be located more than expected in association with early successional cover and edges, as we assumed that most Bobcat activity was associated with their foraging behavior. Similarly, we predicted that early successional cover and edges would be used more than expected during night and crepuscular periods associated with presumed peaks in Bobcat foraging activities. Field Site Description The Joseph W. Jones Ecological Research Center at Ichauway was an 11,735-ha research facility located in Baker County, 16 km south of Newton, GA. Approximately 24 km of the Ichawaynochaway Creek flowed through the study area, and almost 22 km of the Flint River formed the eastern boundary (Boring 2001). Elevations ranged from 27 to 61 m. The climate was characterized by hot, humid summers and short, mild, wet winters, with average daily temperatures ranging from 11 °C (winter) to 27 °C (summer). Average annual precipitation was 132 cm per year (Boring 2001). Pinus palustris P. Mill (Longleaf Pine) woodlands and limesink wetlands were the dominant habitat types at Ichauway. Other habitats included mixed pine-hardwood areas, food plots, agricultural fields, P. elliottii Enghelm (Slash Pine) flatwoods, riparian hardwood hammocks, and Quercus spp. (oak) sandhill barrens. Natural and old-field P. taeda L. (Loblolly Pine) stands, Nyssa spp. (tupelo) trees, limesink ponds, creek swamps, forested wetlands, riverine areas, shrub-scrub upland, and human/cultural (i.e., resident quarters) areas were also found at Ichauway (Boring 2001). The understory was dominated by Aristida stricta Michx (Wiregrass) and oldfield grasses (e.g., Andropogon spp.), but >1000 vascular plant species occurred on the site (Drew et al. 1998, Goebel et al. 1997). Quail management practices were used at the study site, and included food plots consisting of Brachiaria ramose Stapf (Brown Top Millet), Triticum aestivum L. (Winter Wheat), Vigna spp. (cowpea), Sorghum vulgare Persoon (Grain Sorghum), and Sorghum spp. (Egyptian wheat) (Godbois et al. 2004). Supplemental feeding for quail with Grain Sorghum occurred on approximately 60% of the study area each year between November and May (Godbois et al. 2004). Fields were disced to improve quail food availability, and limited predator removal was practiced. Bobcats were not harvested during the study. 2010 J.D. Kirby, J.C. Rutledge, I.G. Jones, L.M. Conner, and R.J. Warren 319 Prescribed burning was performed on a 2-year rotation, usually during winter and early spring, on approximately 4000 to 6000 ha throughout the entire site (Godbois et al. 2004). Prescribed burning was used to control understory vegetation, reduce hardwoods, manage wildlife habitat, reduce fuel buildup, promote Wiregrass seed production, prepare sites for pine regeneration, and for experimental research and educational activities (Boring 2001). Methods Bobcat capture, handling, and radiotelemetry We trapped Bobcats using Victor No. 1.75 offset and No. 3 Soft Catch® traps (Woodstream, Lititz, PA) from December 2000–May 2004. Captured animals were netted and administered an intramuscular injection of ketamine hydrochloride (10 mg/kg body weight; Seal and Kreeger 1987). We classified animals as adult or juvenile (<1 year) based on secondary sex characteristics, length, and weight (Crowe 1975). We fitted adults with a 180-g VHF radiocollar (Advanced Telemetry Systems, Isanti, MN); some VHF radiocollars were outfitted with motion sensors. Each Bobcat received a uniquely numbered ear tattoo. Bobcats were allowed to recover in portable pet kennels placed in a secluded barn or other covered structure. They were kept and monitored for 8–24 hr after immobilization to ensure full recovery before release at the trap site. All trapping procedures were approved by the University of Georgia Institutional Animal Care and Use Committee (IACUC #A990159). Using radiotelemetry, we began monitoring Bobcats 2–7 d after release. We obtained locations by triangulation, taking >2 radiotelemetry azimuths from known reference points with a 3-element Yagi antenna (Sirtrack, New Zealand) and hand-held receiver (Wildlife Materials, Carbondale, IL). We located each Bobcat 4–6 times per wk, with >8 hr between each location. To ensure equal sampling throughout the diel period, we shifted the start time of each telemetry data