The Auk 118(2):422-431, 2001
RECLAIMED
COAL
MINE
FOR HENSLOW'S
GRASSLANDS
SPARROWS
AND
IN THE
THEIR
AMERICAN
SIGNIFICANCE MIDWEST
ROBBA. BAJEMA,1 TRAVIS L. DEVAULT,2 PETERE. SCOTT,AND STEVENL. LIMA3 Department of Life Sciences, IndianaStateUniversity,TerreHaute,Indiana47809,USA
ABSTRACT.--Present methodsof surfacecoal-minereclamationin the Midwest produce large grasslands,someof which exceed2,000ha in extent.Total "mine grassland"production in southwesternIndiana aloneis well in excessof 70 squaremiles (180 km2).Our work in 19 reclaimedcoal mines in southwesternIndiana indicatesthat mine grasslandsharbor many Henslow'sSparrows(Ammodramus henslowii). We basethat conclusionon point-count and line-transectsurveysthat yielded between 200-300 singing male Henslow'sSparrows during the 1997and 1998breedingseasons. Thosesurveyresultsimply an uncorrectedpopulation densityof -0.10 malesper hectare,and a correcteddensityof -0.16 malesper hectare (correctingfor undetectedmales).Extrapolatingthis correcteddensity to total habitat coveragesuggestsan overall populationof a few thousandHenslow'sSparrowsin the mine grasslandsof southwesternIndiana. Small-scalevegetationalsurveyssuggestthat muchof the within-mine variation in Henslow'sSparrow abundancereflectslocal vegetativestructure, with males preferring sitestypically associatedwith that speciesof bird: tall, dense grass-dominated vegetationwith a substantiallitter layer.Managementfor this kind of vegetative structurecould greatly increasethe number of Henslow'sSparrowsinhabiting reclaimedmines.Midwestern mine grasslandscould play a significantrole in stabilizingthe populationsof Henslow'sSparrowsand other grasslandbirds. Received 23 August1999,accepted5 February2001.
FEWLARGEPOPULATIONS of Henslow'sSpar-
so,derivebasicestimatesof populationdensity
rows (Arnrnodramus henslowii)are known to ex-
and size.
ist, especially east of the Mississippi River (Pruitt 1996). Our study, however, indicates that reclaimedsurfacecoal-minegrasslandsin southwesternIndiana harbor suchpopulations of Henslow'sSparrows.Grasslandbirds have
The potential importanceof mine grasslands for Henslow'sSparrowsreflectsthe fact that midwestern grasslands(native or otherwise) have declined precipitously in the last 150 years (Samsonand Knopf 1994, Warner 1994, Noss et al. 1995). Against that backdrop of grasslanddestruction,the Henslow'sSparrow has declinedperhapsmostsignificantlyof any grasslandbird (Herkert 1995), and more than most forest-dwelling Neotropical migrants (Knopf 1994, Peterjohn et al. 1994, Herkert 1995).Althoughseveralfactorsmay contribute to that overallpopulationdecline,suchas state of wintering habitat (Plentovichet al. 1999), disappearanceof suitablebreedinghabitathas probablybeen the most critical determinantof the declineof this species(Askins 1993).That decline can readily be understoodin terms of the natureof suitablebreedinghabitatfor this species:large,undisturbedgrasslands(Herkert 1994b, Walk and Warner 1999) with relatively little woody vegetationand a significantlitter layer (Zimmerman 1988; see Herkert 1998).
been recorded
in the reclaimed
surface mines
of Appalachia(Whitmore and Hall 1978,Whitmore 1980,Aliaire 1981,Wray et al. 1982),including Henslow's Sparrows (Peterjohnand Rice 1991; see also Koford 1999), but the con-
servationpotentialof mine grasslandsin general is far from being completelyevaluated.In fact, it appearsthat large mine grasslandsof the Midwest have gonevirtually unnoticedby avianbiologists.Hence,our overallgoalwas to evaluatethose midwestern mines as potential habitat for Henslow'sSparrows,and in doing
• Presentaddress:Departmentof Biology,Aquinas College,Grand Rapids,Michigan 49506,USA. 2present address: Department of Forestry and Natural Resources,Purdue University, West Lafayette, Indiana 47907, USA.
