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Wilson Bull.,
107(4), 1995,
pp.
615-628
ARE RED-TAILED HAWKS AND
GREAT HORNED OWLS
DIURNAL-NOCTURNAL DIETARY COUNTERPARTS?
CARL D. MARTI’ AND MICHAEL N. KOCHERT*
ABSTRACT.-Red-tailed Hawks
(Buteo jamaicensis)
and Great
Homed Owls
(Bubo vir-
ginianus)
are common
in North America where they occupy a wide range of habitats, often
sympatrically. The
two species are similar in size and have been portrayed as ecological
counterparts, eating the same prey by day and night. We tested the trophic similarity of the
two species by comparing published dietary data from across the United States. Both species
ate primarily mammals and birds, and mean proportions of those two prey types did not
differ significantly between diets of the two raptors. Red-tailed Hawks ate significantly more
reptiles, and Great Homed Owls significantly more invertebrates. Dietary diversity was not
significantly different at the level of prey taxonomic class, and diet overlap between the two
species averaged 91%. At the prey species level, dietary overlap averaged only 50%, and
at that level Red-tailed Hawk dietary diversity was significantly greater than that of Great
Horned Owls. Mean prey mass of Red-tailed Hawks was significantly greater than that of
Great Homed Owls. Populations of the two species in the western United States differed
trophically more than did eastern populations. We conclude that, although the two species
are generalist predators, they take largely different prey species in the same localities re-
sulting in distinctive trophic characteristics.
Received 19 December 1994, accepted 15 May
1995.
Red-tailed Hawks (Buteo jamaicensis) and Great Horned Owls (B&o
virginianus) have been portrayed as ecological counterparts that take the
same kinds of prey by day and night (Bent 1938, Craighead and Craig-
head 1956, Austing 1964, Austing and Holt 1966, Springer and Kirkley
1978). Both are widespread, common raptors in North America (Johns-
gard 1988, 1990), and both occupy a wide range of habitats, often sym-
patrically. They are similar in size (Red-tailed Hawk mean mass = 1126
g; Great Horned Owl mean mass = 1354 g; Dunning 1984), and both
are considered to have generalized diets, i.e., they do not specialize on
specific prey types (Errington et al. 1940, Steenhof and Kochert 1985).
Coexisting species segregate their feeding niches primarily by differ-
ences along three dimensions: the habitat used for foraging, the kind of
food eaten, and the time of day that foraging occurs (Cody 1968, Schoe-
ner 1974, JaksiC 1988). Schoener (1974) considered time of activity to
be the least influential of these niche dimensions, and JaksiC (1982) con-
cluded that time of activity was not adequate to separate niches of hawks
and owls. However, judging from the similarity in body size and habitat
’ Dept. of Zoology, Weber State Univ., Ogden, Utah 84408.2505.
’ Raptor Research and Technical Assistance Center, National Biological Service, Boise, Idaho 83705
615
616
THE WILSON BULLETIN *
Vol.
107, No. 4,
December 1995
usage between Red-tailed Hawks and Great Horned Owls (Hagar 1957,
McInvaille and Keith 1974, Houston 1975, Petersen 1979, Minor et al.
1993) time of foraging activity seems likely to be the most important
factor differentiating the niches of the two.
Our objective was to determine if trophic characteristics of the two
species support the contention that they are dietary counterparts. We test-
ed whether time of activity produced substantial differences in trophic
characteristics between the two raptors.
METHODS
We searched the literature for dietary data with the requirement that the geographic area
and date of data collection potentially enabled both species to exploit the same prey re-
sources. Additionally, we required that each sample contained at least 100 prey items, and
had vertebrate prey identified to genus or species, and most invertebrate prey to order. For
geographic analyses, we considered samples from New York, New Jersey, Connecticut,
Ohio, Wisconsin, and Michigan as representing eastern populations, and samples from Wy-
oming, Utah, Idaho, Montana, Washington, and California as representing western popula-
tions. We calculated the following trophic estimators from the selected data sets: (1) Food-
niche breadth (FNB) as estimated by Levins’ (1968) modification of Simpson’s index (FNB
= lap,Z, where
p, =
the frequency of each prey type in a diet) was calculated at both
coarse and fine resolution. The coarse level (FNB,,), where prey categories were taxonomic
classes, provided an indication of the versatility of the predator, i.e., larger values at this
level indicate that the predator is capable of detecting, capturing, and handling diverse kinds
of prey (Greene and JaksiC 1983). The fine resolution (FNB,,), where prey categories were
species or genera for vertebrate prey and order for invertebrate prey, provided greater dis-
crimination between the two raptors’ diets. (2) Geometric mean prey mass (GMPM; Sokal
and Rohlf 1981, p. 42) was estimated using prey weights in Steenhof (1983).