collection period by 2 hours each wk. We followed this diel schedule for each season. We performed accuracy tests for all field personnel, and the standard deviation associated with bearing accuracy was 7°. To minimize error due to animal movement between readings, we attempted to get close to each animal and limited time between consecutive bearings to <15 min, though most consecutive bearings were taken within 7 min (Cochran 1980, Kenward 1987, White and Garrott 1990). We determined activity (active or inactive) by a change in the motion switch of the transmitter or, on collars lacking the motion-sensitive switch, by a change in signal intensity (i.e., fluctuation in signal pitch or strength) when movement was detected (Chamberlain et al. 1998, Patterson et al. 1999). We defined an animal as active if the transmitter motion switch was triggered or if the transmitter exhibited a change in signal intensity while triangulating (Amstrup and Beecham 1976, Chamberlain et al. 1998, Theuerkauf et al. 2003). Radiocollared Bobcats were distributed throughout the study area. 320 Southeastern Naturalist Vol. 9, No. 2 Data analysis We used the FORTRAN program EPOLY (L.M. Conner, Joseph W. Jones Ecological Research Center, Newton, GA, unpubl. data) to convert radiotelemetry bearings into Universal Transverse Mercator coordinates. We calculated 95% minimum convex polygon (MCP; Mohr 1947) seasonal (e.g., fall = 20 Sep–20 Dec; winter = 21 Dec–20 Mar; spring = 21 Mar–19 Jun; summer = 20 Jun–19 Sep) home ranges for Bobcats with ≥30 locations per calendar season using CALHOME (Kie et al. 1996). We performed cover-type analyses using ArcGis software (ESRI 2004). We identified habitat using photo interpretation, classified habitat into eight cover types (agriculture/food plot [20.2%], shrub/scrub [1.6%], hardwood [10.8%], pine regeneration [4.4%], pine [31.8%], mixed pine-hardwood [24.5%], wetland [5.0%], and urban/barren [1.5%]), and digitized habitat to create a cover-type layer. We intersected Bobcat locations and home ranges onto the cover-type layer using ArcGis (ESRI 2004). We determined habitat use at Johnson's third order of selection (habitat selection within an individual’s home range) to investigate whether activity status and time of day affected habitat selection seasonally (Johnson 1980). We considered daytime as 2 hr after sunrise until 2 hr before sunset, and nighttime as 2 hr after sunset until 2 hr before sunrise. Finally, we defined crepuscular periods as the time period 2 hr before and 2 hr after sunrise and sunset. To simplify calculations, we assumed sunrise and sunset for each day of a season occurred at the same time, determined as the time of sunrise and sunset on the median day of each calendar season. We accounted for Daylight Saving Time during the appropriate seasons. We used a Euclidean distance technique to test for habitat selection by comparing the mean distances between animal locations and cover types to corresponding expected distances (Conner and Plowman 2001, Conner et al. 2003). We generated random locations for each home range using the random-point generator in the ArcGis Animal Movement Extension, and we calculated the mean distance (m) from random locations within each Bobcat home range to each cover type using the NEAR command in ArcGis (ESRI 2004). We also calculated mean distances from each Bobcat location to each cover type in the home range. For third-order habitat selection, we created eight distance ratios for each Bobcat (one for each cover type). Distance ratios were calculated as the average distances from Bobcat locations within the home range (i.e., “used” distances) to each cover type divided by the average distances from random locations within the home range (i.e., random distances) to each cover type. We assessed third-order habitat selection as a function of sex, season, time of day, and activity status using a four-factor MANOVA with year treated as a block. We expected the ratio of used distances to random distances to equal 1.0 if habitat use was random (i.e., no selection; Conner and Plowman 2001, Conner et al. 2003). If the ratio was <1, then animals were closer to the cover type than expected; whereas, if the ratio was >1, then the animals were farther from the cover than expected. Thus, we considered 2010 