Home Ranges of Red-Cockaded Woodpeckers in Coastal South Carolina

HOME RANGES OF RED-COCKADED WOODPECKERS IN COASTAL SOUTH CAROLINA

ROBERTG. HOOPER,• LAWRENCEJ. NILES,2'3 RICHARDF. HARLOW,• AND GENE W. WOOD2 •USDAForest Service, Southeastern Forest Experiment Station, Department of Forestry,Clemson University,Clemson, South Carolina 29631 USA, and •TheBelle W. BaruchForest Science Institute of ClemsonUniversity, Georgetown, South Carolina 29440 USA

ABSTR•CT.--Totalobserved ranges of 24 groupsof Red-cockadedWoodpeckers (Picoides borealis)varied from 34 to 225ha and averaged86.9 ha. Groupsdefended portions of these rangesfrom adjacentgroups year-round. Home rangeswere derivedfrom totalobserved rangesby subtractingextraterritorial areas and areasreceiving limited use.The percentage of year-roundhome-range boundaries determined by intergroupconflicts varied from 0% to 54% and averaged23.6%. Year-round home ranges averaged 70.3 ha and variedfrom 30 to 195ha. The portion of year-roundhome rangesused in all samplingperiods varied from 15% to 65% and averaged30.5%. The amountof potentialforaging habitat per groupfor all groupswithin 2,000m of a studygroup's colony, a measureof populationdensity relative to availablehabitat, accountedfor 70% of the variation in size of year-roundhome ranges (P < 0.0001).Various measures of groupsize and habitatquality were weaklyrelated to size of year-roundhome ranges.Measures of group size and populationdensity together accountedfor 80% of the variation in size of year-round home ranges. Received14 January 1982, accepted2 March 1982.

THE Red-cockadedWoodpecker (Picoides bo- the extensiveareas used by groupsand the fact realis) is dependent upon live southern pines that forestmanagement continually alters home for nestingand roostingcavities (Jackson et al. ranges,a better understandingof home-range 1979) and, to a large extent, for foraging sites requirementsis criticalto the species'manage- (Hooper and Lennartz 1981). The scarcityand ment and survival. The objectiveof this study decline of pines suitable for cavity excavation was to examine relationships between home- are the majorreasons the specieswas classified rangesize, group size, populationdensity, for- as endangered (Federal Register, 13 October aging resources,and habitat quality in order 1970,35, 199: 16047).A scarcityof suitablefor- to test several hypothesesof why year-round aging habitat, however,may alsolimit the oc- home-rangesize varied in coastalSouth Car- currenceof the species.This has been indicat- olina. ed by colonyabandonment where large areas of foraging habitat were clearcutfrom around METHODS colonies. Obviously the home range must en- Studyareas.--Eighteen of 24 homeranges studied compasssome minimal amount of suitable for- were on the Francis Marion National Forest, in South aging habitat. Studies by Baker (1971), Crosby Carolina. The area has at least 400 groups of Red- (1971), Skorupa and McFarlane (1976), Wood cockadedson about 64,000 ha of habitat (Oscar Stew- (1977), Nesbitt et al. (1978), and Sherrill and art pers. comm.). This is one of the largest popula- Case (1980) found that 17 seasonalranges av- tions in existence.Major foragingareas consisted of eraged38.9 ha and varied from 14 to 91 ha. One loblolly(Pinus taeda) and longleaf(P. palustris)pine year-roundrange in south Floridawas 159 ha stands. These stands have been periodically pre- scriptionburned since 1944 and have been under (Patterson and Robertson 1981). Considering even-agemanagement since 1950. Ages of pine stands rangedfrom lessthan 1 to 100 yr. Mixed standsof gum (Nyssa spp.), cypress(Taxodium spp.), oak (Quercusspp.), red maple (Acer rubrum),and other 3 Present address:Georgia Department of Natural specieswere interspersedamong the pine stands. Resources, Game and Fish Division, Route 1, Box Six of 24 home ranges studied were on Hobcaw 547, Fitzgerald, Georgia 31750USA. Barony,3 km east of Georgetown,South Carolina.