Overlap between diets of the two raptors was assessed by Pianka’s index (1973): 0 =
~P&KP,%,z)‘~2,
where
p, =
the frequency of a prey type in one of the raptor’s diet and
q,
=
the frequency of the same prey type in the other raptor’s diet. Overlap was also calculated
at coarse and fine resolutions using the same criteria as for FNB. Paired t-tests were used
to compare means of trophic estimators.
RESULTS
The thirteen data sets meeting our selection criteria (Appendix 1) re-
vealed that both Red-tailed Hawks and Great Horned Owls fed largely
on vertebrates in five taxonomic classes, but they also consumed small
numbers of arthropods. Mammals were numerically dominant in all of
the Great Horned Owl diets, and in all but one Red-tailed Hawk diet (Fig.
1; Orians and Kuhlman 1956). Avian prey was second in numeric im-
portance overall for both species (Fig. l), but was exceeded by reptiles
in four studies on the Red-tailed Hawk (Fitch et al. 1946, Knight and
Erickson 1976, Fitzner et al. 1981, Marti et al. 1993b), and by arthropods
in three studies on the Great Horned Owl (Fig. 1; Fitch 1947, Smith and
Marti and Kochert. *
HAWK/OWL DIETARY COUNTERPARTS
617
- Mammals
- Birds
I Reptiles
Arthropods
80 -
70
80
50
40 -
30 -
20 -
10
O-
90
80
70
80
50
40
30
NY OH WI WI MI MI WY UT ID MT WA WA
COLLECTION SITES
CA
Proportions by number of major prey types in diets of Great Horned Owls and
Red-tailed Hawks. Collection sites arc arranged from east to west and correspond to the
order of data sets in Appendix I.
618
THE WILSON BULLETIN
l
Vol.
107, No. 4, December 1995
TABLE
1
SUMMARY AND COMPARISON OF TROPHIC CHARACTERISTICS OF GREAT HORNED OWLS AND
RED-TAILED HAWKS (CALCULATED FROM DATA SOURCES IN APPENDIX I)
Great Horned Owl
Red-tailed Hawk
Trophic characteristic
Mea” SD Meall
SD P
P
Food-niche breadth (class) 1.51
Food-niche breadth (species) 5.44
Geometric mean prey mass, g
76.0
% mammals in diet 79.5
% birds in diet 14.2
% reptiles in diet 0.4
% amphibians in diet 0.1
% fishes in diet 0.5
% arthropods in diet 5.0
Mean overlap in prey class (SD)
Mean overlap in prey species (SD)
0.42 1.78
3.28 6.66
64.7 175.0
15.3
69.4
14.2 17.7
0.8 11.4
0.4 0.2
1.0 0.2
9.3 1.2
0.91 (0.1)
0.50 (0.3)
0.49 1.81 0.10
4.58 2.36 0.04
137.4 3.00 0.01
18.9 1.73 0.11
16.3 0.77 0.46
14.5 2.80 0.02
0.3
0.25 0.81
0.5 1.43 0.18
3.2 2.15 0.05
Murphy 1973, Marti et al. 1993b). Although both raptors occasionally ate
amphibians and fish, neither was important in their diets (Table 1).
The heavy reliance on mammalian prey by both raptors resulted in high
dietary overlap between them at the prey class level (Fig. 2). Dietary
overlap at the prey species level was considerably lower indicating that
the two often ate different mammal species at the same localities (Fig.
2). The most common prey for Red-tailed Hawks typically was a diurnal
mammal while for Great Horned Owls it was a nocturnal mammal
(Table 2).
FNB,, was also quite similar between the two species demonstrating
that they have corresponding capabilities in detecting and capturing prey
at the broad category of taxonomic class (Fig. 3). Mean differences be-
tween FNB,, in paired samples of Red-tailed Hawks and Great Horned
Owls were not significant (Table 1). FNB,, of the Red-tailed Hawk was
larger than the Great Homed Owl FNB,, in eight of the paired samples,
but the converse was true in five. FNB,, of the Red-tailed Hawk was
significantly greater than that of the Great Horned Owl (Table 2). Ten of
the 13 paired FNB,, values were higher for the hawk (Fig. 4), showing
that it usually preyed upon a greater diversity of prey species than did
the owl. Red-tailed Hawks took larger prey on average in 11 of the 13
paired samples (Fig. 5), and the GMPM of the Red-tailed Hawk overall
was significantly larger than that of the Great Horned Owl (Table 1).
