j RaptorRes.35 (4) :296-303
¸ 2001 The Raptor Research Foundation, Inc.
NESTING
ECOLOGY
OF BURROWING
BLACK-TAILED
PRAIRIE
IN SOUTHEASTERN
OWLS
OCCUPYING
DOG TOWNS
MONTANA
MARCO RESTANI 1
Fishand Wildli•[b
Program,Department
ofBiology,
MontanaStateUniversity,
Bozeman,
MT 59717 U.S.A.
LARRY R. RAu
Bureauof Land Management,III
GarryowenRoad,Miles'City,MT 59301 U.S.A.
DENNIS
L. FLATH
MontanaFish,Wildlife,and Parks,PO. Box 173220, Bozeman,
MT 59717 U.S.A.
ABSTR^CT.--Detailed
investigations
of the relationshipbetweenBurrowingOMs (Athenecunicularia)and
black-tailedprairie dogs (C}n0my,ludovitianu,)are rare, but suchinformation is necessaryto manage the
population declinesof oMs reported throughout much of the westernUnited States.In 1998 we studied
nest-siteselection,productivity,and food habits of Burrowing Owls breeding on prairie dog townsin
southeasternMontana. We located 13 breeding pairs, sevenof which nested on private land. Nesting
density(1 pair/110 ha) on prairie dog townswaslow comparedto densitiesin other regions.Fewhabitat
characteristicsdifibred between nest sitesand random points, but power in statisticaltestswaslow. Nesting
densityand habitat use suggestedthe population of owlswaswell below carryingcapacity.Productivitywas
2.6 young/pair. OMs fed on iuvertebrates(mainlygrasshoppers
and beetles),mammals(mice and voles),
birds (blackbirdsand buntings),and amphibians(fi•ogs).Plague (Yersinia
pestis),poison,and habitatconversionhavefragmentedprairie dog habitatand potentiallythreaten owl persistencein our studyarea.
KEYWORDS: BurrowingOwl;Athene cunicnlaria; blach-tailed
prairie dom Cynomysludovicianus;plague,,
Yersiniapestis;food habits;habitatselection;
Montana.
Eco16giadel anidamiento de Bahos Cavadoresocupando poblados de perros de la pradera de cola
negra en el sudestede Montana
RtsSUmEN.--Investigaciones
detalladasde la relacitn entre BahosCavadores(Athenecunicularia)
y perros
de la pradera de cola negra (Cynomys
ludovicianus)
son raros, pero tal informacitn es necesariapara
manejar el descensode la poblacitn de bahos reportado en la mayorfa del occidente de los Estados
Unidos.En 1998 nosotrosestudiamos
la seleccitndc sitiosnido, productividad,y habimsalimenticiosde
Bahos Cavadoresreproducifindoseen coloniasde perros de la pradera en el sudestede Montana. Localizamos 13 parejasreproductoras,siete de las cualesanidaban en terrenos privados.I•a densidadde anidamiento (1 parcja/110 ha) en lospobladosde perrosde la praderaam bajaen comparacitna densidades
de otrasregioncs.Pocascaracteristicas
del habitat difErian entre los sitMsnido y puntosal azar,pero el
podcr de las pruebasestadfsticas
rue bajo. La densidadde anidamiemo y el nso de habitat sugiereque la
poblacitn de bdhosestababien por deb•oode la capacidadde carga.La productividadfi•e 2.6 jtvenes/
]):u'tj:t.Losbahosse alimentaronde inverwl)•a(h•s(l n incipalmentesaltmmn•tcs
y t,sc,u'al):tj(•s),
mamffcros
(ratonesy campafioles),aves(mirlos y verdetones),y antibios (ranas). I,a peste ( Yersinia
pe.qi.9, el veneno
y la transformacitndel habitat ha fragmentadoel habitat de los perros dc la pradcray potencialmente
ha puestobajo amen•a la persistenciade los bahos en nuestra area de esmdio.
