j RaptorRes'.36(1):58-65
¸ 2002 The Raptor ResearchFoundation, Inc.
TROPHIC
NICHE
OF NORTH
AMERICAN
GREAT
HORNED
OWLS
LEE A. CROMRICH,1 DENVE}•W. HOLT, AND SHAWNEM. LE^SUP.
E
Owl ResearchInstitute, P.O. Box 39, Charlo, MT 59824
ABSTRACT.--The
trophic niche of Great Horned Owls (Bubovirginianus)wassummarizedusing22 North
American studiesreporting >100 prey items each. Twenty-oneof these studieswere reviewedfrom the
published literature, and one, our Montana data, is presented here for the first time. More than 92
speciesfrom four taxonomicclasses
havebeen recordedfrom 19278 prey items.Mammalsconstituted
>93.3% of preyfrom all studies,with six studiesreporting 100% mammalianprey.Food-nichebreadth
rangedfrom 2.09-19.15 (• = 5.17) for combinedstudies,2.12-19.15 (• = 6.29) for breedingseasons,
and 2.09-4.72 (• = 3.50) for non-breedingseasons.Evennessvaluesranged from 0.408-0.840 (• =
0.620) for combinedstudies,0.420-0.703(• = 0.596)for breedingseasons,
and 0.408-0.724(• = 0.609)
for non-breeding seasons.Estimatedmassesof individual prey speciesranged between 2 and 6300 g.
Birds were only a minor part of the owl diet, although a variety of specieswere eaten.
KEYWOP,
DS: diet,food-niche
breadth;GreatHornedOwl;Bubo virginianus;NorthAmerica.
Nicho
trofico
del Gran
Buho
Cornado
Americano
RESUMEN.--E1
nicho tr6fico de (Bubovirginianus)fue compendiadousando22 estudiosnorte americanos
reportando >100 items presa cada uno. Veintiuno de esosestudiosfueron revisadosen la literatura
publicada,y uno, nuestrosdatos de Montana, se presentan aqui pot primera vez. M•ts de 92 especies
de cuatroclasestaxon6micashan sidoregistradas
a partit de 19278 itemspresa.Los mamiferoscons-
tituyeron>93.3% de presasen todoslos estudios,con seisestudiosreportando100% de presasmamiferas.La amplituddel nichoalimenticioestuvieron
en el rangode 2.09-19.15(• = 5.17) paraestudios
combinados,2.12-19.15 (• = 6.29) para estaciones
reproductivas,
y 2.09-4.72 (• = 3.50) para temporadas no reproductivas.La masa estimadade especiespresa individualmente estuvoentre 2 y 6300 gr.
Las avesfueron tan solo una parte menor de la dieta del bfiho, aunque una variedadde especiesfueron
consumidas.
[Traducci6n de Cfsar Mftrquez]
The Great Horned Owl (Bubovirginianus)is perhaps the most widely-distributed owl in North
America and it has been considered to be an opportunistic feeder. Indeed, the Great Horned Owl
has becn reported to have the broadest diet of any
North American owl species (Marti and Kocheft
sons between a few Neotropical and Nearctic localities, but compared owl diets from only two
regions in North America. Our paper summarizes
the trophic niche of Great Horned Owls from 22
North American studies;21 from published literature, and one, our original Montana data.
Our objectives were to: (1) determine Great
Horned Owl trophic niche from west-centralMontana and (2) compare trophic niche among North
1996, Houston et al. 1998). However, the owl's tro-
American
America (Houston et al. 1998, Holt et al. 1999).
Numerous
Horned
studies
Owls
have
of
the
been
food
habits
conducted
of
in
Great
North
phic niche has not been reviewed continent-wide.
Earhart andJohnson(1970) summarizedprincipal
food habits of Great Horned Owls from published
literature, but did not identify prey to the species
level, provide prey numbers, or discusstheir conclusions.Jaksic and Marti (1984) made compari1 E-mail
address: øwlmøntana@charlø'net
58
studies.
