The document summarizes research estimating the breeding population of Booted Eagles in the Cape Province of South Africa. Key points: - Researchers collected data over 25 years on 150 known nest sites to determine the eagles' breeding range and density. - Using a geographic information system and digital terrain models, they defined suitable nesting habitat as areas with broken, hilly terrain based on topographical characteristics of known nest sites. - By calculating the mean inter-nest distance of 9.7 km, they estimated a total breeding population of 702 nest pairs in the study area. However, in more intensely studied core areas, densities were even higher, so the estimate is considered conservative.

1. j. RaptorRes'.35(1):15-19
¸ 2001 The Raptor ResearchFonndation,Inc.
ESTIMATING THE BREEDING POPULATION OF BOOTED EAGLES
IN THE CAPE PROVINCE, SOUTH AFRICA
DAVID PEPLER
DepartmentofNature Conservation,FacultyofAgriculturalandForestrySciences,Universityof Stellenbosch,
PrivateBagX1, Matieland7602, SouthAfrica
ROB MARTIN
DepartmentofForestScience,FacultyofAgriculturalandForestrySciences,UniversityofStellenbosch,PrivateBagXI,
Marieland7602, SouthAfrica
HUBERTUSJ. VANHENSBERGEN
DepartmentofNature Conservation,FacultyofAgriculturalandForestrySciences,UniversityofStellenbosch,
privateBagX1, Matleland7602,SouthAfrica
ABSTRA(;T.--Dataon the breeding range of Booted Eagles (Hieraaetuspennatus)were collectedover 25
yr in the Northern, Western,and EasternCape Provinces,SouthAfrica, to estimatethe breedingpop-
ulation. Basedon the distribution of 150 known nest sites,we used information from digital terrain
•nodelsto define topographicalcharacteristicsof nest sites.This information wasused to identify the
total suitablenestinghabitatin the studyarea.Bycalculatingthe mean inter-nestdistance,weestimated
the totalnestingpopulationthroughextrapolation.With ameaninter-nestdistanceof9.7km,wearrived
at an estimateof 702nests.In coreareasthatwehavestudiedintensively,wefound evenhigherbreeding
densities and therefore consider our estimate to be conservative.
KEYWORDS: BootedEagle;,Hieraaetuspennatus;breedingdensity;SouthAfrica;GIShabitatdelineation;pop-
ulation estimation.
Estimaci6nde la poblaci6n reproductivade Hieraaetuspennatusen la provinciadel Cabo, Surafrica
REsumEN.--Recopilamosdatossobreel rango de reproduccionde Hieraaetuspennatusdurante 25 aftos
en el norte, oestey estede la Provinciadel Cabo,SurMricapara estimarla poblaci6n reproductiva.Con
baseen la distribuci6n de 150 sitiosde nidos,utilizamosinformaci6n de modelosdigitalesdel terreno
para deftnit las caracteristicastopogr•tficasde los sitiosde anidaci6n. Esta informaci6n fue utilizada
para identificar el total del habitatpropicio para anidaci6nen el Mea de estudio.A1extrapolarel c•lculo
de la media de la distanciaentre nidos,estimamosel total de la poblaci6n anidante. Con una media
de distanciaentre nidosde 9.7 kin, 11egamosa un estimativode 702 nidos.En lasMeascentralesque
hemos estudiado intensivamente,encontramosdensidadesafn mas altas,pot 1o tanto consideramos
que nuestro estimativoes conservador.
[Traducci6n de C6sarM•rquez]
In contrastto the Palearcticregion,where the bi-
ology of the Booted Eagle (Hieraaetuspennams)is
well-known(Cramp and Simmons1980, del Hoyo
et al. 1994, Su/trezet al. 2000), only its breeding
biologyhasbeen studiedin southernAfrica (Steyn
and Grobler 1981, 1985). This breeding popula-
tion wasonlyrecentlydiscovered(Martin and Mar-
tin 1974, Brooke et al. 1980) and the first modern
breeding record was confirmed in 1973 (Martin
and Martin 1974). The extent of the breeding
range was clarified by models of seasonalityand
associatedbreedingby Boshoffand Allan (1997)
and Harrison et al. (1997) but the range is com-
plicatedbecausethere appearto be three separate
populations of Booted Eagles in southern Africa
(BoshoffandAllan 1997). Thesepopulationscon-
sistof nonbreedingsummermigrantsfrom the Pa-
learctic region, a relict breeding population from
the Waterberg in Namibia, and a breeding popu-
lation in the Cape Province (del Hoyo et al. 1994,
Brooke et al. 1980). To further complicatethe sit-
uation, someeaglesoverwinterin the southwestern
CapeProvince(PepletandMartin 1997).To date,
only one estimate of breeding population (400
15