675 The Auk 99: 6754582. October 1982 676 HOOPERET t•L. [Auk, Vol. 99

Hobcaw has about 3,000 ha of forest land, 3,000 ha ified into standsof similar speciescomposition, age, of salt marsh, and 1,000 ha of fresh water and brack- and density. Depending on stand size, 3-20 plot cen- ish marshesand abandonedrice fields. Forestvege- ters were locatedsystematically from a random start- tation was similar to that on the Francis Marion Na- ing point. At eachplot center,stems from •3 to <13 tional Forest except that pine stands tended to be cm dbh were recordedby speciesand dbh on a 0.02- older.A detailedanalysis of thevegetation was made ha plot. Stems•13 cm dbh were sampledwith a 1-m by Barry and Batson(1969). factor wedge prism. Trees in the prism plot were Determinationof ranges.--Year-roundranges of 24 recordedby dbh and species. groups of Red-cockadedswere determined by following group membersuniquely marked with col- RESULTS AND DISCUSSION ored plastic leg bands. Red-cockadedgroups were family units consistingof a mated pair, their off- TERRITORIAL BEHAVIOR spring of the year, and, in somegroups, adult male helpers. A group traveled severalkilometers daily The 24 groupsengaged in 146 conflictswith but returnedto its cavitieseach evening. After birds other groups.Intergroup conflicts varied in ob- left their cavitiesin the morning, they were contin- served intensity and duration. Conflicts aver- uouslymonitored for 2-14 h. This was done on 627 aged28.1 min for 87 timed encountersand had days,for a total of 3,080h of monitoringtime. Sam- a range of about 1-200 min. In the overtly less pling intensity was similar in all groupsso that it agonistic conflicts, two groups would forage would not be a variable in determining range size. about 10-100 m apart and were silent or mildly The locationsof groups were recorded at 5-min intervals. On the Francis Marion, l:24,000-scale ae- vocal, as comparedto more intense conflicts. rial photographswere griddedto 15.2 m, with each The more aggressiveconflicts were character- tenth line shown on the photograph.Major (152 x ized by increasedvocalizations, aerial chases, 152 m) grid points were locatedin the forestwhen chaseson the trunks of trees, wing spreading, natural featurescould not be used for accurateplot- and loud pecking (comparedwith normal for- ting of the group's location. On Hobcaw Barony, aging). At different times, breeding males and group locationswere plotted each5 min on forest females,adult helper males, and juveniles were vegetationmaps with a scaleof 1:12,000. seen engaged in these activities during con- On the Francis Marion, movements of six different flicts. Although both male and female Red- groupswere followed from May of one year to May cockadedsdrum, especiallyduring encounters of the next for 3 yr starting in 1976 and ending in 1979. On Hobcaw Barony, six groupswere followed with singlealien birds, they were not observed from September1976 to September1977. The year to do so during intergroupconflicts. In 17 cases, was divided into five samplingperiods based on the two birds from opposinggroups made bodily behaviorof the Red-cockadedand arbitrarydivision. contactby graspingbills or feet and fluttering Theseperiods were nestling(1 May-15 June),post- toward the ground, sometimeslanding there fledging(16 June-15September), fall (16 September- together. Conflicts ended when one of the 31 December)and spring (1 January-30April). Three groups withdrew. of the 24 groupsdid not nest the year we studied Conflicts between groups occurred year- their movements,and thus they did not havea nest- round (Table 1), but only three were seenwhen ling or postfledgingperiod. For thosethree groups, young were in the nest. Only one group had movement was monitored during a comparableperiod calledsummer (1 May-15 September).Using ob- no observed intergroup conflict and three servationsfor the nestlingand posffledgingperiods, groupshad only one. In contrast,another group we determineda summer home range for the other engagedin 17 encounterswith adjacentgroups. 21 groups. The mean number of conflictsper group was Total observedrange was determinedby connect- statisticallysimilar among sampling periods. ing outermostpoints at which the group was ob- There appearedto be a reductionin territorial served. Territorial boundaries were determined from encountersin spring,however, when fewer and advances and retreats of respective groups during significantlybriefer conflictswere observed. intergroup conflicts.Extraterritorial areas were por- Typically, a group would not honor its ter- tions of the total observedrange that were across ritorial boundaryif the adjacentgroup was not territorial boundaries. Limited-use areas were portions of the total observedrange visited only once aware of its presence. During all periods, by the group. Home range was determinedby sub- groupsleft their territory and traveledinto ad- tracting extraterritorialand limited-use areasfrom jacentones. On theseforays, the residentgroup total observedrange. was sometimes encountered, resulting in re- Measurementof vegetation.--Vegetationwas strat- treat by the trespassinggroup. The longestre- October1982] Red-cockadedWoodpecker Home Ranges 677

