The University of Notre Dame
Response of Raptors to Reduction of a Gunnison's Prairie Dog Population by Plague
Author(s): Jack F. Cully, Jr.
Source: American Midland Naturalist, Vol. 125, No. 1 (Jan., 1991), pp. 140-149
Published by: The University of Notre Dame
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Am.Midl. Nat. 125:140-149
Response of Raptors to Reduction of a Gunnison's
Prairie Dog Population by Plague
JACK F. CULLY, JR.'
ofNew Mexico,Albuquerque87131
Museum ofSouthwestBiology,Dept. ofBiology,University
ABSTRACT.-Raptors were counted at approximately2-week intervalsbetween March
and November1985-1987in theMorenoValley,ColfaxCo., New Mexico.Duringthat
thevalleyand sequentially
pestis)sweptthrough
period,an epizooticof plague(Yersinia
25 km2
in threeareasofapproximately
gunnisoni)
killedGunnison'sprairiedogs(Cynomys
overthe
each. Red-tailedhawk (Buteojamaicensis)numbersdid notchangesignificantly
theirnumdeclinedin abundance,
goldeneagles(Aquilachrysaetos)
studyperiod.Although
hawks(B. regalis)
ofprairiedogsin 1987.Ferruginous
withtherecovery
bersdidnotrecover
whereprairiedogswere abundant,but their
were abundantduringautumnmigration
prairie
declineofprairiedogs.Gunnison's
withthepopulation
significantly
declined
numbers
forferruginous
hawksduringtheirmigration
foodresource
dogsappearedtobe an important
theMorenoValley.
through
INTRODUCTION
The importanceof large rodentsand lagomorphsas preyformanyraptorspecies during
and Murphy,
thebreedingseason is well documented(Howard and Wolfe,1976; Woffinden
1977, 1989; Janes, 1985; Bednarz, 1987; Schmutzand Hungle, 1989). There is less information on the diet of raptors during migrationand winter,despite the fact that raptor
mortalityin the nonbreedingseason may be high and may affectpopulationstability(Hall
et al., 1988; but see Schmutzand Fyfe,1987; Cully, 1988).
Between 1985-1987 an epizootic of plague (Yersinia pestis) reduced numbersof Gunnison's prairie dogs (Cynomys gunnisoni) in the Moreno Valley, New Mexico, 105?16'W,
36030'N, (Cully, 1988, 1989). This eventprovidedan opportunityto studythe numerical
relationshipbetweenprairie dogs and migratoryraptors.The plague outbreakconstituted
a serendipitous"natural experiment"in thatthe epizooticprogressedthroughthreephases
fromthe northend of the valley to the south,and decimatedprairie dogs over threelarge
discreteareas. This paper describeschangesin raptorabundance in each area thatoccurred
as the preyresourcedeclinedand then recovered.
If prairie dogs regulatelocal raptorpopulationsin the Moreno Valley, then dependent
raptorpopulationsshoulddecreasein thesame areas ofthevalleywhereprairiedog numbers
declined.Dependentraptorpopulationsshouldthenincreaseas prairiedog numbersrebound
followingtheepidemic.Small changesin prairiedog densitymightnotaffectraptornumbers
if the raptor populations are held below the carryingcapacity of the prey base by other
extrinsicfactors.However, even it the raptorswere below carryingcapacitypriorto plague,
but dependenton prairie dogs, the near total eliminationof prairiedogs by plague should
be followedby statisticallydetectablereductionsin raptor numbers.Response of raptor
numbersto changingprairiedog numberswould not occur if raptornumbersare regulated
by factorsotherthan prairie dog abundance, or if raptor species forageopportunistically
I Presentaddress:VectorBiologyLaboratory,
of
of BiologicalSciences,University
Department
NotreDame, NotreDame, IN 46556
140
1991
CULLY:
RAPTORS AND PRAIRIE DOG
PLAGUE
141
by switchingfromone preyspeciesto anotherdependingon the relativeabundance ofother
preyspecies. If raptorsrespondto reducedprairie dog numbersby movingelsewhere,this
would also be recordedas a local decline in raptorpopulation.
