Download Food-niche Relationships Between Great Horned Owls and

January1984]
ShortCommunications
new buffer systems,or better staining regimes may
reveal the expression of enzymes not seen in our
study. Rather, the total scorefor each tissue over all
the taxa we examined
is more
informative.
than
blood
but
less informative
than
formedin the Cornell Laboratoryfor Ecologicaland
Evolutionary Genetics.
LITERATURE CITED
There-
fore, a quantitative comparisonwas made by assigning a ! to all tissueswith faint expressionand a 2 to
those with good expressionand then totalling these
numbersfor all enzymes in each tissue(see bottom
of Table !). Again, feather pulp was more informative
175
internal
organs.
ARISE, J. C., J. c. PATTON, & C. F. AQUADRO. !980.
Evolutionary geneticsof birds: comparativemolecular
evolution
in
New
World
warblers
and
rodents. J. Hered. 7!: 303-3!0.
BAKER,
M. C. !981. A musclebiopsy procedurefor
use in electrophoreticstudiesof birds. Auk 98:
392-393.
There are only two serious limitations to using
feather pulp for electrophoreticanalysis. First, the
birds must either be molting when sampled or be
subjectedto plucking to stimulate feather regrowth
and then recapturedat a later date. Second,in small
birds such as Bobolinks, warblers (Parulidae), or
chickadees(Parusspp.), only tiny amounts of tissue
can be obtained from the flight and tail feathers,necessitatingthe useof two or more feathers.Mengden
and Stock(!976) point out, however, that small quantities of tissuemay be put into cell culture to increase
the amount available; such tissue cultures may also
be stored for indefinite periodsof time. Alternatively, other formsof electrophoresisthat requiresmaller
quantitiesof tissue(e.g. celluloseacetate)may be performed when only a few loci need to be examined.
Especiallyusefulto field workers,the tissueis conveniently "pre-packaged,"and may be placed, whole
or with the upper shaft removed, directly onto dry
ice or into a portable liquid nitrogen flask. In terms
BARROWCLOUGH,
G. F., & K. W. CORBIN. !978.
netic
variation
lidae.
Auk
and differentiation
Ge-
in the Paru-
95: 691-702.
HARRIS, I-I., & D. A. HOPKINSON. 1976. Handbook
of
enzyme electrophoresisin human genetics.New
York, American
Elsevier.
MAY, g., J. E. WRIGHT, & M. STONEKING. !979. Joint
segregation of biochemical loci in Salmonidae:
resultsfrom experimentswith Salvelinus
and re-
view of the literature on other species.J. Fish.
Res. Board Canada
36: !!14-!126.
MINGDEN,G. A., & D. STOCK.!976. A preliminary
report on the application of current cytological
techniquesto sexing birds. Intern. Zool. Yearbook !6: 136-14!.
RYTTMAN,
I-I., & I-I. TELGELSTROM.
!981. Low degree
of isozyme variation within and between Herring Gull (Larusargentatus),
LesserBlack-backed
Gull (Larusfuscus)
and their Britishand Swedish
subspecies.Hereditas 94: !61-!64.
of easeof samplecollectionand minimizationof stress
SHERMAN,
I•. W. !98!. Electrophoresis
and avian geto birds, we suggestthat feather pulp is a viable alneological analysis.Auk 98: 419-422.
ternative tissuefor non-destructivesampling in aviS•BLE¾,
C.G. !96!. The electrophoreticpatternsof
an electrophoreticstudies.
avian egg-white proteins as taxonomic characWe are grateful to StevenBloom,Irene Brown, Mike
ters. Ibis 102: 2!5-284.