METHODS
In Montana, we collected pellets and prey remains annually from 10 territories in the Missoula and Mission
valleysduring the breeding and non-breeding seasons
from 1987-95. Prey was identified using local dichotomous keysfor mammals (Hoffmann and Pattie 1968) and
by comparing feather and body parts of prey with museum specimensat the Philip L. Wright Zoological Museum (Universityof Montana). Numbersand proportions
MARCH 2002
GREAT HORNED OWL TROPHIC NICHE
of prey typeswere then compared betweenbreeding and
non-breeding seasonsfor Montana. Comparisonsof trophic niche were then made among other availableNorth
American
data
tributed. However; we found no method to determine
the statisticalsignificancebetweennarrow and wide foodniche breadth. We compared owl trophic niche from 22
North American studieswith >100 prey items each (Tables 1-3). We then divided these studiesinto breeding
seasonand non-breeding seasondiets. To compare trophic niche among studies,we primarily used prey identified to the species.Prey identified to genus were included if they occurred frequently or exhibited an
unusual body mass.Insects,arachnids, and unidentified
reptiles, birds, and mammals were eliminated from trophic niche comparisonsbecause they were either not
•dentified to genusor occurred only rarely (<1%) in the
diet.
Food-nichebreadth (H') wascalculatedfor each study
using the antilog of the Shannon-Weinerdiversityindex
(Marti 1987). We used this equation becauseit is linearly
related to the number of prey categoriesin the sample.
Evennesswas calculated using Alatalo's (1981) modification of Hill's (1973) equation: Evenness= (N2 - 1)/
(Nt - 1), where N• = exp H' and N,• = 1/]57.Evenness
valuesrange from zero to one. An evennessvalue of one
•ndicatesprey proportionsin the diet are equal.We compared food-niche breadth and evennessvaluesfrom all
studiesaswell as thosefrom breeding and non-breeding
seasonsusing the Mann-Whitney U-test (Fowler and Gohen 1990).
Spearmanrank correlation (Fowler and Cohen 1990)
wasused to examine the relationship between the number of mammalian speciesand number of prey itemswith
food-niche breadth values among studies.We did so to
if wider
food-niche
breadth
values were asso-
ciated with increased numbers of prey or speciesin the
diet. The Spearman rank correlation was also used to
examine the relationship between number of prey items
and food-niche
Table 1. The number of individual prey consumedby
Great Horned Owls during breeding and non-breeding
seasons in Montana
from
1987-95.
sets.
We defined trophic niche as the relationshipbetween
owlsand their prey. We followed Marti's (1987) definitions for trophic diversity in which a broad food-niche
breadth (FNB) has a high number of prey species,which
are nearly equally distributed, and a narrow food-niche
breadth has a low number of prey speciesunequally dis-
determine
59
breadth
because
food-niche
breadth
val-
ues can fluctuate with sample size, thus influencing the
results.
A relative-sizecategoryof the main prey classeseaten
by the owls was derived using body massestimatesof
mammals (Whitaker 1992) and birds (Dunning 1984).
Standard mean prey biomassestimateswere not calculated based on speciesbecauseof unfounding factors. For
example, standard prey biomassestimatesare usuallyderived from the adult age classand do not considerother
age classes
in the population.Further,mean preybiomass
estimatesgenerally give whole carcassmassesand do not
consider that only specificportions of some medium to
large prey are eaten (Holt 1993, 1994).
No.
BREEDING
SPECIES
SEASON
OF PREY
NON-BREEDING
SEASON
Mammals
Microtuspennsylvanicus
1264
902
Microtus montanus
1158
470
Microtusspp.
Peromyscus
maniculatus
Thomomys
talpoides
72
37
54
159
52
39
Ondatra
13
14
zibethicus
Mustelafrenata
Sylvilagusnuttallii
Tamias amoenus
1
2
1
--
2
--
--
Tamiasciurus hudsonicus
Glaucomys
sabrinus
1
--
1
Sturnusvulgaris
30
4
Phasianus
12
4
Birds
Fulica
colchicus
americana
Picapica
Turdus migratorius
Colaptesauratus
Xanthocephalus
xanthocephalus
Anas platyrhynchos
Anas spp.
Bombycilla
spp.
Sturnellaneglecta
Asio otus
Porzana
Railus
Bonasa
carolina
limicola
umbellus
Agelaiusphoeniceus
6
1
7
1
3
2
12
2
1
1
I
--
1
--
1
--
1
--
1
•
--
1
-2
--•
1
Other
Catostomus
spp.
crayfish
squawfish
Total
4
6
2
2696
-1
-1654 = 4350
non-breeding season. Although collectively the
owls ate a wide variety of prey (N = 28), they ate
predominately small mammals, particularly voles
(Table 1).