METHODS
Raptor counts.-I conducted43 roadside counts of raptors (excluding turkeyvultures,
Cathartesaura), at intervalsof approximatelytwo weeks fromMarch 1985 to November
1987. Counts were made on morningswith winds less than 20 KPH and usually began
between09:00 and 10:00 MDT. The surveyroutewas the same in all cases, was approximately30 km long and took 2-2.5 hours to complete(Fig. 1). I stoppedat approximately
1-kmintervalsto scan forraptors.On mostmornings,at moststops,no raptorswere counted.
If a raptorappeared betweenregular stops,I pulled offthe road to identifyit and record
its location. I was carefulnot to count a raptor more than once. These counts probably
underestimatedthe total number of raptors presentin the valley, but procedureswere
standardizedto provideaccuraterelativeestimatesofraptordensity.The speciesand location
of each raptorwas noted so that it could be assigned to one of threeareas (north,middle,
south) withinthe valley. Raptors seen in each sectionof the valley were thentabulatedby
species.
-Two studysiteswereestablishedin theMoreno
Studysitesand estimation
ofpreynumbers.
Valley prior to the plague epizooticto monitorprairie dog numbers.The Midlake study
site was establishedadjacent to Eagle Nest Lake in October 1984, and the Val Verde site
was establishedin October 1985 (Fig. 1). In October 1984, I droveor walked throughall
suitable habitat east of Highway 64 to determinethe distributionof prairie dogs in the
valley. I estimatedthat 40% of the grasslandhabitatwas populated by Gunnison's prairie
dogs, and thatthe densityat Midlake was typicalof coloniesgenerallyin 1984, and in the
middleand souththirdsofthevalleyin July1985. Prairiedog densityand colonydistribution
west of Highway 64 appeared to be similarto that east of the highway.
I establishednew studyareas at two additionaltownsto monitorprairiedog population
recoverysubsequentto plague. Vega was establishedin the northin April 1986 following
the epizooticin 1985 (Fig. 1). South Entrancewas establishedin May 1987 followingthe
epizooticin the middle of the valley.
Val Verde, like Midlake, was monitoredbeforethe epizooticand, because some prairie
dogs survived,monitoringcontinuedafterwardsas well. Afterplague passed througheach
area, I searchedthe plague-affectedareas forsurvivingprairie dogs. In addition,in areas
throughwhich the plague had passed, I recordedany prairie dogs observedduringraptor
surveys.When I foundnew colonies I estimatedthe area of the colonyand the numberof
prairie dogs.
At Midlake I staked a 6 ha grid,and at Vega and South Entrance I staked 2 ha grids
to map burrows.I attemptedto trap and markall prairiedogs in the markedareas in order
to obtain estimatesof population size. Prairie dogs were live-trappedand marked with
numberedear-tagsand dye marks in the fur. Population size was estimatedat each area,
each yearby dividingthetotalnumberofindividualscaughtand markedbythearea trapped
and then multiplyingthe estimatedarea of the colonyby the estimateddensity.I did not
mark a grid or map burrowsat Val Verde, but trapped prairiedogs on approximately0.5
ha in 1985 and 1 ha in 1987. The populationof the studysite colonieswas thenused as a
relativeindex of prairiedog populationin the north,middleand southsectionsofthevalley.
Because there were numerous prairie dog colonies in each sectionof the valley prior to
plague, but the study colonies were the largest of a reduced number of colonies in each
142
THE AMERICANMIDLAND NATURALIST
125(l)
.......
.2~~~~~~~.........
~~4~A
~~
u 64
yareno...*-rCr-*
JIc,efl ~
VEA
S
A
A
ML
A
4
p<NM
0.
