Published by the Wilson Ornithological Society INFLUENCE OF PRESCRIBED FIRE ON WINTER ABUNDANCE OF BACHMAN S SPARROW

INFLUENCE OF PRESCRIBED FIRE ON WINTER ABUNDANCE OF BACHMAN S SPARROW JAMES A. COX 1,3 AND CLARK D. JONES 1,2 Published by the Wilson Ornithological Society

The Wilson Journal of Ornithology 121(2):359 365, 2009 INFLUENCE OF PRESCRIBED FIRE ON WINTER ABUNDANCE OF BACHMAN S SPARROW JAMES A. COX 1,3 AND CLARK D. JONES 1,2 ABSTRACT. Prescribed fire is used extensively to manage breeding habitat for Bachman s Sparrow (Aimophila aestivalis), but little is known about the effects of prescribed fire on winter habitat requirements. We used conspecific recordings in conjunction with point counts to assess relationships between winter sparrow abundance and use of prescribed fire. Counts of sparrows conducted over three winters were higher (0.59 0.42) (x SD) when surrounding vegetation was burned the previous breeding season than in areas burned 18 months earlier (0.27 0.38). Year-to-year abundance estimates for individual stations increased an average of 0.39 ( 0.54) individuals per count when surrounding vegetation was burned the previous breeding season and decreased 0.22 ( 0.59) individuals per count when vegetation was not burned. Sparrow counts were positively correlated with percent bare ground cover surrounding census stations and negatively correlated with increases in percent grass cover, grass standing crop, height of grass, and shrubs 1 m in height. Prescribed fire may improve winter foraging conditions for this ground-dwelling species by reducing dense grass cover at ground level. Increased flowering responses that many dominant plants exhibit following burns also may improve winter food resources. We observed color-marked birds (n 18) in the same areas used during the breeding season and confirmed the maintenance of year-round home ranges by some individuals. Received 22 September 2007. Accepted 30 September 2008. Winter habitat preferences of Bachman s Sparrows (Aimophila aestivalis) are vague because individuals are difficult to observe outside the breeding season (Dunning 2006). Use of conspecific recordings increases detections of Bachman s Sparrows during winter (Cox and Jones 2004), and surveys using conspecific recordings could help in evaluating changes in winter sparrow abundance in relation to differences in habitat, climate, and management practices including use of prescribed fire. Better information on winter habitat associations is listed as a research priority by organizations concerned with conservation and management of this declining species (Mitchell 1998, Meyer 2006). We conducted winter surveys of Bachman s Sparrows using conspecific recordings to assess relationships between winter sparrow abundance and use of prescribed fire during the breeding season. Bachman s Sparrows nest and forage on the ground (Dunning 2006), and appropriate breeding season conditions are best maintained using frequent ( 3 years) prescribed fire (Tucker et al. 2004). There has 1 Tall Timbers Research Station, 13093 Henry Beadel Drive, Tallahassee, FL 32312, USA. 2 Current address: Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602, USA. 3 Corresponding author; e-mail: jim@ttrs.org been little research on the influence of prescribed fire on wintering birds (King et al. 1998, Vickery et al. 1999), and relationships between winter sparrow abundance and prescribed fire may differ from relationships observed during the breeding season. The objective of our study was to compare sparrow abundance in winter in areas that differed in terms of the timing of recent prescribed fires. We compared winter counts (Nov Jan) conducted in areas where vegetation was burned in May of the previous breeding season (i.e., 6 10 months before Nov Jan counts) with counts conducted in areas where vegetation was burned two breeding seasons prior, also in May (i.e., 18 22 months prior to counts). We also quantified vegetation features and evaluated relationships between sparrow abundance and changes in vegetation structure during post-fire succession. Our study sites coincided with areas where large colormarked populations (n 350) have been established for breeding-season studies (Cox and Jones 2007). Observations of colormarked individuals during winter surveys allowed us to compare winter locations with known breeding-season home ranges (Cox and Jones 2007). METHODS Our study was conducted in southwest Georgia (USA) on Pebble Hill (Grady Coun- 359