During the breeding season,the owlsconsumed
28 speciesof prey. Of these however,they ate predominately small mammals, especially Microtus
RESULTS
voles(92.5%, N = 2494). During the non-breeding
The Montana study yielded 4350 prey items: season,the owls ate only 16 speciesof prey, again
2696 from the breeding seasonand 1654 from the consumingpredominatelyMicrotusvoles(92.5%, N
60
CROMRICHET AL.
VOL. 36, NO. 1
Table2. Landscape
dietsof GreatHornedOwlsin NorthAmerica.
Percentof preyin taxonomic
classes
calculated
from 22 studies,representing19278 preyitems.
PERCENT
NO. OF PP,E¾ OSTEICHTHYES
Breeding season
2696
1896
1300
398
356
276
209
142
119
CRUSTACEA
AVES
MAMMALIA
LOCATION
SOURCE
0.2
---
0.2
---
3.09
-1.4
6.6
100.0
98.6
MT
OR
ID
This study
Maser & Brodie 1966
Marti & Kochert 1996
-0.8
3.3
----
1.5
---9.2
--
2.8
2.2
31.5
-20.4
30.3
95.7
96.9
65.2
100.0
70.3
69.7
UT
WY
NY, NJ, CT
MB
MI
OH
Smith & Murphy 1973
Craighead& Craighead1969
Bosakowski& Smith 1992
Bird 1929
Craighead& Craighead1969
Springer& Kirkley1978
-0.1
---
1.4
1.0
98.6
98.9
MI
MT
1.9
95.6
MT
--
0.3
2.4
99.7
97.6
CA
IN
Craighead& Craighead1969
This study
Seidensticker 1968
Rudolph 1978
---
---
100.0
100.0
NE
YT
Rickart 1972
Weir & Hanson 1989
95.2
98.1
98.1
WI
CO
WA
Errington 1932
Marti 1974
Irafight& Jackman 1984
Non-breeding season
1845
1654
---
756
2.5
584
210
---
161
122
---
Breeding and non-breedingseasons
2571
-0.1
2152
--809
0.1
-568
--
--
273
178
---
0.7
--
4.4
1.7
1.7
Kirkpatrick& Conway1947
--
100.0
CA
Barrows 1989
21.2
--
78.0
100.0
WI
OK
Orians & Kuhlman 1956
Tyler &Jensen 1981
= 1531). The decreasedprey speciesdiversitydur- speciesoverall,Microtus(N = 10 studies),Peromys(N = 2), Sig'rnodon
(N =
ing the non-breedingseasonreflected the fewer cus(N = 6), Perog'nathus
speciesof preyavailableduringthe fall andwinter 1), and Lepus(N = 1) speciesrepresentedthe
months in Montana.
highestpercentageof preyin all studies.Overall,
The 22 studiescombinedyielded 19 278 prey food-nichebreadth valuesrangedfrom 2.09-19.15
items (Table 2) from eightwestern,sixcentral,and (i -- 5.17, SD + 3.61) (Table 3). Food-niche
three eastern states,and two Canadian provinces.
Studiesfrom New York and Pennsylvania(Latham
1950), and Alberta (Rusch et al. 1972, McInvaille
and Keith 1974, Adamcik et al. 1978) were also re-
breadth values for the breeding season(range =
viewed but omitted from trophic calculationsbecausedominant prey specieswere not alwaysidentified to genusor species.
The owlsconsumed>--92prey speciesfrom four
taxonomic classes:Osteichthyes,Crustacea,Aves,
and Mammalia (Table 2). Mammals composed
-->65.2%of the prey from each study,constituting
93.3% of the total prey from all studies.Six studies
reported 100% mammalianprey (Table 2).
Althoughthe owlspreyedon a broadnumberof
were not significantlydifferent (Mann-WhitneyU
2.12-19.15, / = 6.29, SD ___5.39, N = 9) and non-
breedingseason(range = 2.09-4.72,i = 3.50,SD
_+0.82, N = 7) were similar. Food-niche breadths
= 24.5, P > 0.05) between seasons.
The broadest food-niche breadth (FNB-- 19.15)
wasfrom New York, New Jersey,and Connecticut,
where15 mammalspecies
constituted
65.2%of the
diet, with Peromyscus
representing14.3% (Bosakowski and Smith 1992) (Table 3). Fourteen other
mammal, 20 bird, and two fish speciescomprised
the remainder. The broad FNB in this study,com-
pared to other studies,may be explainedby the
MARCH 2002
GREAT HORNED
OWL TROPHIC NICHE
61
Table 3. Trophic parameterscalculatedfrom twenty-twostudiesrepresenting19 278 prey items.