38
Raptor Survey Sites
KM_____
Raptor Survey Route
Trap Site
-Paved
C
1
2
3
Roads
Dirt Roads
0
LIForestf
FIG. 1.-Map of MorenoValleyshowingthelocationof theraptorcensusroute,and theareas
affected
VV = Val
by plagueeach year.The trapsitesare ML = Midlake,SE = SouthEntrance,
Verde,and Vega. The heavylines (1) and (2) dividethe valleyintothe threesectionsaffected
sequentially
byplague:northin winter1984-1985,middlein summerand autumn1985 and south
duringsummer1986
region afterthe plague, the population declines that occurredas a resultof plague were
greatlyunderestimatedby the trappingcounts.
Analysis.
-Although otherraptorspecies were seen duringcounts,threespecies,the redtailed hawk (Buteoj'amaicensis),ferruginoushawk (B. regalis), and golden eagle (Aquila
were presentin high enough numbersforstatisticalanalysisduringthe period
chrysaetos),
when prairiedogs were active.Numbersofred-tailedhawks,ferruginous
hawks,and golden
eagles were subjectedto a two-wayanalysisof variance(ANOVA), using SPSSPC + (Noofdata to equalize variances(Sokal and Rohlf,
rusis,1988) afterlogarithmictransformation
1981). The treatmenteffectsof theANOVA were area (north,middleand south)and year.
If raptorabundance is a functionof prairiedog abundance, as hypothesized,the ANOVA
should have a significantinteractioneffectbecause the northand south had high prairie
dog densitiesin different
years. To testwhethersignificantinteractionpatternscorrelate
with prairie dog density,hawk densityand prairiedog densitywere testedforcongruency
by use ofa Spearman's rankcorrelationtestusing SYSTAT (Wilkinson,1989). The prairie
dog densitiescalculated fromtrappingdata were not independentbetweencountswithin
years,however,so statisticalsignificanceshould not be attributedto this test.In all other
statisticalanalyses the alpha level was set at 0.05.
1991
CULLY:
143
RAPTORS AND PRAIRIE DOG PLAGUE
TABLE1.-Prairie dogdensity
estimates
basedon thenumberofindividualprairiedogscaptured
at thefourstudysites.The plagueepizootics
occurred
in winter/spring
1985 at Vega,summer/fall
1985 at Midlakeand SouthEntrance,
and summer1986 at Val Verde
Date
Number
trapped
Area
trapped
Oct. 1984
July1985
1.5 ha
6.0 ha
S. Entrance
Sept. 1985
May 1986
Sept. 1987
Oct. 1987
45
168
Val Verde
Oct. 1985
13
Site
Midlake
Vega
25
7
0
19
July 19863
Aug. 19864
Aug. 1987
30
18
4
6.0 ha
6.0 ha
1.0 ha
2.0 ha
0.5 ha
1.0 ha
1.0 ha
2.0 ha
May 1986
Oct. 1986
Oct. 1987
17
69
116
2.0 ha
2.0 ha
2.5 ha
Density
perha
30
28
4.1
1.2
0
Colonyarea' Colonypop.
100 ha
100 ha
9.5
262
30
18
2.0
100 ha
100 ha
0 ha
3 ha
200 ha
200 ha
5 ha
10 ha
8.5
34.5
46.4
2 ha
2 ha
5 ha
3,000
2,800
410
120
0
28
5,200
6,000
90
20
17
69
232
I Estimated fromU.S. Geological Survey Maps
2
In October-November
1985,adultfemalesand mostyoungwerealreadyhibernating.
See text
3July 1986,beforeplague
4September 1986,following
plagueepizootic
RESULTS
Prairie dog populations.
-Based on visual observationsof fur-markedindividuals,more
than 90% of the prairie dogs on the 6-ha study site at Midlake were captured between
October 1984 and July 1985. In October 1984, the prairiedog densityat Midlake was 30/
ha (Table 1). I estimatedthat prairiedog coloniesoccupied a total area of 40-50 kiM2,and
that the total prairie dog population in the Moreno Valley was 100,000-150,000 at that
time.