360 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 121, No. 2, June 2009 ty; 30 46 N, 84 06 W) and Arcadia (Thomas County; 30 46 N, 84 00 W) plantations. The properties contained extensive areas ( 500 ha) of undisturbed ground cover (Ambrose 2001) and mature forests dominated by longleaf pine (Pinus palustris). Approximately 50% of the available sparrow habitat on each property was burned in May during each year of study. The size of burn blocks varied from 10 to 60 ha, and fires typically removed 98% of the ground cover vegetation within burn blocks (Tall Timbers Research Station, unpubl. data). Four point-count stations were spaced 200 m apart along four transects on each property (n 32 stations). Previous winter surveys conducted on these properties suggested a distance 200 m eliminated spatial autocorrelation among counts (JAC, unpubl. data). Census stations were aligned with burn blocks to ensure that ground cover vegetation within 100 m of each station either was burned extensively ( 80%) or negligibly ( 20%) during each year of study (hereafter called burned and unburned treatments). Counts were distributed equally between burn treatments on both properties, and the fire frequency used (i.e., 50% of each property burned in an alternating manner) resulted in survey stations alternating between burned and unburned treatments during the 3 years of study (i.e., stations with 20% burned in May 2005 and 2007 had 80% burned in May 2006 and vice versa). Census stations were visited six times from November to January during each year of study (2005 2006, 2006 2007, and 2007 2008). Surveys were performed a half-hour before and up to 2.5 hrs after sunrise, and the order in which we visited transects varied with each survey. A recording containing primary and excited songs, and aggressive chitter notes (Dunning 2006) was played at each station for 2 min. We counted Bachman s Sparrows that responded with distinctive chip notes (Cox and Jones 2004, Dunning 2006) within 100 m. Locations of color-banded individuals (Cox and Jones 2007) that flew to exposed perches in response to recordings were geographically referenced using a handheld global positioning system (Trimble Explorer III, Trimble Inc., Sunnyvale, CA, USA) for comparison to breeding-season home ranges (Cox and Jones 2007). The average number of sparrows recorded each year was used as the estimated abundance for a station. We subtracted the estimated abundance for a given year from the estimate obtained at the same station the following year to assess the influence of burn treatments. The differences were analyzed using ANOVA with two burn treatments (burned and unburned), two sites (Pebble Hill and Arcadia), and two time intervals (winter 2005 06 vs. winter 2006 07 and winter 2006 07 vs. winter 2007 08) as independent factors. We controlled for variation in soil conditions, proximity to other habitat types, and other site-specific features (but not for other effects such as overall breeding season productivity) by comparing changes as census stations alternated between burned and unburned treatments. Plots of least squares means were used to interpret higher-level interactions (Wilkinson 1998). The probability of detecting sparrows at census stations was analyzed using the singleseason model in program PRESENCE (Hines 2006) with years, burn treatments, and properties treated as covariates. The multi-season model available in PRESENCE (Hines 2006) was not used because sparrows were detected at 95% of the stations sampled (i.e., extinction and colonization events were minimal). Models comparing detection probabilities with year, burn treatment, and site covariates were evaluated using Akaike s Information Criterion (AIC, Akaike 1973). Models within AIC 2 of the best fitting model were considered part of our confidence set (Burnham and Anderson 2002). Distance-based methods for evaluating point count data (e.g., Buckland et al. 2001) were not used because conspecific recordings likely caused birds to move closer to the observer and therefore violated assumptions associated with these procedures. Shrub and ground-cover measurements were collected at four plots established 25 m from the centers of each station along major compass headings (n 128) in each year of study. Woody shrubs were quantified at two heights ( 1 and 1 m) following James and Shugart (1970). Woody shrubs 1 m were quantified separately because they may provide escape cover in winter (Dean and Vick-