FOOD-NICHE
NO. OF PREY
BREADTH
Breeding season
2696
EVENNESS
LOC•kTION
SOURCE
3.27
0.644
MT
This study
1896
2.12
0.465
OR
Maser & Brodie
1300
8.12
0.686
ID
Marti
0.420
0.566
0.670
UT
WY
NY, NJ, CT
Smith & Murphy 1973
Craighead & Craighead 1969
Bosakowski& Smith 1992
1966
& Kocheft
1996
398
356
276
4.47
2.85
19.15
209
2.94
0.687
MB
Bird 1929
142
119
4.55
9.10
0.527
0.703
MI
OH
Craighead& Craighead1969
Springer& Kirkley 1978
Non-breedingseason
1845
2.94
1654
3.43
0.669
0.649
MI
MT
Craighead & Craighead 1969
This study
756
3.91
0.622
MT
Seidensticker
584
210
2.09
4.72
0.724
0.631
CA
IN
Rudolph 1978
Kirkpatrick& Conway1947
161
3.84
3.56
0.558
0.408
NE
Rickart 1972
YT
Weir & Hanson
WI
Errington 1932
122
Breeding and non-breeding season
2571
4.89
0.629
2152
1968
1989
6.36
0.605
CO
Marti 1974
809
5.27
0.602
WA
Kifight & Jackman1984
568
3.89
0.840
CA
Barrows
273
6.98
0.604
WI
Orians & Kuhlman
178
5.87
0.720
OK
Tyler & Jensen1981
1989
1956
large number of bird, as well as mammal, species cantly different (Mann-Whitney U = 30, P > 0.05)
included in the diet, or the relativelysmall sample between seasons.
size (N = 276).
A weak positive correlation existed between the
Food-niche
breadth
calculated
from
Ohio
number of mammalian species in the diet and
(Springerand Kirkley 1978) wasalsobroad (9.10)
compared to other studies(Table 3). In this study,
six mammalspeciesconstituted69.7% of the owl's
diet with Microtusrepresenting26.1%. Sixmammal
and 12 bird speciesrepresented the remainder of
the diet.
The
narrowest
FNBs
came
from
Califor-
nia (2.09), Oregon (2.12), Wyoming (2.85) and
Manitoba (2.94), respectively(Table 3). In all these
cases, small mammals dominated the diet (Bird
1929, Maser and Brodie 1966, Craighead and
Craighead1969, Rudolph 1978).
Evennessvaluesoverall ranged from 0.408-0.840
(• = 0.620, SD _+ 0.101). Evenness values for the
food-niche breadth values (r s = 0.299, P • 0.01).
A weak negativerelationship occurred between the
number of prey items and food-niche breadth val-
ues (r• = -0.207, P > 0.01), suggestingthat sample sizeswere not influencing the results.
Mammal prey biomassranged from 2 g, (masked
shrew [Sorexcinereus])
to 6300 g, (stripedskunk [Mephitismephitis]
) (Whitaker1992).The majorityof prey
ranged from 2-1800 g and included shrews,voles,
mice,rats,pocketgophers,squirrels,and rabbits.The
dominant prey from each study,Microtus,Peromyscus,
Perognathus,
Sigmodon,
and Lepusrangedin bodymass
breeding (range = 0.420-0.703, /= 0.596, SD ___ from 16-85 g, 10-43 g, 16-47 g, 80-120 g, and 18003600 g, respectively(Whitaker 1992). Other medium0.105, N = 9) and non-breeding season(range =
0.408-0.724, • = 0.609, SD + 0.102, N = 7) were sized mammals, including yellow-bellied marmot
also similar (Table 3). Evennesswas not signifi- (Marmotaflaviventris)and white-tailedjackrabbit (Le-
62
CROMRICH ET AL.
pustownsendii)
rarelyoccurredin the diet and ranged
from 2200-4500 g.