The Vega studysitewas notyetestablishedin 1984, but theprairiedog densityat colonies
in the northappeared to be comparableto densityat Midlake. I estimatedthatprairiedog
coloniesin the norththirdof the valley covered1000 ha in 1984, fora totalpopulationsize
of 30,000. Plague struckthe prairie dogs in the northernthirdof the valley duringwinter
and spring1985 (Fig. 1), and the prairiedog populationin the northdeclinedat thattime.
A carefulsearchin June 1985 locatedisolatedindividualsand one colonyofabout 20 prairie
dogs thathad disappeared one monthlater. There were scatteredprairiedogs in the north
in late summer,but the population densitywas less than 1 per ha. A small colonyof 17
prairie dogs formedat Vega in spring 1986. The Vega colonygrew during 1986-1987 to
morethan 200 prairiedogs by October 1987 (Table 1). In 1987 therewere also othersmall
colonies establishedin the north.
On 15 July 1985, therewas no noticeabledecline in prairie dog densityin the middle
sectionof the valley. However, by mid-September1985, the populationwas in decline and
I was able to countonly 25 prairie dogs on 6 ha at Midlake. All were gone fromMidlake
by July 1986 (Table 1). By 1 September1986, I was able to locate only 2 prairiedogs in
a carefulsearch of 2 km2adjacent to Eagle Nest Lake. Based on visual searches,the total
THE
144
AMERICAN
MIDLAND
125(1)
NATURALIST
hawksandgoldeneaglesthroughout
ofred-tailed
hawks,ferruginous
TABLE2.-Seasonal abundance
(Summer),and
theentirevalleyandoverall threeyears(1985-1987)on countspriorto 1 September
foreach raptor
are shownseparately
(Autumn).Means and standarddeviations
after1 September
speciesin each area in Figure2. n = thenumberofraptorcounts,x = themeannumberand SD iS
between
thestandarddeviationofhawksper count.F is theresultofANOVA testsfordifferences
seasons
Speciesandseason
n
S
SD
F
P
Red-tailedhawks
Summer
Autumn
30
13
1.90
7.69
1.71
5.19
30.62
<0.0001
30
13
0.43
6.23
0.82
5.26
35.54
<0.0001
30
13
1.47
1.69
1.59
1.87
hawks
Ferruginous
Summer
Autumn
Goldeneagles
Summer
Autumn
0.157
0.694
prairiedog populationin themiddlesectionofthevalleydid notexceed 25 in October 1986.
A small colonyat South Entrance increasedto approximately30 by October 1987.
I estimatedthatmorethan 30,000 prairiedogs were presentat thesouthend ofthevalley
(including Val Verde) in September 1985. In October and November 1985, afteradult
femalesand mostjuvenile prairie dogs were hibernating,I captured 1 juvenile femaleand
12 adult male prairie dogs on 0.5 ha at Val Verde (Table 1). In July 1986, I trapped 30
prairiedogs thereon approximately1 ha, but the proportionofmarkedindividualsin traps
did not exceed 50%, so my estimateof the population prior to plague at the Val Verde
colony was too low (Table 1). This colony was destroyedby plague within the month
followingtheJuly 1986 trappingsession. In August I trappedat an isolatedcolonynearby
and estimatedthe population to consistof 90 prairie dogs. The epizootic spread to that
colonyand throughoutthe southernarea in summer 1986 and by October 1986 the population in the southwas reducedto <100 prairiedogs. The epizooticcontinuedinto 1987,
when 4 prairie dogs were captured on 2 ha at Val Verde (Table 1). In August 1987, I
estimatedthattherewere 25-50 prairie dogs in the south thirdof the valley.