[Traduccitn de Victor Vanegasy (;•sar Marquez]
The recent finding that the petition to list the
black-tailedprairie dog (Cynomys
ludovicianus)
un• Presentaddress:Department of BiologT,RockyMountain College, Billings, MT 59102 U.S.A.
E-mail address:testahim@rocky.
erin
296
der the U.S. Endangered SpeciesAct is "warranted
but precluded" drawsnational attention to the status and management of a declining speciessubjected to poisoningcampaigns,recreational shooting, and introduced plague (Yersinia pestis).
Decreases of prairie dog populations and their
DECEMBER 2001
BURROWIN(; OWl, ECOI,OGY IN MONTANA
habitat are thought to be responsible for similar
declines of closelyassociatedspecies(Miller et al.
1994, Samsonand ICuopf 1994), most notably the
black-footedi•rret (Mustelanigripes)and mountain
plover (Charadrius montanus). Burrowing Owls
(Athene cunicularia) in the Great Plains south of
Canada also rely on prairie dog habitat (Butts and
Lewis 1982, Plumpton and Lutz 1993a, Desmond
et al. 1995), and many statesreport recent declines
m owl abundance (James and Ethier 1989, Marti
and Marks 1989,Jamesand Espie 1997).
Although Burrowing Owls nest extensivelyin
prairie dog burrows, few studies have reported
habitat characteristicsimportant in nest-site selection or factors influencing owl densitywithin prairie dog towns. In Colorado, differences between
nest burrows and random burrows in surrounding
burrow density,town size,and distanceto road varied from year to year (Plumpton and Lutz 1993a);
however, owls favored areas with lower vegetation
than wasavailableat random on prairie dog towns.
297
rolling to fiat (elevation 680-865 m). Vegetation was
dominated by grasses(Agropyron
smithii) and shrubs (Artemisiatridentataand A. cana). Riparian areas supported
scatteredcottonwood (Populustremuloides)
and willow (Salix spp.), while open standsof ponderosapine (Pinusponderosa)andjuniper (Juniperus
scopulorum)
dominatedhilly
terrain. Climate was semi-arid. The study area was an
even mixture of public (federal and state) and privatc
land that supportedlivestockgrazing.Recreationalshooting of prairie dogsoccurredyear round, but wasconcentrated during spring and early summer.
Beginning in mid-May 1998, we used spotting scopes
(15-45x) and binoculars (10x) to survey prairie dog
townsIbr BurrowingOwls.We made no attempt to search
tbr owlsoff of prairic dog towns.Wc scanncdtownsfrom
a vehicle or on foot, concentrating efibrt in early morn-
ing (0500-1000 H) or late atternoon (1700-2200 H), the
daytime periods when owls are most active and visible
(Haug and Oliphant 1990). Presenceof territorial pairs,
whitewash,castpellets, molted feathers,and prey remains
were used to identify nest burrows. We used a GPS receiver (Carmen XL 12) to plot the locationsof nest burrows on USGS 7.5 min topographic maps. Individual
towns were visited repeatedly (every 2 wk) throughout
the field season(May-August) to minimize the possibility
of overlooking secretive or non-breeding owls and to
monitor nesting success.
larger prairie dog towns, but spaced themselves
At nest burrows (N = 13), we measured elevation
randomly within smaller towns (Desmond et al. (nearest m with a GPS receiver) and percentage slope
1995). Density of prairie dog burrows did not af- with a clinometer. From the nest burrow, we used a tape
In Nebraska, owls nested in loose colonies within
fect spacingpatterns of nesting owls,and a positive
relationship existedbetween town sizeand number
of nesting pairs (Desmond and Savidge1996).
Plague, poisoning, and habitat conversion have
reduced and fragmented prairie dog towns across
the Great Plains, including Montana (Flath and
Clark 1986), but how these processesaffect nestsite selectionand population ecologyof Burrowing
Owls
remains
unknown.