Cox and Jones PRESCRIBED FIRE AND BACHMAN S SPARROWS IN WINTER 361 TABLE 1. Analysis of variance comparing differences in annual Bachman s Sparrow counts among burn treatments ( 20 vs. 80% burned), intervals (winter 2005 2006 vs. 2006 2007 and winter 2006 2007 vs. 2007 2008), and sites (Pebble Hill vs. Arcadia Plantation). Source SS df MS F P Site 0.40 1 0.40 1.38 0.24 Burn 5.66 1 5.66 19.344 0.01 Interval 0.56 1 0.56 1.92 0.17 Site*burn 0.57 1 0.57 1.94 0.17 Site*interval 1.44 1 1.44 4.99 0.03 Burn*interval 0.65 1 0.65 2.23 0.14 Site*burn*interval 0.02 1 0.02 0.07 0.80 Error 16.37 56 0.29 ery 2003). Grass standing crop was quantified by measuring the resting height of a 1.5-kg disk after it was dropped from a height of 70 cm over 10 randomly selected locations (Bransby and Tainton 1977). Disk resting height is higher with greater grass standing crop (Bransby and Tainton 1977). A 1-m 2 cover grid with 16 cells (Bonham 1989) was used to quantify percent cover for different groundcover categories. The grid was dropped at 16 random locations per plot and the number of cells dominated by four classes of vegetation (bare ground [including dead litter], forb, grass, or woody) was recorded. Pine and hardwood basal area surrounding stations was estimated using forest inventories conducted in 2005 (Tall Timbers Research Station, unpubl. data). Relationships between winter sparrow abundance and timber and ground-cover characteristics (pooled by station) were assessed using Pearson correlations with P-values adjusted for multiple comparisons (Wilkinson 1998). RESULTS The average ( SD) number of Bachman s Sparrows observed at all stations was 0.46 ( 0.48), but counts varied by burn treatments and other factors (Table 1). Sparrow abundance increased an average of 0.20 ( 0.47) and 0.57 ( 0.55) for the two time intervals when vegetation surrounding census stations was burned the previous May. Counts decreased 0.21 ( 0.60) and 0.22 ( 0.59) when vegetation was not burned for the same time intervals. Overall, counts averaged 0.59 ( 0.42) sparrows at stations when surrounding vegetation was burned during the previous breeding season, while counts averaged 0.27 ( 0.38) sparrows at stations when vegetation was not burned. Significant interactions were observed between sites and time intervals (Table 1). Comparison of least squares means indicated the interactions stemmed from (1) larger negative values observed on Pebble Hill and (2) larger positive values observed on Arcadia during the first time interval (winter 2005 06 vs. winter 2006 07). The average difference observed on Pebble Hill during the first time interval was 0.12 ( 0.41), while the average observed on Arcadia was 0.22 ( 0.42). Differences observed on Pebble Hill during the second time interval averaged 0.25 ( 0.60), while the average observed on Arcadia was 0.10 (SD 0.78) for the same interval. Four models were included in the confidence set emerging from program PRES- ENCE. The best fitting model suggested detection probabilities co-varied with site, burn treatments, and years (AIC weight 0.285); however, the confidence set included a model with constant detection probability ( AIC 1.32, AIC weight 0.147), which suggested detection rates were similar by sites, years, and burn treatments. Detection probability was 0.38 (SE 0.03) for this model. Vegetation measurements in areas burned the previous breeding season (Table 2) had less grass cover, lower average grass height and grass standing crop, and fewer woody shrubs 1 m in height (Table 2). Ground cover associated with burn treatments also contained a higher percentage of bare ground (Table 2). Percent bare ground was positively correlated with winter sparrow counts (P 0.10, Table 3), while four measures of ground cover vegetation were negatively correlated

362 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 121, No. 2, June 2009 TABLE 2. Average vegetation measurements (x SD) recorded at census stations (n 32) during each year of study. Sites burned in May of the previous breeding season ( Burned ) were compared to sites burned 18 months earlier also during May ( Unburned ). P-values are based on t-tests (df 95) adjusted for multiple comparisons using Bonferroni approximations (Wilkinson 1998). Component Unburned Burned P Bare ground a 20.1 16.1 52.7 16.5 0.01 Forb cover a 11.8 7.1 13.1 9.2 0.37 Grass cover a 33.9 16.6 12.9 6.5 0.01 Woody cover a 34.1 16.8 21.4 13.7 0.17 Grass height b 95.0 15.6 55.6 13.7 0.01 Grass standing crop c 17.5 4.2 6.7 0.9 0.01 Shrubs 1 m d 5.7 4.5 2.9 2.3 0.05 Shrubs 1 m d 8.4 6.3 6.8 5.4 0.45 a Percent cover based on cells (n 16) within a 1-m 2 grid dominated by component. b cm above ground level. c Resting height above ground (cm) of a 1.5-kg disk dropped from a height of 70 cm. d Shrub strikes measured at two heights along 20-step transects. with counts (P 0.10, Table 3). Sparrow numbers were lower at stations where grass standing crop, percent grass cover, grass height, and shrubs 1 m in height were all greater. Several vegetation measurements were correlated (e.g., r 0.64 0.77 for grass measurements) and reflected a transition from bare ground to increased vegetation cover as post-fire succession proceeded. All observations of color-marked individuals (n 18) occurred within 50 m of home ranges documented during the breeding season. Most winter observations (n 16) also occurred in areas burned the previous breeding season. In addition, two color-banded male-female pairs were observed together in January 2007 and nested successfully the following breeding season. TABLE 3. Pearson correlations comparing abundance of wintering Bachman s Sparrows at census stations with vegetation characteristics surrounding the station. P-values have been adjusted for multiple comparisons (Wilkinson 1998). Component Correlation P Bare ground 0.34 0.06 Forb cover 0.02 1.00 Grass cover 0.40 0.01 Woody cover 0.09 1.00 Grass height 0.35 0.04 Grass standing crop 0.53 0.01 Hardwood shrubs 1 m 0.33 0.08 Hardwood shrubs 1 m 0.00 1.00 Pine basal area 0.18 0.61 Hardwood basal area 0.01 1.00 DISCUSSION Dunning (2006) described Bachman s Sparrow breeding habitat as ephemeral based on decreases in sparrow numbers observed as time since burning increased. Winter habitat also appears to be ephemeral and can change quickly as post-fire succession proceeds. Winter counts were consistently higher at census stations where surrounding vegetation was burned the previous breeding season. Sparrow abundance at individual census stations also increased 50% when surrounding vegetation was burned. The variation in sparrow abundance we observed likely resulted from (1) changes in winter food resources and/or (2) changes in the structure of ground-cover vegetation as post-fire succession occurred. The winter diet of Bachman s Sparrow contains a high proportion of grass seeds and other plant materials (Allaire and Fisher 1975). We did not quantify food availability, but some common plants (e.g., Aristida and Tephrosia) on our study areas produce more seeds following burns (especially burns conducted after Apr; Hiers et al. 2000). Platt et al. (1988) and Streng et al. (1993) also found burns conducted after April generally increased the dominance of fall-flowering forbs and grasses; insect abundance in fall also may be greater following burns conducted after April (Brennan et al. 1997). The influence of ground-cover vegetation structure was suggested by positive correlations between sparrow abundance and percent