Raptorpopulations.-Red-tailed hawks, ferruginoushawks and golden eagles were all
commonspecies in the Moreno Valley during the threeyears of this study,and all were
observedkillingprairie dogs (Cully, 1988). The red-tailedhawk was the most abundant
species duringthe summerand theirnumbersincreasedduringautumn migration(Table
2). Golden eagles were also presenteach year duringspring,summerand autumn when
prairiedogs were activeabove ground,but goldeneagle numbersdid not change seasonally
(Table 2). Ferruginoushawks were most commonduringautumn migration.They were
irregularand uncommonvisitorsduring springand summer.Prior to autumn migration,
which began in September,the mean numberof all raptorsseen per 30-km countwas 6.8
(n = 30 counts,SD = 4.09). During the September-Octobermigrationperiod,the mean
numberof raptorsper count increasedto 21.1 (n = 13 counts,SD = 7.84).
throughout
During bothsummerand autumnthered-tailedhawkswereevenlydistributed
amongyearswas nearlysignificant(Table
thevalley.The summerANOVA fordifferences
CULLY:
1991
145
RAPTORS AND PRAIRIE DOG PLAGUE
hawksand
and autumnred-tailed
TABLE 3.-Results ofANOVA ofareasand yearsforsummer
hawks
ferruginous
Species/season
Red-tailedhawks
Summern = 30
Autumnn = 13
hawks
Ferruginous
Summern = 30
Autumnn = 13
Sourceofvariance
df
F
P
Overall
89
2
2
4
1.437
1.140
3.044
0.782
0.194
0.325
0.053
0.540
Overall
38
2
2
4
1.761
1.639
1.147
2.129
0.125
0.211
0.331
0.102
Area
Year
Interaction
Area
Year
Interaction
Inadequatenumbersforanalysis.
11.077
38
Overall
Area
Year
Interaction
2
2
4
4.129
11.139
14.520
<0.001
0.026
<0.001
<0.001
3), but that statisticprobablyreflectsthe low numberof red-tailedhawks in the south in
1985 (Fig. 2) when prairiedogs were abundantthere.The interactiontermin theANOVA
(r = -0.102) did not
was not significant,and the Spearman rank correlationcoefficient
indicatecorrelationof red-tailedhawk numberswith prairie dog abundance.
Golden eagles were less commonduringall seasons than red-tailedhawks, and during
autumn they were less commonthan ferruginoushawks. Golden eagle numbersdid not
betweensummerand fall, so seasonal data were lumped foranalysis.
change significantly
They were most commonin the southerntwo-thirdsof the valley duringall 3 years (Fig.
among areas (F = 8.923, n = 129, P <
2). Golden eagle numbersdifferedsignificantly
0.001) and years (F = 5.282, n = 129, P = 0.006), but the interactionwas not significant
(F = 1.204, n = 129, P = 0.313).
in the numbersof ferruginoushawks counted
There were highlysignificantdifferences
in summervs. autumn (Table 2). Prior to autumnmigration,ferruginoushawks were rare
in theMoreno Valley withtwofewdata forstatisticalanalysis.The autumncounts,however,
among years and areas, and the interactiontermwas
showed highlysignificantdifferences
also significantduring autumn (Table 3). During autumn migrationferruginoushawk
numbersin the threesectionsof the valleywere positivelycorrelatedwithestimatedprairie
= 0.658), although significancelevels
dog density(Spearman rank correlationcoefficient
cannotbe inferredas discussedabove.