3Address correspondenceto this author. E-mail:
Such habitat is scarce in the Midwest, where
the Henslow'sSparrow'sbreedingrangeis concentrated (Pruitt 1996, Herkert 1998).
[email protected]
422
April2001]
CoalMinesandHenslow's Sparrows
423
habitat consists of relatively dense, undisturbed grass-dominatedvegetationwith a significantlitter that produceslargeblocksof grasslandhabitat. layer (reviewedby Herkert 1998).If a minehad a sigThosemine grasslandsare foundmainly in the nificant extent of such vegetation,we established fixed survey routes. Only very recently reclaimed Illinois coalbasin (portionsof Indiana, Illinois, mines, or a few small mine grasslandsconverted to and Kentucky)and portionsof Ohio, and rep- cattle grazing operations,lackedsuchhabitat (Bajeresent the main result of mine reclamation ma and Lima 2001). We note that our working defi(Brothers 1990). Newly reclaimed mines are nition of "suitablehabitat" was meant to be broadly typicallystockedwith quickgrowing,cool-sea- inclusive so as to not exclude any potential Henson grassessuchas tall fescue(Festucaarundi- slow'sSparrowhabitat. We presenta more refined nacea),orchard grass (Dactylisglomerata),and perspectiveon habitat suitability below. Surveymethods.--We used two survey methods:5 smooth brome (Bromusinermis), as well as varmin, unlimited distance point counts,and unlimited ious legumes such as clovers (Melilotusspp.) distanceline transects(seeBibby et al. 1992, Ralph and alfalfa (Medicagosativa),to establisha veget al. 1993).Pointcountswereperformedin a fashion etative structure and minimize soil erosion similar to the BreedingBird Survey(Priceet al. 1995) (Brothers 1990). Tree planting and crop pro- and took advantageof the gravel-roadnetworkwithductionmay thenfollow,but oftenin only a rel- in most reclaimedmines.A surveyroute was estabatively small portion of the reclaimedarea;the lishedby stoppingevery0.5 km alongmine roadsin bulk of a reclaimed mine usually remains as suitable Henslow's Sparrow habitat. That distance grassland (Brothers 1990). Furthermore,soil betweenpoint countseliminatedthe risk of censusconditions and isolation from forested habitat ing the samesparrowstwice although still allowed (Hardt and Forman 1989) often inhibit woody adequatecoverageof an area. During a point count, plant invasion,allowing those grasslandsto all Henslow'sSparrowsdetectedby song were recorded,as was the time of day. Locationof the point
Ironically, surfacecoal mining in the Mid-
west
is a form
of environmental
disturbance
persistfor many yearswith no management. Severalaspectsof midwesternmine grasslands make them particularly attractivefor the conservationof grassland birds in general. First, mostmidwesternmine grasslandsexceed the apparentsize thresholdsfor eventhe most area-sensitivegrasslandspecies(Samson1980, Herkert 1994a, Vickery et al. 1994, Bollinger
count itself was recorded
with
the use of Garmin ©
Global PositioningSystemunits (10-30 m accuracy). A total of 249pointscountswereestablishedin 1997, with 248 points establishedin 1998; most points (>98%) were in the samelocationsboth years. Line transectswere conductedin the larger "roadless" units of Henslow'sSparrow habitat. Transects beganat least200 m from the nearestpoint-countlocation,and were walked slowlyat -2 km per hour. 1995, Walk and Warner 1999, Winter and FaaExactshapeand length of a giventransectwere dicborg 1999;see also Horn et al. 2000). Second, tated by terrain and extent of suitablehabitat, but althoughreclaimedminesare not alwaysman- mosttraverseda rectangularpattern in a large field. aged as grasslands,soil conditionsmay leave GPS units were used to map the location of each them largely unsuitablefor other purposes.Fi- transectand Henslow'sSparrow detections.Overall, nally, entire reclaimedmines are often owned a total of 36 transects(averagelength = 1,220 m, by a singleentity, which makeslong-termcon- range 300-4,200 m) covering 44.0 km were estabservationof thoselarge grasslandsmore feasi- lished in 1997,with 39 transectscovering45.5 km in ble. In short,midwesternmine grasslandspro- 1998(averagelength = 1,170m, range300-4,200m); enhanced
access to some mines allowed
us to estab-
vide a unique opportunity for the large-scale, lish a few new transects in 1998, otherwise all trandiverse grassland management approach to sectswere in the samelocationsacrossyears. conservationof grasslandbirds (Herkert et al. We conductedour surveysfrom mid-May through 1996, Sampleand Mossman1997). mid-July during both 1997 and 1998 breeding seaMETHODS