2. 16 PEPLERETAL. VOL.35, NO. 1
•Stellenbo•
Figure 1. The studyareain southernAfrica.
pairs) hasbeen madefor the CapeProvince(Mar-
tan and Martin 1991).
We collecteddata on the breeding range and
densityof the Booted Eagle population in the
CapeProvinceoverthe past25 yr throughdirect
observations. We used elevation information from
digitalterrain modelsto define the topographical
characteristicsof BootedEaglenestingsitesbased
on the distribution of 150 known sites to deter-
mine the total area of suitable habitat. To estimate
the total nesting population, we establishedthe
nestingdensitywithin suitablehabitatby calculat-
ing the mean inter-nestdistancefollowing the
method of Pepler et al. (1991). In this method a
plot of the cumulativesum of deviationsfrom the
runningmean (Lombaard1989)indicateschanges
in densitywith distance.Thismethodismoretyp-
icallyusedin the analysisof time-seriesdatabut is
appliedin thiscaseto the distanceseries.A system-
atic deviationfrom zero is indicativeof a change
in trend, in this case inter-nest distance.
STUDY AREA AND METHODS
The studyareacoveredtheportionof southernAfrica
southof 31% and westof 26øE(Fig. 1). The total land
areawas263 532 km2.The townof Stellenbosch(33ø55'S,
18ø52'E)wasusedasour base.Our studyareacovered
mostof the perceivedbreedingrangeasdescribedby
Steyn(1982) and Boshoffand Allau (1997). The study
wasconductedannuallyfrom 1975-93.Timing of field-
work wasrestrictedto the breedingseason,which was
typicallyfrom Septemberto December.
Within our studyarea,BootedEagleshuntedandbred
•n both hilly and open country,preferringhabitatscon-
sistingof namakaroo,succulentkarooandfynbos(Low
and Rebelo1996) and,especially,the ecotonesbetween
thesehabitat types(Boshoffand Allan 1997). In recent
years,however,we observedBootedEagleshunting in
suburbanareas(Peplet and Martin 1996), and it isrea-
sonableto assumethat recordsof breeding from within
these areas will be found in due course.
We searchedfor nestsfrom roadsthat gaveaccessto
lectedthat affordedthe greatestpossiblefield of view.
The totaldistancecoveredduring the courseof thefield-
workwasin excessof 500000 km (approximately2 km
traveledper km2 of the studyareaor 0.1/km peryr).
Occupied nest siteswere confirmed when one or both
adultswereseencarryingnestingmaterialto a specific
site,preyitemswereseenbeingcarriedtosites,oryoung
were observedat sitesbefore they fledged.Nestswere
typicallysituatedbehiud siualltreesor shrubsgrowing
on cliffs.Whitewasharound occupiednest siteshas a
uniquestreakedappearancethat helpeduslocatethem
from greater distances.
Breedingsiteswereplottedon1:950000topographical
mapsandsubsequentlydigitizedintoa formatcompati-
blewithanARC/INFO GeographicInformationSystem
(Environmental SystemsResearch Institute, Inc., 380
New YorkStreet,Redlands,CA 99373-8100,U.S.A.). El-
evationdata for the studyarea were obtainedfrom na-
tionaldigitalelevationdata (Departmentof LandInfor-
mationSystems,PrivateBagX10, Mowbray7705,South
Africa)and thesedatawerealsoimportedinto theGIS.
The vertical interval for the elevation data was 950 m.
Booted Eagle nest sitesare associatedwith mountain-
ouscountrywith broken terrain (Steyn1989). Elevation
alonewasnot a goodindicator of suitablehabitatbecause
the eaglesdo not neston high plateaus.Steepslopes
werealsoindicatorsof suitablehabitatbutevenlysloping
areasare not usedfor nesting.The broken terrain used
byBootedEagleswasidentifiedusingtherateof change
of slopewhich wasdetermined from the secondderiva-
tive of the function describingthe surface.The function
wascalculatedfor eachpoint from the eightelevations
immediatelyadjacentto a point aswell asthe elevation
of the point itself.The valuereferredto a pointat the
centerof a grid of nine pointswith a totaldimensionof
678 x 678 m. High valuesof thisparameterindicated
the rapidlychangingslopesassociatedwithbrokenhilly
countrywhile excludingplateausand smoothinclines.
Lowvaluesindicatedconstantslope.A numberofvalues
of thisparameterweretrieduntil one,whichbyinspec-
tion of the area coveredon the mapjust includedthe
distribution of the majority of the known nests,was
found. The term "mountainous," in the context of this
analysis,wastakeuasanyareawith a secondderivativeof
heightgreaterthan,or equalto, 0.9.The cellsizefor the
analysiswasfixed at 226 x 226 m (51 076 nl2) sincethis
was the scale at which the second derivative was calculat-
ed. Finally,we calculatedthe total number of cellsin the
mountainousand nonmountainousareasand the per-
centageof the studyarea that wasmountainous.
Sincethesurveywascarriedoutfromroads,itwaspos-
sible that undetected nests in areas isolated from roads
might cause an overestimate of the inter-nest distance
andanunderestimateof thedensity.Similarly,thesurvey
wasspreadovera wide areaand it waslikelythat nests
closer to our base iu Stellenbosch could have been more
likely to be found. To determine if thiswasthe case,we
calculated the cumulative sum of deviations from the
runningmean (CUSUM, Lombaard1989,Pepleret al.
1991) for inter-nest distances based on the observations
ordered in increasingdistancefrom a road and alsoon
increasingdistancefrom Stellenbosch.

