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Are red-tailed hawks and great horned owls diurnal-nocturnal dietary counterparts?

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-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
Content may be subject to copyright.
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 Simpsons 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 Piankas index (1973): 0 =
~P&KP,%,z)~2,
where
p, =
the frequency of a prey type in one of the raptors diet and
q,
=
the frequency of the same prey type in the other raptors 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
Lepl.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
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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
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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
... According to Schoener (1974), coexisting species segregate themselves primarily by selecting different foraging habitat, activity time, and prey species. Studies on competitive interactions among ecologically similar and evolutionarily related owls have reported habitat displacement (Marks and Marti 1984, Capizzi and Luiselli 1988, Gerhardt et al. 1994, Ganey and Block 2005, Gutiérrez et al. 2007, Riegert et al. 2009, Bolboaca et al. 2013, Kajtoch et al. 2015 and temporal shift (Schoener 1974, Korpimäki 1987, Marti and Kochert 1995, Leveau et al. 2004, Scott et al. 2005, Figueroa et al. 2009). Temporal segregation is considered an important axis of food-niche partitioning, because it can reduce competition by allowing access to prey active at different times (Marti and Kochert 1995, Leveau et al. 2004, Riegert et al. 2009. ...
... Studies on competitive interactions among ecologically similar and evolutionarily related owls have reported habitat displacement (Marks and Marti 1984, Capizzi and Luiselli 1988, Gerhardt et al. 1994, Ganey and Block 2005, Gutiérrez et al. 2007, Riegert et al. 2009, Bolboaca et al. 2013, Kajtoch et al. 2015 and temporal shift (Schoener 1974, Korpimäki 1987, Marti and Kochert 1995, Leveau et al. 2004, Scott et al. 2005, Figueroa et al. 2009). Temporal segregation is considered an important axis of food-niche partitioning, because it can reduce competition by allowing access to prey active at different times (Marti and Kochert 1995, Leveau et al. 2004, Riegert et al. 2009. Jaksić (1982) tested the validity of this theory by predicting higher dietary overlap among synchronous (active at the same time) species than among asynchronous (active at different times) species and found that activity time did not adequately explain niche separation. ...
... At the class level, prey categories were taxonomic classes that provided information on versatility of the owls. For the mammal level comparison, prey categories were the mammal species or genera, to provide a finer comparison of the mammalian portion of the diets of the three owl species (Marti and Kochert 1995). For biomass calculations, we multiplied the number of prey items of each prey species by the mean mass of the prey species. ...
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The Forest Owlet (Heteroglaux blewitti), Spotted Owlet (Athene brama) and Jungle Owlet (Glaucidium radiatum) are small owls coexisting in the East Kalibhit Forests in the Khandwa District of Madhya Pradesh State, India. We compared their diets to better understand their food habits, dietary overlap, and the mechanism of their coexistence. The diet of the three species differed significantly. The diet of the Forest Owlet was dominated by small mammals (52.2%, by frequency), whereas invertebrates dominated the diets of the Spotted Owlet (58.1%) and Jungle Owlet (84.8%). The Forest Owlet consumed a wider array of prey (food-niche breadth [FNB] = 0.40) compared to the Spotted Owlet (FNB = 0.13) and the Jungle Owlet (FNB = 0.06). The dietary overlap was highest between the Forest Owlet and Spotted Owlet (56%), lower between the Spotted and Jungle Owlet (28%), and lowest between the Forest and Jungle Owlet (22%). Diets also differed significantly among the three owls in terms of the mean prey mass of the species consumed. The Forest Owlet is a generalist predator feeding on various sizes and types of prey; the Spotted Owlet feeds on large and medium-sized mammals, as well as invertebrates, and the Jungle Owlet is a specialist feeding mainly on invertebrates and small mammals. Our results suggest the three species coexist by consuming prey of different sizes.
... These raptors include Great Horned Owl (Bubo virginianus) Barred Owl (Strix varia), Eastern Screech-Owl (Otus asio), Red-tailed Hawk (Buteo jamaicensis), Red-shouldered Hawk (Buteo lineatus), and American Kestrel (Falco sparverius). The species of these two groups are often referred to as ''ecological equivalents'' (perhaps erroneously) due to similarities in anatomy, habitat use, hunting style, and general prey types (Table 1) (Bent 1937(Bent , 1938Voous 1988; but see Marti and Kochert 1995). What might account for the coexistence of these wide-ranging raptors? ...
... Both the group and species differences between grip force and hunting behavior of owls and hawks suggest at least a partial basis for resource partitioning in the eastern deciduous forests. The differences also provide evidence that the similarly sized raptors are not simple ''ecological equivalents,'' in agreement with the conclusions of Marti and Kochert (1995). Each raptor studied has a unique force production that would allow for a degree of prey specialization. ...