DISCUSSION
to obtainbecause thesebirdstravelover
Data on foodhabitsof large raptorsare difficult
long distances.Bednarz (1988) followedgroups of radio-taggedHarris' hawks (Parabuteo
uncinctus)duringtwo wintersand observed30 killsin 403 h ofobservation.Callopy (1983b)
observed126 attackson preyby goldeneagles ofwhich 23 were successful.Otherfoodhabit
studiesoflarge raptorshave focusedon preydeliveredto nests(Luttichetal., 1970; Wakely,
1978; Ensign, 1983; Callopy, 1983a; Gilmer and Stewart,1983; Janes, 1985; Steenhoffand
THE
146
AMERICAN
125(1)
NATURALIST
MIDLAND
Golden Eagle
*
North
L
Middle
5
Q) 4 -
South
3
zE)
C:
d 2
Q)
1985
1987
1986
Autumn
Summer
Ferruginous Hawk
10
5
~~~~~~~~~~~~~Q)
4.
z
E
Oj
6
Qj
1985
1986
4
1985
1987
Red-taiiled
1986
1987
1986
1987
Hawk
10
504
Q)~~~~~~~~~~~~~~-
1985
1986
1987
1985
FIG. 2.-Number of raptorsseen on countsduring1985-1987. The histogramsrepresentthe mean
number per count at the North, Middle, and South sectionsof the valley. The verticalbar is 1 SD.
Note that the scale for autumn red-tailedhawks and ferruginoushawks is double that of summer.
Golden eagle data are pooled forspring,summerand autumnbecause therewere no seasonal differences
in numbers
1991
CULLY:
RAPTORS AND PRAIRIE DOG
PLAGUE
147
observations(Cully, 1988). Food habitdata formigratory
Kochert,1988), or on opportunistic
raptorsare virtuallynonexistentbecause of the transientbehaviorof the birds.
to examine
The plague epizooticin the Moreno Valley providedan unusual opportunity
the relationshipbetweenraptorsand prairiedogs because ofthe dramatic,spatiallydefined,
changes in prairie dog density.Prairie dog mortalitydue to plague exceeded 99% (Cully,
in Gunnison'sprairiedogs caused byplague in Colorado
1989), and was similarto mortality
(Lechleitneret al., 1962, 1968; Fitzgerald, 1970; Barnes, 1982; Rayor, 1985). The prey
communitywas well suitedto thisanalysisbecause otherlarge rodentor rabbitpreyspecies
groundsquirrels(Spermophiwere rarein thevalley.Gunnison'sprairiedogs,thirteen-lined
meadow voles (Microtuspennsylvanicus)and deer mice (Peromyscus
lus tridecimlineatus),
maniculatus)were the available prey species in the grasslandwhich was the focusof this
study.Botta's pocketgophers (Thomomysbottae)occurredlocally but were more common
at the edge of the surroundingforestthan in grassland. During fouryears of fieldwork
(approximately1500 h of observation)I saw only threecottontailrabbits(Sylvilagussp.),
this species is also more commonat the forestedge, and one black-tailedjackrabbit(Lepus
in the grassland.Woodrats (Neotomasp.), golden-mantledgroundsquirrels(S.
californicus)
lateralis)and least chipmunks(Eutamiasminimus)were commonin the surroundingforest,
but absent fromthe grassland.
Red-tailed hawks preyedon prairie dogs (Cully, 1988), but there was no statistically
significantrelationshipbetweenred-tailedhawk numbersand spatial or temporalchanges
in prairiedog numbers.Red-tailedhawks used alternatepreyspeciesin the Moreno Valley;
groundsquirrelsonce each. In the valley
I saw themkill meadow voles and thirteen-lined
bottomhabitat,thisspecieswas mostfrequentlyseen associatedwithmesicgrasslandwhere
voles were abundant. Red-tailed hawks were frequentlyseen flyingalong the forestedge,
and theyprobablyalso took preythat occurredthere.Red-tailed hawks have a varied diet
(Luttich et al., 1970; Janes, 1985), a featurethat probablyallowed themto persistin the
Moreno Valley withoutspatial or temporalpopulationchanges despitethe radical fluctuations in the prairie dog populations.