TABL•1. Frequencyand durationof conflictsof 23a groupsof Red-cockadedWoodpeckers with adjacent groups.

Summer Fall Spring Intergroupconflicts (1 May-15 Sept.) (16 Sept.-31Dec.) (1 Jan.-30Apr.) Number of conflicts Number per groupb 2.3A 2.3A 1.7A Minimum-maximum •6 •6 0-7 Total 54 53 39 Duration of conflicts Mean per conflictc (min) 26.5A 38.8B 16.9C Minimum-maximum (min) 0-123 0-200 0-50 Number conflicts timed 28 32 27

One groupout of 24 had no observedintergroup conflicts. Meansfollowed by the sameletters do not differ significantly(F = 0.95;df = 2, 84; P > 0.05). Meansfollowed by differentletters differ significantly(F = 3.4•5;df = 2, 84; P < 0.05). treat was about 700 m. On other occasions, the with isolated groups, and Skorupa and Mc- resident group was not encountered. Twice, a Farlane worked in an area with a low popula- trespassinggroup examinedseveral cavities and tion density. Thus, the potentialfor intergroup pecked briefly at resin wells before returning conflictsappears to have been limited in their to its own territory. study areas. Movements across observed territorial boundariesaveraged less than those into lim- SIZE OF RANGES ited-use areas (X = 180.8 and 260.3 m, respectively; t = 2.12, df = 84, P < 0.05). Probably Year-roundranges.--The largest total ob- one reason for this difference was the termi- servedrange among the 24 groupsof Red-cock- nationof extraterritorialforays upon encounter aded Woodpeckerswas 225 ha, and the small- with the resident group. Many of the excur- est was 34 ha. Mean size of total observedrange sionsinto areasof limited use would probably was 86.9 ha, with a standard deviation of 44.2 have been shorter had the group being fol- ha. Only one groupwe studiedhad a total ob- lowed encounteredthe resident group. In other served range larger than the 159 ha reported words, many of the limited-use areas were for a group in south Florida (Patterson and probably part of adjacent territories, but we Robertson 1981). Year-round home ranges av- failed to see intergroup conflictsat those lo- eraged 70.3 ha, with a standard deviation of cations.For example,in the secondlongest ex- 35.7 ha. The largestyear-round home range was cursion(620 m) into a limited-use area, a group 195 ha, and the smallest was 30 ha. On the went 430 m beyond the cavity tree used by average,year-round home rangesconsisted of anothergroup for nesting.Because both groups 82% of the total observedranges, but this var- were still active for at least the two following ied from 60% to 99%. The two measures of breedingseasons, and becausethe two groups range were highly correlated(Pearson product- had one encounter in a different place before moment correlation coefficient = 0.98, P • the above excursion, it seemed certain that a 0.0001). conflict and retreat would have occurred if the Extraterritorialareas averaged 8.4 ha, and the two groups had met. largestwas 24.7 ha. In two groupswe did not Accounts of territorial behavior of Red-cock- observe an extraterritorial area, but those adeds in the literature tend to support our ob- groups did have limited-use areas. Areas of servation that the speciesis strongly territorial limited use averaged 10.4 ha, and the largest (Ligon 1970,Lay et al. 1971,Nesbitt et al. 1978, was 31.0 ha. Seven groups had no observed Sherrill and Case 1980). Crosby (1971),Baker limited-use area, but all seven of those had ex- (1971), Patterson and Robertson (1981), Sko- traterritorialareas. It is possiblethat somepor- rupa and McFarlane (1976), and Wood (1977), tions of the limited-use areas were extraterri- however, did not report intergroup conflicts. torial and that we simply failed to seeconflicts Crosby and Patterson and Robertson worked in those areas. 678 HOOPElET AL. [Auk, Vol. 99

TABLE2. Mean and extremevalues for size of home rangesof 24 groups of Red-cockadedWoodpeckers during different periods (ha).