In
1998
we
initiated
a
study in southeasternMontana to elucidate nestsite selection of Burrowing Owls occupyingblacktailed prairie dog towns. We also estimated productivity and quantified food habits.We selecteda
study area previouslymapped for prairie dogs because presence/absenceof Burrowing Owls had
been recorded during visits (R. Richardson, D.
Tribby, K. Wittenhagen, Jr. unpubl. data). Thus,
some data were available to determine the population
trend
of owls.
STUDY AREA AND METHODS
The study area in southeasternMontana (Custer and
Prairie counties;46ø44'N, 105ø38'W) encompassedapproximately400 km2, of which 1425 ha (3.6%) wasoccupied by black-tailedprairie dogs.We surveyedthe prairie dog complex within the Custer and Harris Greek
watersheds,areas being considered for black-footed ferret reintroduction. The badland topography was gently
to measure
distance
to the
nearest
active
and
inactive
prairie dog burrows (+0.05 m), nearestedge of the prairie dog town (+0.5 m), and nearest road (+0.5 m). We
also counted active (presence of fresh diggingsand/or
scat) and inactive prairie dog burrowswithin a 30-m radius of the nest burrow (0.28 ha circle) to index prairie
dog activity (Biggens et al. 1993). Size of prairie dog
towns was obtained from habitat mapping with a GPS
receiver conducted from July-September 1996 (K. Wlttenhagen,Jr. and D. Tribby unpubl. data). We useda GPS
receiver to measure distance (+50 m) from the nest burrow to the nearest neighboring nest burrow.
We
also measured
these
same
habitat
variables
at 13
burrowsselectedhaphazardlyfrom prairie dog townsnot
occupied by nesting owls.We selectedburrows by dividing randomly selectedprairie dog townsinto progressively smaller quadrants bisected by the cardinal directions
(numbered 1-4, chosen using a random numbers table).
The number of quadrants required to narrow down to a
single potential nest burrow depended upon the size of
the prairie dog town. We picked only thoseburrowswith
openings large enough tbr nesting owls.
Terminology describing reproductive successand productivityof Burrowing Owls tbllowed Steenhof (1987).
successthlpairs fledged at least one young, and productivity estimatesincluded both successfuland thiled pairs
We assumedevery owl pair attempted to breed (i.e., laid
eggs). Multiple visits (10-20) to individual nest sites
throughout the breeding seasonpermitted accurate determination of nesting success(young fledged per pmr).
We estimated nesting chronology from age of emerged
youngbasedon plumage (Priest1997), assumingan in-
298
BIOLOGY
u) 35
•
0
25
•
20
•-
15
•
lO
•
5
VOL. 35, NO. 4
We log transformed data prior to analyses(SPSS1998)
to achieve normal distributions (Kolmogorov-Smirnov
test) and homogeneityof variances(Levene's test). However,we have presenteduntransformed data (x ñ SE) in
this paper to facilitate interpretation. Becauseof the relatively small number of nesting pairs (N = 13) and concerns of low statisticalpower,we opted to reduce Type II
errors by assigningstatisticalsignificanceat P <- 0.10
when comparinghabitat variablesbetweenoccupiedand
• Occupied
•
• Unoccupied
_
random
sites.
RESULTS
z
Prairie dog towns on our study area averaged
19.5 _+ 3.6 ha (range : 0.4-198.3 ha, N = 73).
Most occupied prairie dog habitat surveyedwason
Size of prairie dog town (ha)
privateland (65%), followedby federal (30%) and
F•gure 1. Size (ha) of black-tailedprairie dog townsoc- state (5%) lands. Prairie dog townsaveraged11.0
cupied and unoccupiedby BurrowingOwlsin southeast_+1.9 ha on privatelands (N = 30) and 17.3 _+5.3
ern Montana, 1998.
ha on public lands (N = 19; t = 0.64, df = 47, P
o
0-9
30-39
60-69
90-99
120-129
150-159
180-189
: 0.53).
cubation period (first egg to first hatch) of 30 d and
fledging at 40 d (Haug et al. 1993).