Denison, Tom Gavin, JoshHamilton, and Jerry Wald~ SMITH, J. K., & E.G. ZIMMERMAN. !976. Biochemical
vogel for their help in obtaining the birds. Scott
geneticsand evolution of North American blackCamazine and Paul Sherman made many helpful
birds,family Icteridae.Comp.Biochem.Physiol.
commentson earlier drafts of the manuscript.Fund53B: 319-324.
ing was provided in part by a Sigma Xi Grant-in-Aid
of Researchto J. E. Marsden. This work was per- Received14 February1983, accepted25 July 1983.
Food-nicheRelationshipsBetweenGreat Horned Owls and Common Barn-Owlsin
EasternWashington
RICHARDL. KNIGHTl AND RONALDE. JACKMAN
2
•Department
of WildlifeEcology,
University
of Wisconsin,
Madison,Wisconsin
53706USA,and
2681749thNE, Seattle,Washington
98115USA
GreatHorned Owls (Bubovirginianus)
and Common ships from our analysisof 622 Common Barn-Owl
Barn-Owls(Tyto alba)have recentlybecomesympat- pellets,from 6 nestingand 2 roostingsites,and 234
ric in the PacificNorthwest(Stewart!980, Smithand Great Horned Owl pellets, from 4 nesting and 3
Knight !98!) and provide an opportunity for exam- roostingsites,collectedbetween October 1977-June
ining resourcepartitioningbetweentwo membersc•f .- 1979•in EsquatzelCoulee, Franklin County, Wasl•-._
the samefeedingguild. We quantifieddiet relation- ington.The studyareaandits raptorpopulations
are
176
ShortCommunications
described in Knight and Smith (1982). Fitch (1947),
Rudolph (1978), and Jaksi• and Y/tfiez (1980) have
examinedfood-nicherelationshipsbetweenthesetwo
specieswhen sympatricin California and Chile, providing the opportunity for comparisons.
Prey remains in pellets were identified to species
level, and mean fresh weights were obtained from
the Bureauof Land Management (1979). The follow-
[Auk,Vol. 101
(Kritzman 1977: 107) is temporally unavailable for
the strictly nocturnal Common Barn-Owl (Marti 1974,
Rudolph 1978). Common Barn-Owls are the fourth
most important prey item, in terms of total prey biomass, in the diet of Great Horned Owls.
MPS of Great Horned Owls (œ _+ 2 SE = 55.24 _+
9.31 g, n = 872) was significantly greater (P < 0.001)
than MPS of Common Barn-Owls (27.44 _+ 1.70 g,
ing parametersfor eachowl specieswere calculated: n = 2628). This was the result of Great Horned Owls
(1) mean prey size (MPS) (Herrera and Jaksi• 1980), preying on larger prey items such as rabbits and
(2) food-niche breadth (Levins 1968: 43), and (3) foodniche overlap (Pianka 1974: 2142). The numbers of
prey speciesin each owl species'diet were used as
entries for computationof niche breadthsand'overlaps.
Small mammals were the major prey of Great
Horned
Owls
and
Common
Barn-Owls
at the Es-
quatzelCouleestudyarea,both in percentageof total
prey items (91.1% and 97.8%, respectively) and in
percentageof total prey biomass(77.9% and 98.3%)
(Table 1). Mammals compriseda lower percentageof
total prey biomassin the diet of Great Horned Owls
than of Common Barn-Owls (X2 = 162.34, P < 0.001).
Two speciesof mammals, northern pocket gopher
(scientific names given in Table 1) and Great Basin
pocket mouse,comprisedover half of the total prey
itemsin the dietsof both owl species(50.1%for Great
Horned Owl and 69.3% for Common Barn-Owl), as
well as a major portion of total prey biomass(42.2%
and 73.9%). Rabbits were the third most important
prey species,in termsof biomass,in the diet of Great
Horned Owls, although they were not found in
Common Barn-Owl pellets. BecauseCommon BarnOwls are smaller than Great Horned Owls (Marti
1974), they may have difficulty in capturing rabbits
(Jaksi• and Y/tftez 1980). Common Barn-Owls do oc-
casionallyeat rabbits(Fitch 1947,Marti 1974);therefore, an alternative explanation for the absenceof
rabbits as a Common Barn-Owl prey item in our study
may be that the largely crepuscularNuttall cottontail
Common
Barn-Owls.