Birds were not a major part of the owl'sdiet, but
a wide variety of specieswere eaten. Waterfowl,
shorebirds, pheasants and allies, and passerines
represented the majority of bird prey. Severalowl
specieswere also reported as prey in nine studies:
Northern Saw-whetOwl (Aegoliusacadicus) (Bosakowski and Smith 1992), Long-eared Owl (Asio
otus) (Marti 1976, Holt this study), Barn Owl (Tyro
alba) (Knight and Jackman 1984), and Eastern
Screech-Owl (Otus asio) (Errington 1932, Orians
and Kuhlman 1956, Craighead and Craighead
1969, Bosakowskiand Smith 1992). Body massesof
avianprey ranged from: 318-1100 g, waterfowl;74415 g, shorebirds;178-1317 g, pheasantsand allies;
88-580 g, owls;and 29-458 g, passetines(Dunning
1984). Passetines constituted most of the avian
prey.
DISCUSSION
Great Horned Owls are generally considered to
be opportunistic feeders, preying on a broader
range of speciesthan any other North American
owl (Craighead and Craighead 1969, Voous 1988,
Marti and Kocheft 1996, Houston et al. 1998). Bo-
sakowskiand Smith (1992) reported such unusual
speciesas a raccoon (Procyonlotor),opossum (Didelphisvirginiana), and a Red-shouldered Hawk
(Buteolineatus);Marti (1974) reported a yellow-bellied marmot and black-tailedprairie dog (Cynomys
ludovicianus);
Errington (1932) reported a striped
skunk; and Rudolph (1978) reported a Brazilian
VOL. 36, NO. 1
(Glaucidium brasilianum) (Proudfoot 1997) and
Northern Pygmy-Owl (Glaucidiumgnoma) (Holt
and Leroux 1996, Holt and Petersen 2000). More
specializedspeciesinclude the SnowyOwl (Nyctea
scandiaca) (Watson 1957, Parmelee 1992), Shorteared Owl (Asiofiammeaus) (Holt 1993, Holt and
Leasure 1993), Northern Saw-whet Owl (Holt et al.
1991, Cannings 1993), and Barn Owl (Marti 1989,
1992).
Other Bubospecieshave a trophic niche similar
to the
Great
Horned
Owl.
Herrera
and
Hiraldo
(1976) reported FNB values ranging from 2.406.68 (• = 4.13, SD _+0.01) for the EurasianEagleOwl (Bubobubo)in Europe.Jaksicand Marti (1984)
found that Great Horned Owls and EurasianEagleOwls followed a similar trophic pattern in North
American and European shrubland. Don•zar et al.
(1989), however, reported limited dietary convergence between these two species.They attributed
discrepanciesin trophic diversityto the differences
betweensimilar North American and Europeanbiomes, variations in the composition and abundance of prey types, and differences in the body
massesof Great Horned Owls and EurasianEagleOwls.
The moderate trophic niche of Great Horned
Owls could be the result of several factors, includ-
ing prey speciessize, diversity,density,availability,
and distribution. Marti (1974) suggestedthat although owlscan capture a broad range of prey sizes,an optimum sizeexistsin terms of how efficiently a particular individual prey item can be found
and caught. He argued that very smallprey is only
free-tailed bat (Tadarida brasiliensis).Llinas-Gutierefficient fbr Great Horned Owlsif it can be caught
fez et al. (1991) reported a wide variety of arach- quickly and easily.Marti felt that prey density,ease
nids, insects,and reptiles in the owl diet, and Roh- of killing, overlap of time of activitybetween prednet and Doyle (1992) reported a Great Horned ator and prey, and learning by individual owls all
Owl feeding on an adult Northern Goshawk (Ac- determine what proportionsof the diet a particular
czpitergentilis).
prey specieswill comprise.
The Great Horned
Owl's moderate
food-niche
The moderate trophic niche (high number of
prey species unequally distributed [see Methods breadth may also reflect the habitat or time of day
section]) of the Great Horned Owl reported here- in which they fbrage. Open areas the Great
in somewhat contrastswith previous studies (see Horned Owl inhabits are frequented by mice,
text). Excluding predominately insectivorousowl voles, lagomorphs, and gophers, which may
species,the Great Horned Owl's moderate food- emerge during the owl's optimal feeding periods
niche breadth (opportunistic feeding) aligns it of evening, night, and early morning (Maser et al.
with speciessuch as the Burrowing Owl (Speotyto 1970). The communitystructureof predatorswithcunicularia)(Haug et al. 1993), Spotted Owl (Strix in a particular habitat may also affect food-niche
occidentalis)(Gutierrez et al. 1995), and Eastern breadth. Marti et al. (1993) found that although
Screech-Owl (Gehlbach 1995), for example. Spe- predators in an area may utilize prey resourcesin
cies apparently more opportunistic than Great different fashions, patterns of resource use do
Horned Owls include the Ferruginous Pygmy-Owl emerge, particularly in terms of predator size.