We collectedpelletsand prey remainsopportunistically
from May-August while visiting nest sitesand surroundmg perching and feeding areas. Entomologistsat Montana State University, Bozeman, used a dissectingscope
(6.4-10X) to sort and identify invertebrate remains to
family, and relied on museum specimensand Borror et
al. (1989) tbr classificationto genus. To savespace,we
have presented invertebrate taxa to only the family level.
We checkedremainsof vertebrateprey againstpellet contents collectedduring subsequentvisitsto the samenests
to minimize duplication.We useda dissectingscope(730X) to identify vertebrate prey, and relied on museum
specimensand Hoffman and Pattie (1968) for classificauon to species.Number of both invertebrate and vertebrate prey were determined conservativelyby presence
of diagnosticbody parts (e.g., legs,mandibles,skulls).We
calculated percentagebiomassusing estimatesfrom Mart• (1974), Rodriguez-Estrella(1997), and museumspecimens.
BurrowingOwlsnestedon 12 of 73 (16%) prairie dog townsthat we surveyedin 1998. We found
13 breeding pairs of Burrowing Owls on ca. 1425
ha (1 pair/110 ha) of prairie dog townswithin the
400 km2 studyarea. No singleadult owlswere observed. Size of prairie dog townsdid not diffkr between towns occupied by owls and townsunoccu-
pied by owls(t = 1.24, df = 71, P = 0.22; Fig. 1).
Burrowing Owls neither prefkrred nor avoided
nesting on prairie dog towns subjected to recreational shooting (X• = 0.00, df = 1, P = 1.00) or to
grazing (X2 = 0.16, df = 1, P = 0.69). Sevenpairs
nested on private land, with three pairs each on
f•deral
and
Most
state land.
habitat
characteristics
did not diffbr
fbr oc-
cupied Burrowing Owl nest sites and random
points (Table 1). Occupied nestswere closerto ac-
Table 1. Itabitat characteristics(x ñ SE) of Burrowing Owl nest sites(N = 13) and random sites(N = 13) on blacktailed prairie dog townsin southeasternMontana, 1998.
OCCUI'II'5I)
HABITAT
VARIABLE
RANI)()M
SITE
SITE
t
P
Elevation (m)
749 ñ 51
752 ñ 58
0.15
0.88
Percentageslope
Nearest active burrow (m)
2.6 ñ 0.5
14.6 -+ 7.1
2.3 ñ 0.4
21.8 ñ 6.4
0.27
1.81
6.7 ñ 1.9
0.79
0.08
0.50
7.7 ñ 2.8
0.68
Number
Nearest inactive burrow (m)
of active burrows
ll ñ 2
9 ñ 2
0.74
0.47
Number
of inactive
32 ñ 3
30 ñ 3
0.56
0.58
burrows
D•stanceto town edge (m)
Town size (ha)
Nearest road (m)
Nearestneighbor (kin)
111 ñ 36
73 ñ 17
0.85
0.40
27.3 ñ 10.1
227 ñ 98
25.4 ñ 7.1
280 ñ 110
0.96
1.29
0.35
0.21
2.2 ñ 0.5
3.3 -+ 0.7
1.28
0.21
DECEMBER 2001
BURROWING OWL ECOLOGY IN MONTANA
Table 2. Prey of Burrowing Owls based on remains
fbund at nest and perch siteson black-tailedprairie dog
townsin southeasternMontana, 1998. Preyare expressed
m number of items (N), percentagefrequency,and percentage biomass. Unidentified items were not included
m
biomass
estimates.
Invertebrates
were
identified
Table
2.
299
Continued.
to
TAXON
N
family, vertebratesto genus or species.