In dietary samples from eastern populations, none of the trophic char-
Marti and Kochert.
l
HAWK/OWL DIETARY COUNTERPARTS
619
m Prey Class Resolution
r
@M Prey Species Resolution
100 -
90 -
80 -
70 -
60 -
50
40
30
20
10
0
1
NY
OH WI WI MI
MI WY UT ID MT
COLLECTION SITES
WA WA
CA
FIG.
2. Overlap between diets of Great Horned Owls and Red-tailed Hawks. Col-
lection sites are arranged from east to west and correspond to the order of data sets in
Appendix I.
TABLE 2
MOST COMMON FREY FOR GREAT HORNED OWLS AND RED-TAILED HAWKS (CALCULATED
FROM DATA SOURCES IN APPENDIX I)
Location”
Great Horned Owl Red-tatted Hawk
1
Peromyscus
Tamias
and
Sciurus
2
Microtus
Tamias
3
Peromyscus Spermophilus
4
Peromyscus
Phasianus
5
Peromyscus
Microtus
6
Microtus Microtus
I
Microtus Spermophilus
8
L‘epl.l.5
Lepus
9
Microtus Spermophilus
10
Peromyscus Spermophilus
11
Perognathus Spermophilus
12
Microtus Coluber
13
Neotoma Spermophilus
‘See Appendix I for geographic locatmn and source of data.
THE WILSON BULLETIN
l
Vol. 107, No. 4, December 1995
-
Great Horned Owl
mss Red-tailed Hawk
2.0
1.5
1.0
0.5
0.0
_.-
NY
OH
WI WI MI MI WY UT ID MT WA WA CA
COLLECTION SITES
Frc. 3.
Dietary diversity at the coarse level of prey discrimination (FNB,,) in diets of
Great Horned Owls and Red-tailed Hawks. Collection sites are arranged from east to west
and correspond to the order of data sets in Appendix I.
acteristics we measured was significantly different between the two spe-
cies (Table 3). In the West, though, five of the seven trophic characteristics
did differ significantly. GMPM diverged most with Red-tailed Hawks
taking significantly larger prey. Dietary overlap at the prey species level
also was much less in the West indicating a stronger divergence in the
kinds of prey eaten (Table 3). Despite the geographic variation in inter-
specific differences between Red-tailed Hawks and Great Horned Owls,
intraspecific trophic characteristics were not significantly different, east
versus west, except for birds in diets of Great Horned Owls (Table 4).
DISCUSSION
Red-tailed Hawks and Great Horned Owls appear to use similar habi-
tats, although we do not know of any studies that simultaneously exam-
ined microhabitat use by the two species. Numerous investigators found
them breeding in the same habitat (e.g., Hagar 1957, McInvaille and Keith
1974, Houston 1975, Petersen 1979, Minor et al. 1993). Nests of the two
species averaged only 51 m apart where nest sites were limited and
clumped (Houston 1975), but even in more homogeneous habitats the two
often nested within 200-300 m of each other (Hagar 1957, McInvaille
and Keith 1974, Minor et al. 1993). Great Horned Owls commonly use
Marti and Kochert. *
HAWK/OWL DIETARY COUNTERPARTS
621
20
18
18
m
Great Horned Owl
m
Red-tailed Hawk
14
12
10
8
8
4
2
0
NY
OH
WI WI MI MI WY UT ID MT WA WA CA
COLLECTION SITES
FIG. 4. Dietary diversity at the fine level of prey discrimination (FNB,,) in diets of Great
Horned Owls and Red-tailed Hawks. Collection sites are arranged from east to west and
correspond to the order of data sets in Appendix I.
nests constructed by Red-tailed Hawks (Orians and Kuhlman 1956, Hagar
1957, Houston 1975, Petersen 1979, Minor et al. 1993).
Both of these raptors are dietary generalists and highly opportunistic
predators capable of taking the same prey over a large range in size and
type. They have the potential to have high overlap in diet, and on a
continent-wide basis we found that diets of co-occurring populations did
overlap extensively at the coarse level of prey discrimination (taxonomic
class of prey). At the fine level (prey species), however, their diets on
average overlapped only 50%-a large niche separation. Both species
have been reported to feed on carrion (Sooter 1942, Stalmaster 1980,
Preston and Beane 1993). How this behavior might affect the trophic
parameters we measured cannot be evaluated because the data on it are
limited and mostly anecdotal.