The food-niche
breadth of Great
Horned Owls was 4.12, considerablygreaterthan the
2.28 of Common Barn-Owls, suggestingthat Great
Horned Owls apply a more uniform predation pressureover a wider range of animals.Food-nicheoverlap was 0.97 (of a maximumof 1.00).This is largely
due to the importance of northern pocket gophers
and GreatBasinpocketmice in both owl species'diets.
On our study area, these two speciesshow a high
degree of ecologicaloverlap in several traits other
than diet. Strongestevidenceof habitat overlap is the
presenceof Common Barn-Owls in the diet of Great
Horned Owls (Herrera and I-Iiraldo 1976). Addition-
ally, Common Barn-Owls and Great Horned Owls
showed few or no differences in nest-site structure,
mean nest height, mean nest-structureheight, landuse patterns near nesting sites,and distancesof nest
sites to human activity (Knight and Smith 1982).
Nesting Great Horned Owls exhibited regular and
Common Barn-Owls random intraspecific distributions. Their interspecific pattern was random, however, indicating no interspecific territoriality. There
were indications of ecological differences between
the two species.Common Barn-Owls may utilize
habitats not frequented by Great Horned Owls, as
evidencedby differencesin prey species(e.g. house
mouse).Common Barn-Owlsare strictlynocturnaland
hunt mainly on the wing, whereas Great Horned
Owls are more crepuscular and hunt primarily by
flights from perches(Marti 1974,Rudolph 1978). Fi-
TABLE1. Prey itemsfrom 234 pelletsof GreatHorned Owls and from 622 pelletsof CommonBarn-Owlsin
EsquatzelCoulee, easternWashington. Common names are in parentheses.
Great Horned
Prey
Weight
(g)
Number
of prey
(%)
Owl
Common
Barn-Owl
Total prey
biomass
(%)
Number
of prey
(%)
Total prey
biomass
(%)
Mammals
Sylvilagusnuttallii
(Nuttall cottontail)
Spermophilus
washingtoni
(Washingtonground squirrel)
Thomomys
talpoides
(northern pocketgopher)
514
1.3
12.4
--
--
172
0.1
0.4
--
--
200
7.7
29.7
5.44
39.3
TAIlLE 1.
177
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January 1984]
Continued.
Great
Weight
Prey
Perognathus
parvus
(Great Basin pocket mouse)
Dipodomysordii
(Ord's kangaroo rat)
Reithrodontomys
megalotis
(western harvest mouse)
Peromyscus
maniculatus
(g)
Horned
Number
of prey
(%)
Owl
Common
Total prey
biomass
(%)
Number
of prey
(%)
Barn-Owl
Total prey
biomass
(%)
15
43.1
12.5
63.85
34.6
53
1.4
1.4
0.53
1.0
11
1.5
0.3
3.39
1.3
19
16.9
6.2
13.77
9.5
310
0.3
2.1
0.04
0.4
30
13.3
7.7
8.11
8.8
--
--
0.08
0.8
(deer mouse)
Neotoma cinerea
(bushy-tailedwood rat)
Microtus
montanus
(montane meadow mouse)
Rattussp.