J.D. Kirby, J.C. Rutledge, I.G. Jones, L.M. Conner, and R.J. Warren 321 cover types to be preferred if the ratio was <1 and avoided if the ratio was >1. We used univariate t-tests to identify preferred (i.e., animal closer than expected) and avoided (i.e., animal farther away than expected) cover types if the MANOVA indicated selection was nonrandom. Finally, we ranked cover types using ranking matrices (Conner and Plowman 2001, Conner et al. 2003). All statistical analyses were performed with SAS software (SAS Institute 2003). We considered statistical significance to occur at α ≤ 0.10. Results The number of Bobcats monitored ranged from 13–27 individuals seasonally from 21 September 2001 to 20 June 2004 for 11 consecutive calendar seasons (43 total animals: 16 male [M] and 27 female [F]), and we used 12,520 telemetry locations for analysis. Number of individual locations ranged from 156–334 (SE = 7.38) per animal. For analysis of cover-type selection and time of day, we used 7088 daytime telemetry locations and 4332 nighttime telemetry locations. To analyze cover-type selection and activity status, we used 6420 active animal locations and 5000 inactive animal locations. Bobcat cover-type selection did not differ by sex (F8, 195 = 1.37, P = 0.2102), season (F80, 1245 = 1.08, P = 0.3056), or their interaction (F80, 1245 = 0.60, P = 0.9937). Although there were no significant two-way interactions (P > 0.10, all cases), habitat selection occurred (Λ = 0.017, P = 0.001) when data for sex and season were pooled. Bobcats were farther than expected from pine regeneration, pine, and mixed pine-hardwood cover types, but were closer than expected to wetland and urban/barren cover types (Table 1). Urban/barren cover was most preferred, followed by wetland, hardwood, agriculture, shrub/scrub, pine regeneration, pine, and mixed pine-hardwood (Table 2). We found no evidence that activity status (Λ = 0.990, P = 0.981) or time of day (Λ = 0.972, P = 0.647) affected Bobcat cover-type selection. Table 1. Cover type distance ratios for third-order selection using seasonalA home ranges for Bobcats at Ichauway, Baker County, GA, 2001–2004. Cover type ρB t PC Agriculture 0.037 0.75 0.456 Shrub/scrub 0.044 1.41 0.167 Hardwood 0.027 0.57 0.575 Pine regeneration 0.112 2.98 0.005 Pine 0.133 1.77 0.084 Mixed pine-hardwood 0.254 2.44 0.019 Wetland -0.000 -0.02 0.988 Urban/barren -0.043 -2.01 0.051 ASeasons were delineated according to calendar year (i.e., fall = 20 Sep–20 Dec, winter = 21 Dec–20 Mar, spring = 21 Mar–19 Jun, summer = 20 Jun–19 Sep). BAverage distance from random locations within home ranges divided by average distance from random locations throughout study area. Mean ratios <1 indicate cover type preference, and >1 indicate habitat avoidance. CProbability that the mean ratio = 1.0. 322 Southeastern Naturalist Vol. 9, No. 2 Discussion Bobcats selected cover types within their home ranges, but no sex-specific or seasonal differences were detected at this spatial scale. Bobcats used agriculture, shrub/scrub, hardwood, and wetland cover types as expected, but they were farther than expected (i.e., avoided) from pine regeneration, pine, and mixed pine-hardwood cover types. However, Bobcats were closer than expected to urban/barren cover, probably because several animals maintained home ranges in close proximity to the laboratory, research, and residential buildings on Ichauway, the location of most of the urban/barren cover on site. Bobcats likely selected cover types based on prey abundance and availability. Sigmodon hispidus Say and Ord (Cotton Rat), a primary Bobcat prey species in the Southeast, are typically most abundant in early to mid-successional habitats dominated by grass/forb-shrub vegetation in the understory (Boyle and Fendley 1987). Agricultural fields (i.e., food plots and their associated edges) also attract and potentially concentrate rodents such as Cotton Rats (Cummings and Vessey 1994). Hardwood cover types contain a dense herbaceous understory and shrub interspersion necessary to provide essential cover for prey species (Golley et al. 1965, Schnell 1968). Supplemental grains were distributed throughout portions of the study area for quail, which potentially concentrated Bobcat prey such as Cotton Rats and may have influenced cover-type selection. Although a previous