Number Period Meana Maximum Minimum of groups Nestling (1 May-15 June) 27.8A 49 12 21 Postfledging(16 June-15September) 43.1B 114 19 21 Summerb (1 May-15 September) 47.6B 115 22 24 Fall (16 September-31December) 37.5A, B 77 16 24 Spring (1 January-30April) 48.9B 151 22 24 ' Meansfollowed with differentletters differ significantly(P < 0.05) basedon analysesof variance(F = 4.06; df = 3, 86; P < 0.01, and F = 1.83;df = 2, 69; P < 0.17) and Duncan'smultiple range tests. Note that the nestlingand postfledgingperiods were not comparedto the summer period. b The summerperiod home range was the areaused during the nestlingand postfledgingperiods for groupsthat nested. For groupsthat did not nest, it was the area used during the comparableperiod.

Boundariesof year-round home rangesvar- periods of less than a year. Several of these ied in length from 2.7 km to 8.3 km (• = 4.4 ranges are of particular interest when com- km). The percentageof these boundaries de- pared with our data. A group monitored by termined by intergroup conflictsvaried from telemetryfor 4 daysin Octoberhad a range of 0% to 54% and averaged23.6%. Observedter- 91 ha (Nesbitt et al. 1978).The two largesttotal ritorial boundaries were generally well de- observedranges that we measuredin fall were fined, but we were unable to delineate entire 85 ha and 77 ha. Use of telemetry did not ap- territories for three possible reasons. First, pear to account for the size of the range mea- home-rangeboundaries were extensive,and we sured by Nesbitt and his coworkers,because might not have recognizedthe significanceof they measuredtwo other rangesby telemetry a given intergroup conflict to the extent the that were only 58 ha and 59 ha. Recently, woodpeckersdid. Second, the low population Nesbitt (pers. comm.) studied five total ob- density around some groupsmight have pre- served ranges in south Florida during July- cluded the establishment of territorial bound- October that averaged 144 ha. He suggested aries. And third, we undoubtedlyfailed to see the sparse distribution of suitable foraging all territorial encountersthat occurredduring areas accounted in part for the large size the year. of those ranges. Crosby (1971)reported that a Rangesize for differentperiods.--Home ranges group studiedin March-Junehad a range of 15 were similar in size during the summer, fall, ha, and Skorupaand McFarlane(1976) reported and spring periods (Table 2). Of these three ranges of 16 ha and 18 ha for two groups in periods, home ranges in fall were on the av- June-July.These three ranges were smallerthan erage21 and 23% smallerthan in summerand any we studied during a comparableperiod. spring, respectively,but the differenceswere Groups shifted the area they used within not significant. Only three groups had their year-round home rangesamong sampling pe- largesthome range in fall, however, compared riods. In the fall, groups used an average of with 10 groups, each with their largest home 56% of their year-round home ranges, com- rangesin spring and summer. One group had pared with 71% in summer and 69% in spring. home rangesof equal size in fall and spring. For groups that nested, an average of 44% of These results are contrary to those of Skorupa the year-round home ranges was used when and McFarlane (1976), who found home ranges young were in the nestand an averageof 60% to be smaller in summer. after they fledged. The portion of the year- In groups that nested, the summer period round home range used in all periods averaged was divided into the nestlingand postfledging only30.5% andvaried from 15% to 65%. Other periods.Home rangeswere significantlysmall- studiesinvolving more than one samplingpe- er when young were in the nest than during riod alsoreported shifts in ranges(Skorupa and the postfledgingand spring periods(Table 2). McFarlane 1976, Wood 1977). Home rangesdid not differ significantlydur- ing the nestlingand fall periods. DETERMINANTS OF HOME RANGE SIZE Of the 18 total observed ranges reported in Before examination of the data, we framed the literature, all but one were based on sample four hypothesesthat we could test about the October1982] Red-cockadedWoodpecker Home Ranges 679

TABLE3. Mean and extremevalues for major vegetationcharacteristics in 24 year-rounghome ranges of Red-cockadedWoodpeckers.