Trophic differences between the two species were much more pro-
nounced in the West than in the East. A previous broad-scale analysis of
the trophic structure of raptor assemblages (Marti et al. 1993a) concluded
that, on a regional basis, both the Red-tailed Hawk and Great Horned
Owl consumed far more species of prey and had broader food-niche
breadths in the western United States than in the central or eastern U.S.
That same pattern held for entire assemblages of many raptor species
622
THE WILSON BULLETIN
l
Vol.
107, No. 4,
December 1995
600
F
m
Great Horned Owl
mm Red-tailed Hawk
500
400
300
200
100
0
NY
OH
WI WI MI MI WY UT ID MT WA WA CA
COLLECTION SITES
FIG. 5. Geometric mean prey mass of Great Horned Owls and Red-tailed Hawks. Col-
lection sites are arranged from east to west and correspond to the order of data sets in
Appendix I.
(Marti et al. 1993a). The species density of both birds and mammals
increases from east to west in North America (Cook 1969, Page1 et al.
1991), and, at least for mammals, the size of geographic ranges decreases
toward the West. These patterns could help explain why raptor food-niche
breadths calculated for large regions should be broader in the West than
in the East. Our present much finer scale analysis found that food-niche
breadth was narrower for both Red-tailed Hawks and Great Horned Owls
in the West compared to the East. A possible explanation is that greater
diversity of available prey in the West may permit local populations of
these two raptors to increase their diet segregation in that region.
JaksiC (1982) believed that time of activity, in general, did not result
in diet differences sufficient to separate the niches of hawks and owls.
Carothers and JaksiC (1984) proposed that interference competition rather
than exploitation competition was the force causing the die1 difference in
activity between hawks and owls. Time of activity, however, does seem
to be the niche dimension that causes the greatest divergence in diets of
Red-tailed Hawks and Great Horned Owls. Our findings show that diets
of the two species at the same locality are similar to each other in most
trophic characteristics, but that these two raptors concentrate their pre-
Marti and Kochert. * HAWK/OWL DIETARY COUNTERPARTS
623
TABLE 3
TROPHIC CHARACTERISTICS OF GREAT HORNED OWLS (GHO) VERSUS RED-TAILED HAWKS
(RTH) IN EASTERN AND WESTERN POPULATIONS (CALCULATED FROM DATA SOURCES IN
APPENDIX I)
Trophic
characterlstlc
Eastern U S. Western U.S.
GHO RTH GHO
RTH
Food-niche breadth (class)
Mean (SD) 1.64 (0.44) 1.62 (0.36)
Paired-r (P) 0.15 (0.89)
Food-niche breadth (species)
Mean (P) 6.30 (4. IO) 7.70 (6.49)
Paired-t (P)
1.32
(0.24)
Geometric mean prey mass, g
Mean (P) 75.2 (23.8) 151.1 (177.0)
Paired-r (P) 1.10 (0.32)
% mammals in diet
Mean (SD) 73.6 (17.3) 71.2 (21.4)
Paired-t (P) 0.25 (0.81)
% birds in diet
Mean (SD) 24.3 (15.5) 24.2 (21.4)
Paired-t (P) 0.01 (0.99)
% reptiles in diet
Mean (SD)
0.1 (0.2) 4.2 (5.8)
Paired-t (P) 1.71 (0.15)
% arthropods in diet
Mean (P) 1.0 (2.0)
0 (0)
Paired-t (P)
- (-)
Mean overlap in prey class
(SD)
0.92 (0.1)
Mean overlap in prey spe-
cies (SD) 0.58 (0.2)
1.41 (0.41)
1.92 (0.57)
2.44 (0.05)
4.70 (2.48)
5.77 (2.22)
2.44 (0.05)
76.1 (88.9) 195.4 (102.9)
5.03 (0.002)
84.6 (12.4) 67.8 (18.4)
2.39 (0.05)
5.6 (3.9)
12.1 (8.4)
2.16 (0.07)
0.7 (1.0)
17.6 (17.1)
2.66 (0.04)
8.5 (11.9) 2.2 (4.2)
2.05 (0.09)
0.90 (0.1)
0.42 (0.3)
*N=
13
dation on a different array of species by being active at different times
of the day.