PER-
PER-
CENT
CENT
FRE-
BIO-
QUENCY
MASS
Aves
PER- PERCENT CENT
FRE- BIOTAXON
N
QUENCY
MASS
<1
<1
Chilopoda
Scolopendromorpha
Arachnida
Scorpiones
Araneae
<1
<1
<1
<1
Non-insectarthropod
<1
<1
Insecta
Sturnella
neglecta
Calamospiza
melanocorys
Undetermined
18
22
1
1
2
<1
25
11
Mammalia
Spermophilus
richardsonii
6
• 1
18
Perognathusfasciatus
4
<1
<1
Peromyscus
spp.
Onychomys
leucogaster
36
2
3
<1
11
1
Microtus
spp.
Zapus
hudsonius
12
2
1
<1
8
<1
16
1
Unknown rodents
Total
1202
100
100
Odonata
Family undetermined
1
<1
311
4
26
<1
9
<1
tive prairie dog burrowsthan were random points.
Neither number of activeprairie dog burrows nor
total number of burrows (inactive + active) cor-
8
<1
<1
relatedwith town size (P > 0.30, N = 26). Statis-
3
<1
<1
with low and nonsignificant
P-values
(i.e., nearest
1
<1
<1
roadandnearest
neighbor).
337
28
3
Orthoptera
Acrididae
Gryllacrididae
Gryllidae
Hemiptera
Belostomatidae
Reduviidae
tical power was 0.35 for each of the two contrasts
Goleoptera
Garabidae
Silphidae
Hydrophilidae
Histeridae
Scarabaeidae
Elateridae
Tenebrionidae
Meloidae
Gerambycidae
Ghrysomelidae
Gurculionidae
Burrowing Owls produced 2.6 + 0.4 young/pair
72
3
<1
I
<1
6 <1
produced
atleast
onefledgling.
Onepairfailed
for
<1
11
<1
<1
<1
<1
<1
5
<1
7
<1
3
<1
unknown reasons. Productivity did not correlate
with number of active or inactive prairie dog burrows (P > 0.30, N = 13) within a 30-m radius of
the nest. Productivity also did not correlate with
sizeof prairie dog towns(P = 0.57, N = 13). Owls
nesting on prairie dog towns subjected to recrea-
<1
tional shooting(N -- 6) had productivitysimilarto
12
1
<1
thosenestingpairsnot exposed
to shooting(N =
1
<1
<1
tively;t = 0.65,df = 1, P = 0.54).Bybackdating
1
<1
<1
fromyoungof knownage(N = 7 nests),
weesti-
1
<1
<1
127
1
81
1
42
<1
Diptera
Asilidae
Muscoidea
Lepidoptera
Sphingidae
Hymenoptera
Sphecidae
Eumenidae
Formicidae
Undetermined Hymenoptera
7) (2.3 young/pair versus2.9 young/pair, respec-
mated a • layingdate of 20 May (+_1 d), • hatching
dateof 19June(_+1d), and• fledging
dateof 29
July (_ 1 d).
7
4
16
<1
<1
<1
<1
1
2
Amphibia
liana pipiens
Scaphiopus
bombifrons
(• + SE,N = 13pairs).Twelve
pairs(92%)each
We identified 1053 invertebrateand 149 verte-
bratepreyremains
(Table
2).Themost
common
invertebrate prey were grasshoppers (Orthopotera) and beetles(Goleoptera).Amphibianpreyincluded northern leopard frogs (Rana pipiens)and
28
2
10
plainsspadefoot
(Scaphiopus
bombifivns).
Onlytwo
2
<1
<1
species
of birdsweretaken:LarkBunting(Cala-
300
BIOLOGY
mospiza melanocorys)and Western Meadowlark
(Sturnellaneglecta).
Mice (Peromyscus
spp.) and voles
(Microtusspp.) were the most common mammalian
prey. Most important prey items in terms of biomass were meadowlarks (25%), mice + voles
(20%), and Richardson's ground squirrels ($permophilusrichardsonii;
18%).
VOL. 35, NO. 4
cupied townsin Nebraska (Desmond et al. 1995).