Mean Maximum Minimum

Total basal area of stems •> 13 cm dbh (ms) Pine 831.7 2,310 210 Hardwood a 233.3 690 10

Total number of stems >3 but <13 cm dbh for all species(x 1,000) 68.2 273 10 Hectaresof pine and pine-hardwood standsby ageclass b •<20 6.0 44.6 0 21-40 7.6 70.0 0 41•50 19.3 90.7 0 >60 30.9 100.7 1.0

Includescypress. Standswhere pine basalarea >50% of total basalfor stems•>13 cm dbh.

variation in rangesize amonggroups. The first habitats within a home range of Red-cockad- was that home rangescontained approximately eds were not of equal value to the birds, our the same quantity of foraging resourceseven third hypothesiswas that home-rangesize was though they differed in size. Such a relation- relatedto habitat quality. Possibly,the occur- ship was found for severalnectar-feeding birds rence of young pine stands and hardwood (Gass et al. 1976). Winter territories of Town- standsincreased home-range size becausethey send's Solitaires (Myadestestownsendi), how- took up spacebut contributedlittle in the way ever,varied considerablyin the amountsof food of food. Also, home ranges composedmostly they contained(Salomonson and Balda 1977). of olderpines might be smallerthan thosewith Our secondhypothesis was that home range mostly younger pines. size was related to the density of the surround- For some group-living birds, territory or ing population.Compressible home rangesand home-range size increaseddirectly with group territorieshave been reportedfor many verte- size (Parry 1973, MacRoberts and MacRoberts brates (Wilson 1975:270). Although an inverse 1976, Woolfenden and Fitzpatrick 1978). Thus, correlationbetween food density and territory a fourth hypothesiswas that group size was sizehas been reportedin severalstudies (Myers directly related to home-range size. Larger et al. 1979), in more recent studies this rela- groupscould possibly defend larger ranges or tionship was shown to result from intruder perhaps larger ranges could support larger pressure(Myers et al. 1979, Ewald et al. 1980). groups. Earlier, Krebs (1971) found that territory size in Great Tits (Parusmajor) was not related to TABLE4. Resultsof regressionsbetween year-round food supply but was determinedby the num- range size of study groups and the population ber of birds surviving. Settlementpatterns of densitysurrounding study groups;n = 24 for all models. territoryholders had a stronginfluence on territory size in Song Sparrows (Melospizamelo- Regressionmodel R2 P dia) (Knaptonand Krebs 1974).Sherrill and Case (1980) found a positive relationship between 1. Home range = 106.79 - 3.71 size of Red-cockaded home ranges and mean (number groups) 0.25 0.01 distance to nest trees of surrounding groups 2. Home range = 26.42 + 0.3830 (r = 0.95, n = 4). (ha habitatin 2 kma/numbergroups) 0.70 0.0001 An associationbetween habitat quality (usu- 3. Total range = 124.93 - 3.86 ally statedin terms of food production and for- (number groups) 0.18 0.04 aging efficiency) and territory size has been 4. Total range = 37.65 + 0.4298 foundfor manyspecies of birds (Schoener1968, (ha habitatin 2 kma/numbergroups) 0.57 0.0001 Southern 1970, Wiens 1973, Morse 1976, Best a Hectaresof pine and pine-hardwood stands•>20 yr old within 2 km of nest cavity. For the three groupsthat did not nest, the current 1977, Seastedt and MacLean 1979). Because all roostcavity of the breedingmale was used. 680 HOOPERET AL. [Auk, Vol. 99

TABLE5. Resultsof regressionsbetween year-round home-rangesize of study groupsand variablesasso- ciatedwith habitatquality within ranges;n = 24 for all comparisons.