Other sympatric predators (raptors, mammals, and snakes) that eat the
same species taken by Red-tailed Hawks and Great Horned Owls must
also be considered when attempting to understand and compare the niches
of Red-tailed Hawks and Great Horned Owls. Only four predator assem-
blages in North America containing both Red-tailed Hawks and Great
Horned Owls have been analyzed for such effects (JaksiC 1988, Marti et
Marti and Kochert.
l
HAWK/OWL DIETARY COUNTERPARTS
625
al. 1993b). In two assemblages (Idaho and Wisconsin) the two species
were in different feeding guilds, but in the other two (Michigan and Cal-
ifornia) they were in the same guild (in California they were in different
subgroups within one guild). The small number of such studies does not
provide much insight into what trophic patterns might arise out of inter-
actions among a wider range of predators. But, those analyses may in-
dicate that trophic relationships between Red-tailed Hawks and Great
Horned Owls are also affected by the presence of other predators.
The high overlap in habitat use and prey between Red-tailed Hawks
and Great Horned Owls could certainly lead to exploitation competition.
Competition and its effects, however, have been notoriously hard to quan-
tify in most bird populations (Wiens 1989). We know of only two studies
on competition in raptors that have detected reduced reproductive success
in the presence of a potentially competing species (Nilsson 1984, Kor-
pim%ki 1987). Interference competition between Red-tailed Hawks and
Great Horned Owls may be largely avoided by differences in time of
activity. However, Great Horned Owls begin nesting about one month
earlier than co-existing Red-tailed Hawks (Orians and Kuhlman 1956,
Seidensticker and Reynolds 197 1, McInvaille and Keith 1974, Minor et
al. 1993) and may thus interfere with the hawks’ access to breeding areas
by appropriating nests. The ultimate form of interference competition-
one species killing the other-has been reported between these two spe-
cies, mostly based on circumstantial evidence. The majority of instances
were Great Horned Owls preying on nestling Red-tailed Hawks (Ham-
erstrom and Hamerstrom 1951, Craighead and Craighead 1956, Luttich
et al. 1971, Petersen 1979), but one adult Red-tailed Hawk may have
been killed by a Great Horned Owl (Houston 1975). Red-tailed Hawk
predation on nestling Great Horned Owls was suspected by Fitch (1940)
and Orians and Kuhlman (1956).
Our analysis suggests that time of activity may be the most important
factor that prevents or reduces the degree of competition between these
two species by permiting sympatric populations to prey upon somewhat
different prey arrays. Our results further support the contention that many
raptors are very versatile in diet, and that diets in local areas are, to a
large extent, the result of opportunism.
The answer to the question-are these two species diurnal-nocturnal
dietary counterparts-is strongly affected by the scale used to compare
them. At a coarse level, the two are much alike in habitat use and pred-
atory capability, and could be considered to be day-night equivalents. At
a fine level, dietary differences are much more pronounced. Thus, Red-
tailed Hawks and Great Horned Owls, by being active at different times,
626
THE WILSON BULLETIN *
Vol. 107, No. 4, December 1995
interact with different arrays of prey species, and are not day-night feed-
ing equivalents
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APPENDIX I
SOURCES
AND
CHARACTERISTICS OF DATA SETS USED
FOR ANALYSES
Location Habitat type
Length
of
study
(years)
SGWXl
of data
collection SOWX
1 New York,
New Jersey,
Connecticut
2 Ohio
3 Wisconsin
4 Wisconsin
5 Michigan
6 Michigan
7 Wyoming
8 Utah
9 Idaho
10 Montana
11 Washington
12 Washington
13 California
Deciduous forest
Farmland
Farmland
Farmland
Farmland
Mountain valley”
Shrub-steppe
Shrub-steppe
Mountain valleya
Shrub-steppe
Shrub-steppe
Grassland/chaparral
12
2
Breeding
1
4 Breeding
17 Breeding
3
Breeding
6 Breeding
2 Breeding
3 All year
Breeding
Bosakowski and Smith (1992)
Breeding
All year
Breeding
Winter
Breeding
Springer and Kirkley (1978)
Errington (1932, 1933)
Orians and Kuhlman (1956)
Craighead and Craighead
(1956)
Craighead and Craighead
(1956)
Craighead and Craighead
(1956)
Smith and Murphy (1973)
Marti et al. (1993b)
Seidensticker (1968, 1970)
Fitzner et al. (1981)
Knight and Erickson (1976,
1977)
Fitch et al. (1946), Fitch
(1947)
a Mostly riparian and shrub-steppe with some upland forest and pasture