3. MARCH 2001 ESTIMATION OF BOOTED EAGLES IN SOUTH AFRICA 17
31:00 S
• "•'': '•'""'
• '•(:•',......?'•'•.•:•:?r?•":•'••',-'...•?•*.';•z•'•'*•'•?:•-'•'•.•?•;4.'.... ";
0 200 Kilometers
Figure 2. Breeding distributionof BootedEaglesin southern,Africa.Nestsitesare flaggedand areaswith second
derivatives of the surface function •0.2 are shaded.
31:00 S
ß --..... .-,.• .• , . .::? - :. ....,:•,;•
• ,-•. .• ,.• .' • , •,"":•,i-.,, ,-•e• - ,,'.,-:,'::',:, .?'.•..•2½",•.4,,-
i-:-•/ .....•d•,.. z-.•,.., -'- • ß- -... ,½ - :t •-•. . .....,.. ß
"'•.½- 1.?'"•;,•:-.'2..-%-... f .• "'• .. ,• ' - ß-:..... Ic.
. .•:.•:•',.
0 100 200 Ki10melers
Figure3. •reeding distributionof BootedEaglesin southern.•rica. Nestsitesare [laggedand areasof steepslope
wi• first derivative values •6.5 are shaded.

4. 18 PEPLERET AL. VOL. 35, NO. 1
5O
-50
-100
-150
-200
0 200 400 600 800
Distance from Stellenbosch
F•gure4. Cumulativesumof deviationsfrom the running mean (CUSUM) plot for BootedEagleinter-nestdistances
(km) and distancesfrom Stellenbosch (km).
RESULTS
The breeding distributionof BootedEaglescor-
respondedcloselywith areaswhere the secondde-
rivative of the surface function had values >0.2
(Fig. 2). This differed from areasof steepslopes
where first derivativevalueswere >6.5 (Fig. 3).
The total area of the habitat identified is 61 663
km 2.
CUSUM showeda possiblechangein inter-nest
distance for sites in excess of 200 km from Stellen-
bosch (Fig. 4), sowe basedour calculationsof in-
ter-nest distances on nests at distances <200 km
from Stellenbosch. There was no evidence that
nest detection was based on distance from roads
sinceno changepoint wasevidentin the CUSUM
plot. Therefore, we usedobservationswithin 200
km of Stellenbosch to calculate our estimate of
breeding density.Calculationof this estimatewas
basedon two crucial assumptions.First, that the
area included in our surveywasrepresentativeof
the entire area in terms of the averageinter-nest
distance and, second, that the estimate of inter-
nest distance was accurate. The mean inter-nest
distancewas9.677 km (95% CI = 9.17-10.18) and
the estimateof the total breeding population for
the studyareawas702 pairs(95% CI = 576-879).
DISCUSSION
The resultsof our analysiswere comparableto
the atlasdata of Harrison et al. (1997), especially
with their models of breeding rate basedon sea-
sonalityand breedingin zonesfour and eight.
Our studywascarried out over 25 yr and we as-
sumed that all the recorded nests remained occu-
pied throughout the studyperiod. We made this
assumptionbecausewe have,in a number of cases,
observedthe continuousoccupationof particular
nestingsitesfor periodsrangingfrom1972-99.Be-

5. MARCH 2001 ESTIMATION OF BOOTED EAGLES IN SOUTH AFRICA 19
cause our observations were made from roads, we
made no attempt to samplemanyof the mountain
massifs.Therefore, it was possiblethat densities
within these massifs were lower than we estimated.
Given the very high breeding densityof four
pairsof breedingBootedEaglesin a 3 km2 area
that was recorded by Martin and Martin (1988,
1995) in partsof our studyarea,we consideredour
calculationof the total populationto be a substan-
tial underestimate.This did not imply that sucha
high densityis evenlymaintainedthroughoutthe
entire breeding range, but the severeconstraints
of time and logisticsplacedon a studyof this na-
ture precludesaturationsampling.An exampleof
suchan undersampledareaisthe mountain range
of the Great Escarpmentbetween Beaufort West
(32ø20'S,22ø38'E)and Calvinia(31ø27'S,19ø50'E).
We believethat our datapresentthe firstattempt
at an estimationof an entire breeding population
of Booted Eaglesin Africa. It hasbeen suggested
that this southernbreedingpopulationmay be a
subspeciesof the northern BootedEagle (R. Yosef
andG. Verdoornpers.comm.),but thisneedscon-
firmation. Elsewherein its range only "fairly ap-
proximateestimates"ofbreedingdensityexist(del
Hoyo et al. 1994).With time,weareconfidentthat
additional data will expand our databaseand in-
creasethe accuracyof our breedingdensitycalcu-
lations.
ACKNOWLEDGMENTS
We would like to thank E. Martin and the lateJ. Martin
for their invaluablecontributionsto thisproject.We ac-
knowledgethe supportof the Divisionof ResearchAd-
ministration,Universityof Stellenbosch.We thank B. Op-
perman for technical advice,for plotting distributions
and producing the distributionmaps.
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Received5 December 1998; accepted22 October 2000