MARCH 2002
GREAT HORNED OWL TROPHIC NICHE
63
Owls during a snowshoehare cycle. Can.Field-Nat.92
The diet of Great Horned Owls may vary de156-166.
pending upon the particular region the owls inAtATALO, R.V. 1981. Problems in the measurement of
habit (Marti et al. 1993). Hayward et al. (1993)
evennessin ecology.Oikos37:199-204.
found that coastalGreat Horned Owls in WashingBARROWS,
C.W. 1989. Diets of five speciesof desert owls
ton fed exclusivelyon birds during the summer
West. Birds 20:1-10.
months. Bosakowskiet al. (1989) reported oMs liv- BIRD, R.D. 1929. The Great Horned Owl in Manitoba.
ing in the deciduous forests of New Jersey,New
Can. Field-Nat. 43:79-83.
York, and Connecticut preyed more heavilyupon BOSAKOWSY4,
T., R. SPEISER,AND D.C. SMITH. 1989. Nestbirds than those living in open coniferousforests
ing ecology of forest-dwellingGreat Horned Owls,
of the western
United
States. Desert
owls fed on a
Bubovirginianus,in the eastern deciduous forest bi-
ome. Can. Field-Nat. 103:65-69.
variety of arachnids, insects,and reptiles because
--AND
D.C. SMITH. 1992. Comparative diets of symof their availability and abundance in that biome
patric
nesting
raptors in the eastern deciduousforest
(Jaksic and Marti 1984, Barrows 1989, Llinas-Gu-
tierfez et al. 1991).
Jaksicand Marti (1984) found that the diversity
of Great Horned Owl prey at the classlevel was
very low in the temperateregionsand very high in
the desert regions of North America. They believed this difference reflected the greater representation of mammals in the diet of temperate
owls,thus resulting in a moderate trophic niche.
Great Horned Owls may respond opportunistically to the local profile of prey sizesand densities
(Jaksic and Delibes 1987, Jaksic 1988). LlinaGutierrez et al. (1991) suggestedthat lagomorphs
and rodents were the dominant prey species in
their study compared with other desert studiesin
the region becauseof their high abundance.Rusch
et al. (1972) reported that the diet of Great
Horned Owls was stronglyaffected by changesin
the numbers of snowshoehare (Lepusamericanus).
They found that in yearswith high snowshoehare
populations,owlsexhibited higher predation rates
on snowshoehare and lower predation rates on
mice
and voles.
biome. Can.J. Zool.70:984-992.
CANNINGS,R.J. 1993. Northern Saw-whetOwl (Aegohus
acadicus). In A. Poole and F. Gill [EDS.], The birds of
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CRAIGHEAD,
J.J. AND F.C. CRAIGHEAD.1969. Hawks, owls
and wildlife. Dover Publications, New York, NY U.S A.
DONAZAR,
J.A., F. HIRALDO,M. DELIBES,ANDR.R. ESTRELLA. 1989. Comparative food habits of the Eagle Owl
Bubobuboand the Great Horned Owl Bubovirginianus
in six Palearctic
and Nearcftc
biomes. Ornis Scand. 20:
298-306.
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American
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Assoc. Mono.
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1
EARHART,C.M. AND N.K. JOHNSON.1970. Size dimorphism and food habits of North American owls. Condor 72:251-264.
ERRINGTON,P.k. 1932. Food habits of southern Wisconsin
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FOWLER,
J. AND L. COHEN. 1990. Practical statisticsfor
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The data herein support the general conclusion
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breadth.
ACKNOWLEDGMENTS
We thank the Flathead Indian
DC
Reservation, Montana
U.S.A.
HAUG, E.A., B.A. MILLSAP, AND M.S. MARTELL. 1993. Bur-
Department of Fish, Wildlife and Parks, United States
Fish and Wildlife Service, and many private landowners
rowing Owl ($)beotyto
cunicularia).In A. Poole and F
for access to lands. Marc Bechard, Carl Marti, and Fabian
Academy of Natural Sciences,Philadelphia, PA and
The American Ornithologists' Union, Washington,
Jaksicmade thoughtful commentson the manuscript.
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Received27 October 2000; accepted15 September2001