study at Ichauway found that Bobcats were located closer to agricultural habitats in which supplemental food for quail was distributed (Godbois et al. 2004), diet studies suggested that quail comprised only 1.8% of Bobcat diet, whereas rodents comprised 75.1% (Doughty 2004). Cover-type selection by Bobcats on our study site were consistent with other studies in the Southeast, in which Table 2. Rankings based on pair-wise comparisons between cover-type distance ratios for thirdorder habitat selection, using seasonal home ranges of Bobcats monitored on Ichauway, Baker County, GA, 2001–2004. Mixed = mixed pine-hardwood. Urban/ Shrub/ Pine barren Wetland Hardwood Agriculture scrub regeneration Pine Mixed Urban/Barren + +++A + +++ +++ +++ +++ Wetland -B + B + + +++ +++ +++ Hardwood ---A - + + +++ + +++ Agriculture - - - + + + +++ Shrub/Scrub --- - - - + + +++ Pine --- --- --- - - + + regeneration Pine --- --- - - - - + Mixed --- --- --- --- --- - - AThree plus signs indicate row cover type significantly preferred over column cover type and three minus signs indicate column cover type significantly preferred over row cover type (ttest, P < 0.10). BA single plus or minus sign indicates that the row and column habitats have similar rankings. 2010 J.D. Kirby, J.C. Rutledge, I.G. Jones, L.M. Conner, and R.J. Warren 323 Bobcats preferred agricultural areas (Cochrane 2003, Conner et al. 1992) and bottomland hardwoods (Cochrane 2003, Heller and Fendley 1986). Cotton Rats, other rodents, and lagomorphs are typically most active during crepuscular periods (Buie et al. 1979, Hall and Newsom 1976, Kitchings and Story 1978, Marshall and Jenkins 1966). Bobcats have been documented as most active during crepuscular periods, corresponding with activity peaks of their primary prey species (Anderson 1987, Buie et al. 1979, Chamberlain et al. 1998, Hall and Newsom 1976, Kitchings and Story 1978, Marshall and Jenkins 1966). Because Bobcats tend to be more active when their prey are most active, we expected active animals to select agriculture and other early to mid-successional cover types more than inactive animals. We also expected Bobcats to prefer early to mid-successional cover types during crepuscular periods. Because our data were collected equally throughout the diel period, we were able to assess the importance of these sources of variation on the behavior of Bobcats. Contrary to our prediction, we found no evidence that activity status or time of day affected Bobcat cover-type selection, suggesting that cover types used for foraging also contained suitable shelter and loafing areas. Therefore, on our study area and perhaps elsewhere throughout Bobcat range, we suggest that daytime-only radiomonitoring of our study animals would have been adequate for studying cover-type selection. We speculate that Bobcats did not have to move among cover types to meet refugia and foraging requirements. An analysis of Bobcat time-specific and activity-specific habitat selection on study areas with intrinsic differences in prey abundance among cover types would provide a test of this hypothesis. Future studies to document prey densities in various cover types on our study site would provide evidence to support our suggestion that prey abundance was likely the main factor influencing Bobcat cover-type selection. Finally, we recommend multi-species research and perhaps reanalysis of existing radiotelemetry data sets collected for other species to determine the pervasiveness of temporal differences in cover-type selection. Acknowledgments Funding and other support were provided by the Joseph W. Jones Ecological Research Center, University of Georgia, and the Georgia Department of Natural Resources. M.J. Chamberlain, S.B. Castleberry, R.L. Hendrick, Jr., C. Nielsen, and 2 anonymous reviewers provided valuable comments to earlier versions of this manuscript. We thank the Jones Center Wildlife Lab, especially B. Rutledge, M. Perkins, A. Subalusky, B. Howze, and A. Reid, as well as J. Wade, R. Varnum, and B. Cross, for trapping assistance. Several Jones Center personnel, especially B. Bass, M. Melvin, J. Brock and A. Sheffield, also provided critical help with other aspects of our field work. Literature Cited Amstrup, S.C., and J. Beecham. 1976. 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