Establishing studysites.--Minegrasslandswere located using satelliteimagery and a GeographicInformationSystem(GIS) (ArcInfo) as describedin Bajema and Lima (2001). After locating a given mine grassland,we made contactwith the owners, and theninspectedthe mine to determinewhetherit contained suitable Henslow's Sparrow habitat. Such
sons,which largely ensuredthat all detectedHenslow'sSparrowswere on establishedbreedingterritories (Herkert 1998). During 1998, we performed three completeroundsof surveys(Round1, 12 May6 June;Round 2, 28 May-23 June;Round 3, 23 June10 July).A singlesurveyround, spreadover the entire breeding season,was performed during 1997 (mid-May to mid-July).We confinedour daily surveys to a 5 h period beginning0.5 h before sunrise, the period during which Henslow'sSparrowssing
424
BAJEMAET AL.
mostactively(Hellerand Hughes1997,Koford1999). Survey work was postponed during inclement weather (high wind and rain). Surveyswere conductedby a crew of five experiencedobserversduring ead• breedingseason. We also surveyedpastureand hay-fieldhabitats. These areas constitute
under 15% of most reclaimed
mines,but asmuchas45% in two mines (Bajemaand Lima 2001). We confinedthesesurveysto fields that were intensivelymanagedfor hay but uncut at the time of censusing,and areas that were actively grazed by cattle. We used roadside(5 min) point countsonly, with 50 and 54 points establishedin 1997and 1998,respectively.As before,threesurvey roundswere completedin 1998,with a singlesurvey round
in 1997.
Singingbehavior, correction factors,anddetection distances.--Observations made during extendedvisits to specificpoint-countlocations(in 1997) strongly suggestedthat our surveyswere missingmanynonsingingmaleHenslow'sSparrows.At onetime,there may be threeor four malessingingat a site,whereas severalminuteslater only oneor two may be singing. Point countssignificantlylonger than 5 min could have detectedmany of thosemissedmales (seealso Gutzwiller 1991,McSheaand Rappole1997),but longer countswere not feasiblegiven logisticalconstraintsimposedby remoteness of many minesand amountof territorythat we surveyed.Insteadwe devised a correction
factor for missed males.
Correctionfactorsare implied in the basictheory of point counts(e.g.Barkeret al. 1993),althoughexamplesof their use in estimatingavian abundance arefew.Our particularcorrectionfactorinvolveddeterminingthe proportionof time (s) that the typical male will sing at leastonceduring a 5 min interval (the duration of an individual point count).With s determined,the true numberof males(M) at the typical point-countlocationis relatedto the numberof malesdetectedat that location(m)by m = sM, which rearrangedto M = m/s. In other words, the factor correctingfor missedmalesis 1/s. Robbet al. (1998) describean analogouscorrectionfactorapplicableto spot-mappingsurveysfor Henslow'sSparrows. To estimate the value of s, we d•ose 25 count lo-
cationsthat were known to harbor Henslow'sSparrows and recordedwhetheror not a givenmalesang at leastonceduring eachof 12 consecutive5 min periods(1 h). Theproportionof these12periodsduring which a male sangis an estimateof s. We first estimateds on a per-sitebasisby combiningdatafrom all malesdetectedat a givensite.Becausethenumber of malesdetectedduring a given 1 h observationperiod was not correlatedsignificantlywith the sitespecificestimateof s (Spearmanrank correlations rangingbetween-0.04 and 0.10,P > 0.6), we averaged site-specific s valuesto determineour overall estimateof s.During 1998,we visited "singingsites" at the end of eachsurveyround.We thus determined
[Auk, Vol. 118
s valuesfor each round of censusing.During 1997, we quantifiedsingingbehaviormainlyin themiddle of the breedingseason,correspondingto the midpoint of our singleroundof censusing. Duringboth years, hour-longobservationperiodswere evenly distributed throughoutthe 5 h morning surveying period. We were able to make visual
contact with
almost
all singingmales, hencemonitoringmultiple individuals
was feasible. We nevertheless
excluded
data
from a given male if there was any confusionabout its identity during the 1 h observationperiod. We also excluded
data taken from males so distant
that
we couldnot reliably detecteach(brief) act of singing. Followingthe 1 h observationperiod, we measured distancesto singing perchesof all detected males. We did this at about 50% of observation
sites.