Golden eagles were oftenseen huntingoverprairiedog towns.They were also frequently
observedat theforestedge,particularlyat the southend ofthevalley.Golden eagle numbers
steadilydeclined throughoutthe valley from 1985-1987. Golden eagles probablyforage
hawks so thatas longas preywere available
overlargerareas than red-tailedor ferruginous
somewherein the valley eagles could be foundat theirfavoredperchsitesat the southend
ofthevalley.Golden eagles use large preyspeciessuch as black-tailedjackrabbits,mountain
nuttaiji),prairiedogsand groundsquirrels(Callopy,1983a, b; MacLaren
cottontails
(Sylvilagus
etal., 1988; Cully, 1988). At theSnake RiverBirdsofPreyArea, in Idaho, rabbitsconstituted
more than 90% of prey deliveredto golden eagle nests(Callopy, 1983a), and in Wyoming
62%, prairiedogs 18% and groundsquirrels5% ofpreybiomassdelivered
rabbitscontributed
to golden eagle nests (MacLaren et al., 1988). When theyare available, prairie dogs are
probablyimportantpreyforgoldeneagles in the Moreno Valley. Prairie dogs were missing
fromthe northend of the valley when raptor counts began in 1985, and the low eagle
numbersthen may reflectthe absence of prairie dogs. The steadydisappearanceof golden
eagles from1985-1987 correlateswiththe demiseof prairiedogs thatresultedfromplague.
Althoughtherewas some recoveryof prairie dogs in the northby spring1987, theremay
have been too few prairie dogs at thattimeto supportnestingeagles.
Ferruginoushawks were abundant during migration,were rare at other times of the
year, and did not breed in the Moreno Valley. During autumn,when ferruginoushawks
were abundant,theirspatial distributionwas consistentwith the hypothesisthat the local
148
THE
AMERICAN
MIDLAND
NATURALIST
125(1)
abundance of this species was linked to the availability of prairie dogs during autumn
migrationin the Moreno Valley. Ferruginoushawks primarilytake large prey such as
rabbits(Howard and Wolfe, 1976), groundsquirrels(Ensign, 1983; Gilmer and Stewart,
1983; Wakeley, 1978; Schmutz and Hungle, 1989) and Gunnison's prairie dogs. Unlike
red-tailedhawks and golden eagles, which frequentlyhuntedalong the forestedge, ferruginous hawks were always seen huntingover grassland. During 1984-1985, I saw ferruginous hawks kill prairie dogs six times (Cully, 1988), and I never observedthe species
withotherprey.The patternofferruginous
hawk abundanceduringfallmigrationsuggested
that ferruginoushawks respondedstronglyto the local availabilityof prairiedogs.
-I am indebted
Acknowledgments.
toJamesBednarzwhoconducted
raptorsurveys
duringSeptemI am also grateful
to
berand October1987 and whoprovided
hoursofdiscussion.
manyinteresting
andmanyhelpfulcomments
AnneCully,GaryGrahamandJohnHubbardfortheirsupport
throughto Mr. Les Davis and thelateBill Gallagherand hisfamily
outthisproject.I am especially
grateful
forallowingme to workon theirranches.The manuscript
benefited
fromthecomments
of Daniel
This workwas supported
reviewers.
Witter,
JosefK. Schmutzand anonymous
bya grantfromthe
New Mexico Department
of Game and Fish, Share WithWildlifeProgram,and NIH National
ResearchServiceAwardTrainingGrant2 T32 Al 07030.
LITERATURE
CITED
BARNES,A. M. 1982. Surveillanceand controlof bubonic plague in the United States. Symp.Zool.
Soc. Lond., 50:237-270.
BEDNARZ,J. C. 1987. Successive nestingand autumnal breedingin the Harris' Hawk. Auk, 104:
85-96.
1988. Cooperative huntingin Harris' Hawks (Parabuteouncinctus).Science,239:15251527.
CALLOPY,M. W. 1983a. A comparison of direct observationand collectionsof prey remains in
determiningthe diet of golden eagles. J. Wildl.Manage., 47:360-368.
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SUBMITTED 22 JANUARY 1990
ACCEPTED 3 OCTOBER 1990