Dependent variable Independent variable R 2 P < pine basalarea a 1. Homerange totalbasal area 0.06 0.25 pine basalarea a 2.Home range homerange 0.002 0.82 nonpinebasal area a 3.Home range homerange 0.06 0.24 ha pine stands<20 yr old 4.Home range homerange 0.09 0.50 hapine stands >40 yr old 0.003 0.81 5.Home range homerange hapine stands >60 yr old 0.06 0.24 6.Home range homerange number stems <13 cmb 7.Home range homerange 0.21 0.03 Stems •>13 cm dbh. Stems,all species,>3 but •<13cm dbh.

Foragingresources and sizeof homeranges.- (Table 4, model 1). Becausesalt marshes, open As an index to the quantity of foraging re- water, and large hardwood swamps occurred sourcesin eachrange, we estimatedbasal area within some of the 2,000-m-radius circles sur- of pines •> 13 cm dbh in eachyear-round home rounding study groups, it seemedreasonable range(Table 3). Red-cockadedsforaged almost to adjust the populationdensity for theseareas exclusivelyon live pines and preferredthose that had little or no value to the woodpecker. •>13 cm dbh (Hooper and Lennartz 1981). When this adjustmentwas made, 70% of the Among year-roundhome ranges,there was an variation in home-range size was associated 11-fold difference in the minimum and maxi- with population density (Table 4, model 2). mum pine basal area. Also, a direct relation- Theseresttits support the hypothesisthat home- ship was found between year-round home- range size was inversely related to population range size and pine basalarea (R2 = 0.68, P < density, relative to availablehabitat. Thus, it 0.0001). Thus, contrary to the foraging-re- appears that home-range size was primarily a sourcehypothesis, pine basalarea was highly result of the number of groups in an area di- variableamong home ranges, and home-range viding the amountof availablehabitat. size increaseddirectly with basal area. These Habitat qualityand size of homeranges.--Sev- resultsare evidenceagainst the hypothesisthat en independent variables, possibly related to year-roundhome ranges,regardless of size, habitat quality, were derived from various contain essentiallythe same quantity of for- measurements of the habitats within each home aging resources.They supportthe alternative range. Means and ranges for these measure- idea that somegroups had more resourcesthan ments are presented in Table 2, and the inde- needed for occupancyof their home ranges. pendent variables are defined in Table 5. The Surplusresources, however, could possibly play mean number of stems>3 but <13 cm dbh per a role in group size, reproductivesuccess, and ha had the only significantrelationship to size creation of new territories (Woolfenden and of home range,but only 21% of the variation Fitzpatrick1978j. in home-range size was associatedwith this Populationdensity and sizeof homeranges.- variable. No two-variableregression involving The number of active groups within 2,000 m different measuresof habitat quality was sig- of the nesttree of eachstudy group varied from nificant. In multiple regressionmodels involv- 1 to 22. The relationship between home-range ing population density, however, both mean sizeand numberof groups,however, was weak number of stems(for all species)per ha >3 but October1982] Red-cockadedWoodpecker Home Ranges 681

TABLE6. Multiple regressionmodels of the form: y = a + b•x• + b2x2;where y is the year-roundhome rangein ha, x• is ha of habitatwithin 2 km of studygroup's nest cavity + numberof groupswithin 2 km, and x2varies according to table;n = 24 for all models.

x2 a ñ SEa b• ñ SE b,ñ SE R2b Pc 1. pinebasal area • 86.78ñ 23.33 0.3866+ 0.0475 -76.74+ 28.48 0.77 0.01 total basal area number stems <13 cm e 2. 10.77 + 8.78 0.3542 ñ 0.0489 21.85 ñ 8.16 0.77 0.01 home range 3. Group sizet -11.84 + 12.87 0.4057ñ 0.0449 9.84 ñ 2.92 0.80 0.003

Standard error. R2 for y = a + b•x• was 0.70. Significanceof x• contributionin presenceof x•. P = probabilityof largervalue of t if b• = 0. Basal area for stems 513 cm dbh. Number of stems,all species>3 but <13 cm dbt•. Mean monthlygroup size.