Burrow densities of prairie dog towns in southeastern Montana and Colorado (Plumpton and
Lutz 1993a) were similar,but were 3X higher than
in Nebraska (Desmond et al. 1995). Therefore,
prairie dog towns occupied by nesting owls in
southeasternMontana were relativelysmall and active, habitat conditions that should have minimized
DISCUSSION
the probability of badger predation. Badger pre-
Burrowing Owl Use of Prairie Dog Towns. Two
observationssuggestedthe Burrowing Owl population waswell below carrying capacityof nesting
habitat on black-tailedprairie dog townswithin our
study area. First, density of Burrowing Owls (1
pair/110 ha) was low compared to densities in
Oklahoma (1 pair/0.19 ha, Butts 1971), and /
nearest-neighbordistance on our study area (2.2
kin) greatly exceeded that in Nebraska (0.11-0.13
kin, Desmond and Savidge1996). In fact, only one
prairie dog town supported more than one pair of
dation of owls did not occur during our study,supporting this hypothesis.
Historically,Burrowing Owl occupancyof prairie
dog townson our studyarea washighestin 197879 (27%, 15 of 55 towns;C. Knowlespers.comm.),
random
Lewis 1982, Desmond et al. 2000).
intermediate in 1991 (14%, nine of 66 towns; R.
Richardsonand D. Tribby unpubl. data), and lowest in 1996 (4%, three of 73 towns;K. Wittenhagen, Jr. and D. Tribby unpubl. data). We recorded
an increaseto 16% (12 of 73 towns)occupancyin
1998. Fluctuating population size of Burrowing
owls. Second, the habitat characteristics we meaOwls over the past 20 yr may have reflected the
sured did not differ between nest sites and random
impact of plague. Plague was first confirmed on
points. Prairie dog townsunoccupiedby owlswere our studyarea in 1986, and by the late 1980shad
vacant apparently for reasonsother than habitat significantly reduced prairie dog populations in
suitability,perhaps indicating an owl population in southeasternMontana (cJ. Knowlesunpubl. data).
decline (Schmutz 1997). However, conclusions re- Owl occupancyshouldlag behind fluctuatingprairie
garding habitat suitability remain preliminary be- dog populations(whichit did) if townsdecimatedby
causesomecomparisonslacked adequatestatistical plague providenestinghabitat for 3-4 yr before inpower to detect differences between occupied and activeburrowscollapseor fill in with soil (Buttsand
sites.
The only habitat attribute that appeared to differ between nestsand random points wasdistance
to the nearest burrow occupied by prairie dogs,
which waslessfor nest sites.Why owlsnestednear
active prairie dog burrows remains unknown, but
two previously noted patterns imply an anti-predator benefit. Owls nesting in areas of highest burrow densityin Nebraska suffered lessbadger (Tax•dea taxus) predation than did other nesting owls
(Desmond et al. 2000). Badgerpredation on blacktailed prairie dogs also correlated positively with
town sizein Wyoming (Campbell and Clark 1981).
Density of prairie dog burrows did not correlate
with town size in both Wyoming and southeastern
Montana. The relationships between badger predation and (1) burrow densityand (2) town size
imply that highest predation of owls should occur
on large townswith low burrow density,assuming
badger predation on owls occurs under the same
conditionsas predation on prairie dogs.
Prairie dog towns occupied by Burrowing Owls
111 southeastern
Montana
were
half
the size of oc-
In this study area, rodents and birds composed
most of the Burrowing Owl diet by percent biomass,whereas insectsdominated percent frequency. Owls nesting on prairie dog townsin Colorado
and Wyoming exhibited similar prey use (Marti
1974, Thompson and Anderson 1988, Plumpton
and Lutz 1993b). Use of prey varied seasonally,as
mammalian prey were most important to owlsearly
in the nesting period before insectsbecame available (Marti 1974, Green and Anthony 1989,
Schmutz et al. 1991). Owls appeared to forage for
mammals mostlyat night and concentratedon insectsduring daylight.In Saskatchewan
home range
size decreasedsignificantlyonce insectsbecame
abundant (Haug and Oliphant 1990).