Cox and Jones PRESCRIBED FIRE AND BACHMAN S SPARROWS IN WINTER 363 bare ground, and negative correlations between sparrow abundance and grass standing crop, percent grass cover, and grass height. Extensive grass cover and higher grass standing crop in our area leads to dense matted conditions at ground level where sparrows search for food on foot (Allaire and Fisher 1975, Dunning 2006). Prescribed fires reduce the density of grasses and may improve access to food (Brennan et al. 1997), provided vegetation also has recovered sufficiently to provide concealment from predators (Thatcher et al. 2006). Haggerty (1998) also noted relationships between sparse vegetation and foraging behavior during the breeding season. Some breeding-season studies (Engstrom et al. 1984, Dunning and Watts 1990, Dunning 2006) have linked sparrow abundances to increases in woody shrubs 1 m in height. The relationships we observed suggested subtle changes in winter habitat may take place before woody shrubs exceeded 1 m. Flowering responses of many plants diminish in the second growing season following a burn (Platt et al. 1988, Streng et al. 1993, Provencher et al. 2001), and an additional year of plant growth leads to denser ground cover (e.g., disk measurements for grass standing crop 15 cm). Similar relationships have been observed for Henslow s Sparrows (Ammodramus henslowii), which also winters in fire-maintained pinelands in the southeastern United States (Tucker and Robinson 2003) and forages primarily on the ground (Carrie et al. 2002). Wintering Henslow s Sparrows were more abundant in areas burned recently, and both increased food resources (Tucker and Robinson 2003, Bechtoldt and Stouffer 2005) and/ or open ground-cover structure (Bechtoldt and Stouffer 2005, Johnson 2006) have been suggested as reasons for these differences. The interaction term (Table 1) observed between sites and time intervals suggested other factors also influenced winter counts. The consistency of year-to-year differences calculated for individual census stations affirmed the importance of recent burns, but winter abundance also could be influenced by breeding season productivity, juvenile survival, local climate, and several other factors we did not measure. In addition, the size of burn blocks at Pebble Hill was smaller than that at Arcadia ( 10 vs. 60 ha). Variation in the size of burn blocks created a more heterogeneous mixture of burned and unburned areas near point counts at Pebble Hill. Changes in winter sparrow abundance observed from year to year suggested Bachman s Sparrows may move in response to shifting food resources and ground-cover conditions. Winter observations of color-marked individuals coincided with breeding season home ranges and suggested some individuals did not move. Year-round area use may be linked to prescribed fires since most observations of color-marked birds (n 16) occurred in areas burned the previous breeding season. Observations of two color-marked pairs also suggested pair bonds may be maintained through winter months as suggested by Haggerty (1988). Estimates of Bachman s Sparrow abundance obtained from winter counts may be lower than estimates obtained from breeding season counts (cf. Plentovich et al. 1998), but winter counts could help fill information gaps regarding winter distribution (Dunning 2006) and the effects of different management practices. The seasonal timing of prescribed fires warrants further evaluation because it can influence fall insect abundance (Brennan et al. 1997), and the flowering responses and ground-cover dominance of plants (Platt et al. 1988, Streng et al. 1993). Our results reaffirm the importance of frequent ( 3 years) prescribed fire in maintaining preferred habitat conditions for Bachman s Sparrows during both winter and breeding seasons (Tucker et al. 2004). ACKNOWLEDGMENTS We thank Mr. and Mrs. Jeptha Wade III, and the Pebble Hill Foundation for allowing us to conduct research on these properties. R. E. Masters, K. M. Robertson, and two anonymous reviewers provided helpful comments on an earlier version of this manuscript. We also are grateful for financial support provided by the Nongame Wildlife Grants Program of the Florida Fish and Wildlife Conservation Commission, and the Power of Flight Bird Conservation Program sponsored by the Southern Company and National Fish and Wildlife Foundation. LITERATURE CITED AKAIKE, H. 1973. Information theory and an extension of the maximum likelihood principle. Pages 267 281 in Second international symposium on infor-

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