<13 cm dbh and pine basal area divided by birds in group during period) accountedfor a total basal area (for stems•_-13 cm dbh) pro- nonsignificantamount of the variation in range duced a significantreduction in the variation sizes for each of the five periods (R• • 0.11, in size of home range (Table 6, models 1 and P • 0.12, n = 21-24). Skorupa (1979), how- 2). Thus, we cannotreject the hypothesisthat ever, reporteda positive relationshipbetween habitat quality influencedrange size. At least group size and size of fall home ranges(R • = as we measuredit, however, habitat quality 0.99, n = 5), and Sherrill and Case (1980) found was not a very powerful determinantof home- a nonsignificanttrend (r = 0.75, n = 4) be- range size of the groupswe studied. tween group size and home-rangesize for De- Group size and size of home ranges.--The cember-February.Combining their data to in- groupswe studiedvaried in size throughout crease sample size resulted in a weak the year. Followingnesting, they averaged4.3 relationship (R•= 0.04, n = 9). Their data birds, with 2-9 birds per group. Beforenest- points fell within the clustering of our data ing, groups averaged2.6 birds, with 2-5 birds points; thus, there is little evidencein our data per group. Regressionanalysis did not reveal or data in the literature that group size was a linear relationship between mean monthly relatedto size of home-rangesdetermined dur- group size and year-round home-range size ing differentperiods of the year. when other factors were not considered (R 2 = 0.06, P > 0.36, n = 24). In addition, home CONCLUSIONS rangesof groupswith helpersaveraged 72.4 ha (n = 12), and rangesof groupswithout helpers Groups of Red-cockadedWoodpeckers were averaged68.2 ha (n = 12). These means did territorial toward adjacentgroups throughout not differ significantly(t = 0.29, df = 22, P > the year, and the observedterritorial bound- 0.50). aries were generally distinct. Complete terri- Group size did appearto be significantlyre- tories were not observed, however, possibly lated to year-roundhome-range size when a becauseof their large size and the probability measure of the surrounding population den- that somegroups had not establishedterrito- sity was incorporatedinto the regressionmod- rial boundariesin portionsof their home range el (Table 6, model 3). In other words, if pop- that lackedproximity to othergroups. The size ulation densities were the same around all of year-roundhome rangeswas apparentlyde- homeranges, group size by itselfwould prob- terminedprimarily by the groupsin a localarea ably have a significant effect on year-round dividing the availablehabitat within that area. home-rangesize. Habitat quality and group size were weakly Size of rangesfor the different samplingpe- related to year-round home-rangesize. The riods did not appearto be relatedto group size quantity of foraging resourcesincreased di- in our study. Based on separate regression rectly with size of home range, supportingthe analyses, group size (maximum number of idea that some groups had accessto more re- 682 HOOPERET AL. [Auk, Vol. 99 sourcesthan necessaryfor year-round occu- MAcRoBERTS, M. H., & B. R. MAcRoBERTS. 1976. pancy. Social organization and behavior of the Acorn Woodpeckerin central coastalCalifornia. Orni- ACKNOWLEDCMENTS thol. Monogr. 21. MORSE,D.H. 1976. Variablesaffecting the density We thank J. A. Jackson,S. A. Nesbitt, and J. Ver~ and territory size of breeding spruce-woods ner for helpful criticismof earlierdrafts of this paper. warblers.Ecology 57: 290-301. LITERATURE CITED MYERS,J.P., P. G. CONNORS,& F. A. PITELKA. 1979. Territory size in wintering Sanderlings:the ef~ BAKER,W. W. 1971. Progressreport on life history fectsof prey abundanceand intruder density. studiesof the Red-cockadedWoodpecker at Tall Auk 96: 551-561. Timbers ResearchStation. Pp. 44-59 in The ecol- NESBITT, S. A., D. T. GILBERT, & D. B. 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