Management Implications. Productivity and juvenile and adult survivorshipact in concert to determine the trend of Burrowing Owl populations
(James et al. 1997, Johnson 1997, Clayton and
Schmutz 1999). Some of the mechanisms that af-
fect demographyincluded habitat availability,predation, and food availability.Productivityand pop-
DECEMBER 2001
BURROWING OWL ECOI,OGY IN MONTANA
301
ulation size of Burrowing Owls in southeastern
Montana during 1998 was low and we did not estimate survivorship.Populations in neighboring
Saskatchewanand Alberta with similar or higher
productivity showed significant annual decreases
by owls during the nesting season (e.g., mice and
voles,meadowlarks,grasshoppers,
and beetles).
Finally,managementto benefit Burrowing Owls
should consider historically-basednegative attitudes toward prairie dogs because nesting owls
over the past decade (Hjertaas 1997, Wellicome were evenlydistributedacrossboth public and pri1997b). Our preliminary resultssuggestedthe owl vate lands. Many stateagricultural agencies,includpopulation in Montana may have increasedwithin ing Montana's, continue to consider prairie dogs
the past 5 yr as prairie dogs rebounded from "vertebrate pests" requiring systematic"suppresplague epizootics.However, future monitoring is sion" (Sections 7-22-2207 [6] and 80-7-1101 Monwarranted because productivity and density were tana Code Annotated). The acrimonious debate
both low, and becausesignificantowl declinescon- betweenagricultural and conservationinterestsimpedeseffectivewildlife managementßConservation
tinue nearby in Canada.
Management of Burrowing Owls in southeastern of prairie dog habitat can only proceed through
Montana must consider population ecology and partnershipsbetween private citizens and governhabitat selectionof black-tailedprairie dogs.Man- ment (Samson and Knopf 1994, Holroyd et al.
aging plague is the greatest challenge to prairie 2001). To address both economic and conservadog conservation,and has similar potential to chal- tion concerns, the Montana Prairie Dog Working
lenge managementof Burrowing Owlsin the Great Group is developingand implementing a statewide
Plains. Plague moved through prairie dog townsin conservationplan for black-tailedprairie dogs.Insoutheastern Montana during the mid-1980s (G. centivesto maintain prairie dog habitat on private
Knowles unpubl. data), and reduced prairie dog lands are an important component of the plan, as
populations to a level where plans to reintroduce is the goal to maintain viable populations of associated species,such as the Burrowing Owl.
black-footed
ferrets were halted. Whether
size or
distribution of prairie dog townsinfluencesepizootic severityor movement of plague remains unknown. However, because Burrowing Owls select
the best,not the largest,remaininghabitatpatches
(Buttsand Lewis 1982, Warnock andJames1997),
plague may severely reduce owl populations in
Montana.
Burrowing Owls did not avoid nestingon prairie
dog townssubjectedto recreational shooting,and
productivity of pairs nesting on or off shooting areas was similar. Although owls have been shot in
other areas (Butts 1973), we found no evidence of
shooting mortality in our area. Nonetheless, recreational shooting may have disrupted daytime for-
aging by adults and thus produced subtlenegative
effects.For example, owlsfed extensivelyon diurnal prey (e.g., birds and grasshoppers)when the
food
demand
of owl broods
in southeastern
Mon-
ACKNOWLEDGMENTS
We thank the following ranchers for graciouslygranting accessto private lands: A. Haughian, M. Haughian,
P. Haughian, Q. Haughian, T. Haughian, and D. Reed
C. Stuart and M. Ivie identified invertebrate prey remains, andJ. Rozdilskykindly loaned mammal specimens
from the Burke Museum, University of Washington, Seattle. G. Holroyd, B. Olenick, J. Sidle, and T. Wellicome
provided helpful commentson the manuscript.The U.S.
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