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Prey capture in owls and hawks is largely dependent on the biomechanics of the hindlimbs, and both limb size and grip forces potentially determine the size of prey that can be captured and the extent of possible resource partitioning among sympatric species. Morphological study of six species of sympatric raptors—the owls Otus asio, Strix varia, and Bubo virginianus; and the hawks commonly considered their diurnal “ecological equivalents,” Falco sparverius, Buteo lineatus, and Buteo jamaicensis—revealed that, in both groups, talon closure is effected by two discrete mechanisms that function together in a potentially additive or alternative fashion. Grip force measurements obtained from live owls and hawks using “hydraulic” perches showed that grip force increases exponentially with body size and that owls produce greater forces than hawks. That finding is consistent with the distinctive osteology and myology of their hindlimbs and with their hunting behavior. These data provide some understanding of the different demands of diurnal and nocturnal hunting as well as the mechanism of coexistence for those six species in eastern woodlands.
... Several factors may play a role in causing movement during the nocturnal period. Many animals including bobwhite are depredated by both diel and nocturnal predators (Brennan et al., 2020;Marti & Kochert, 1995), which could cause some individuals to engage in nocturnal movements in response to perceived threats. Further, as bobwhites roost in coveys during the night, disturbances near coveys can result in nocturnal movements (Brennan et al., 2020). ...
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... Hawk and owl species, particularly red-tailed hawks and great horned owls, had the highest prevalence of AR, which is not surprising given the rodent-heavy prey diet of both these generalist predators (Marti and Kochert 1995). Previous studies have reported higher hepatic SGAR concentrations in red-tailed hawks than great horned owls in New Jersey and lower hepatic SGAR concentrations in red-tailed hawks than great horned owls in Ontario (Stansley et al. 2014;Thomas et al. 2011). ...
Article
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Anticoagulant rodenticides (ARs) are used globally to control rodent pest infestations in both urban and agricultural settings. It is well documented that non-target wildlife, including predatory birds, are at risk for secondary anticoagulant exposure and toxicosis through the prey they consume. However, there have been no large-scale studies of AR exposure in raptors in Ontario, Canada since new Health Canada legislation was implemented in 2013 in an attempt to limit exposure in non-target wildlife. Our objective was to measure levels of ARs in wild raptors in southern Ontario to assess their exposure. We collected liver samples from 133 raptors representing 17 species submitted to the Canadian Wildlife Health Cooperative (CWHC) in Ontario, Canada, between 2017 and 2019. Liquid chromatography-tandem mass spectrometry (LC–MS/MS) was used to quantitatively assess the level of exposure to 14 first- and second-generation ARs. Detectable levels of one or more ARs were found in 82 of 133 (62%) tested raptors, representing 12 species. The most commonly detected ARs were bromadiolone (54/133), difethialone (40/133), and brodifacoum (33/133). Of AR-positive birds, 34/82 (42%) contained residues of multiple (> 1) anticoagulant compounds. Our results indicate that AR exposure is common in raptors living in southern Ontario, Canada. Our finding that brodifacoum, difethialone, and bromadiolone were observed alone or in combination with one another in the majority of our sampled raptors indicates that legislative changes in Canada may not be protecting non-target wildlife as intended.
... Red-tailed Hawks and Great-horned Owls overlapped in nest placement along the urban density spectrum, as expected (Minor et al. 1993;Marti and Kochert 1995;. Competition for nest sites between these two species occurred throughout the incubation period resulting in Greathorned Owls overtaking nests from Red-tailed Hawks on multiple occasions. ...
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Chapter
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Chapter
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Food niches of the Long-eared Owl Asio otus and the Tawny Owl Strix aluco were compared. The niche of the Tawny Owl was about three times wider than that of the Long-eared Owl. Both species, however, utilized about the same number of prey categories. When in the same area, the Tawny Owl took more water voles, birds, and amphibians and less field voles than did the Long-eared Owl. Food overlap between the two species was lower for neighbouring than for non-neighbouring owls. This strongly indicates that the species compete for food. Mean prey weights for the two owl species were positively correlated in different seasons, and were either equal for both owls, or higher for the Tawny Owl, during every season of the year. Reproductive output of Long-eared Owls increased with their distance from Tawny Owls' nests. This pattern is suggested to be caused by food competition. The opposite was found for the reproductive output of Tawny Owls. This seems to be the opposite of the prediction if competition is occurring, but could be caused by the fact that the Long-eared Owl is more likely to settle in a Tawny Owl territory in a year when this contains relatively abundant prey, than in a year when it does not. Therefore, an effect on the Tawny Owl would not be possible to demonstrate in this analysis. However, the competition seems to be asymmetrical.
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I have presented an analysis of the species density of the breeding birds of continental Canada, the United States (excluding Hawaii) and Mexico following the method of Simpson (1964) in his analysis of North American mammals; seabirds were not analysed. A map of species density was constructed using a 153 mile square-quadrant grid system super-imposed upon the range data of each species. Many of the same trends that Simpson found for mammals—latitudinal gradient, topographic effect, east-west gradient, peninsula effect—were also displayed in the avian species density pattern. There appeared to be a greater decrease in species density in southeastern United States in birds, and it is suggested that this may reflect historical factors in the form of repeated glaciation. A family density map and a species/family ratio map were also constructed, and shown to reflect trends similar to the species density map. Species density maps were constructed for the seven largest families of birds; these were the Anatidae, Accipitridae, Scolopacidae, Trochilidae, Tyrannidae, Parulidae, and Fringillidae. The characteristic pattern of each family is discussed in relation to historical factors and present ecological conditions.