Vegetational mapping andsurveys.--Large-scale patterns in vegetative cover within each mine were mappedwith the aid of recent,large-formataerial photographsobtainedfrom the controllingmining companies.On-the-groundobserversusedthoseaerial photographsto record the following habitat types: suitableHenslow'sSparrowhabitat (as defined earlier), grazed grassland,intensivelyhayed grassland,forest, shrubland(presenceof several young trees 0.45). We did not detect any male Henslow'sSpar- t = -0.29, df = 24, P; 0.779).Similarly,during rowsin grazedand hayedgrasslands. Our sur- 1998, number of males detectedincreasedby a vey effort in thosehabitatsinvolveda cumu- factor of 1.59 _+0.133, which was also not siglative total of 212 point counts over the two nificantlydifferentfrom the expectedvalueof breedingseasons.We havenevertheless made 1.64 (t-test,combiningdata from all three segcasual observations of a few male Henslow's mentsof the breedingseason;t = -0.30, df = Sparrows in hayed areas. However, we virtu- 53, P = 0.766). A plot of distancesto detectedmales(Fig. 1) ally never detectedthat speciesin areaswith suggestedthat detectionsfadedappreciablyby recentor activecattlegrazing. Singingbehavior, correction factors,anddetec- 125 m from the observer,with only one male tion distance.--Ourstudy of singing behavior detectedbeyond160 m. The detectionof birds showedthat male Henslow'sSparrowssing in- beyond125 m requiredunusuallygoodlistenWethustake125m asourworktermittently.When observedfor 12 consecutive ing conditions. 5 min intervals,the proportionof intervalsdur- ing estimateof detectionradius.Note alsothat
April 2001]
CoalMinesandHenslow's Sparrows
m, those estimatestranslateinto a density of 0.122 ___ 0.031malesper hectare.Our correction factorappliesspecifically to point counts,hence we leavetransectestimatesuncorrected. Density
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estimates for 1998 were determined
8
in the same
mannerfor eachsurveyround(Table3). The 1998 point countestimates(usinground-specific cor•3 4 rectionfactors)rangedfrom about0.14 to 0.18 malesper hectare,with an overallbreedingseaz sonaverageof 0.159malesper hectare.The 1998 0 line-transect estimates (uncorrected) ranged from 0.10to 0.18malesper hectare,with an overDetection distance (rn) all averageof 0.129malesper hectare.Note further that the 1998 averagepoint-countdensity FIG. 1. Frequencyhistogramof detectiondistanccomparescloselyto the 1997valueof 0.164males es for male Henslow'sSparrows.Data were obtained per hectare(whichalsocoveredtheentirebreedduring a study of singing behavior in which visual contactwas made with most males (1998 breeding ing season),suggestinga stablepopulationover the two-yearperiod;the corresponding valuesof seasononly). 0.129 and 0.122 males per hectarefor the 1998 and 1997 transect data support the same few Henslow'sSparrowswere detectedat dis- conclusion. We used correctedpoint-countdensityestitances