(rat)
300
Mus musculus
17
0.7
0.2
1.75
1.1
0.84
1.5
0.04
0.5
--
0.08
0.2
0.1
(house mouse)
Unidentified
Microtinae
50
4.7
4.5
178
0.1
0.4
Anasplatyrhynchos
1,185
0.1
2.6
(Mallard)
Phasianuscolchicus
1,138
0.1
2.5
654
0.1
1.5
104
0.1
0.2
Mustelafrenata
(long-tailed weasel)
Birds
(Ring-necked Pheasant)
Fulica americana
(American Coot)
Charadrius
vociferus
(Killdeer)
Columba livia
332
--
--
(Rock Dove)
Tyto albaa
(Common Barn-Owl)
Sturnusvulgaris
(European Starling)
Sturnellaneglecta
603
79
0.9
--
10.7
96
0.1
0.2•
0.04
Unidentified Fringillidae
26
0.3
0.2
0.23
0.2
Unidentified
56
1.1
1.2
0.27
0.5
207
0.3
1.4
583
0.1
1.3
1
2
0.3
4.0
trb
0.2
1.45
0.1
0.11
tr b
(Western Meadowlark)
Passeriformes
Other
Unidentified
snake
Cyprinuscarpio
(carp)
Unidentified Scorpionidae
Stenopelmatus
sp.
(Jerusalem cricket)
Unidentified
Scarabaeidae
1
0.9
tr b
Unidentified
Locustidae
1
0.2
tr b
Total
872
RemainsfoundbeneathGreatHornedOwl nestingand roostingsites;not foundin pellets.
tr = trace amount; 0.01 g.
99.9
2,628
99.9
178
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[Auk,Vol. 101
TABLE2. Comparisonof three trophic statisticsfor the Great Horned Owl and Common Barn-Owl.
Mean prey
size (g)
Corn-
Food-niche
breadth
Com-
mon
Barn-
Great
Horned
mon
Barn-
Great
Horned
Owl
Owl
Owl
Owl
overlap
Central Washington,USA, 46øN
27
55
2.28
4.12
0.97
This study
Northern California, USA, 41øN
Central California, USA, 37øN
30
52
28
98
1.98
2.95
1.92
8.17
0.99
Rudolph (1978)a
123
266
3.98
6.90
0.24
0.48
Fitch (1947) a
Jaksie and Yafiez
(1980)
Geographicareaand latitude
Central Chile, 33øS
Foodniche
Study
• We calculatedtrophicstatisticsusingTable 1 in Rudolph(1978)and Tables2 and 7 in Fitch (1947).Weightsof prey itemswere obtainedfrom
both papersand the Bureauof Land Management(1979).
nally, Great Horned Owls, becauseof their greater
size and strengh, are able to utilize a wider variety
of potential prey.
The almost complete food-niche overlap between
the two speciesin our study and the low level of
spatialand temporal segregationmay be the result
of their recent sympatry,which probably has been
causedby the expansionof irrigated farming in the
region (Smith and Knight 1981:24). Alternatively,it
may reflect the normal situationwherever the two
speciesoverlap. A comparisonof trophic parameters
betweenthesetwo specieswhere they have coexisted
for differing periods of time is revealing. In areasof
recent sympatry, such as northern California and
Washington(Stewart 1980),there is almostcomplete
food-niche overlap, whereas in areas where both
specieshave coexistedlonger, such as central California and central Chile, overlap valuesare one-half
SouthAmericais in agreementwith the findings of
Herrera
and Hiraldo.
MPS
and food-niche
breadth
for both owl speciesincrease from Washington to
Chile, suggestingthat owls in temperateshrub-steppe
areasprey on smaller more numerousprey than do
owls in mediterranean habitats.For both owl species
there are increases of over four-fold
in MPS between
Washingtonand Chile. Indeed, the MPS of Common
Barn-Owlsin Chile is over twice as large as the MPS
of Great Horned Owls in Washington. This is all the
more surprisingas the adult body weight of Common Barn-Owls decreasesfrom north to south (Jaksie
et al. 1982). This is in sharp contrast to the usual
relationshipsbetween predator and prey sizes(Wilson 1975). In conclusion, we are unable to attribute
the high dietary overlap between the Great Horned
Owl and the CommonBarn-Owl in our study solely
to recentsympatry.Further studiesof trophic rela-
tionships among New World owl speciescommunities and prey populationsare needed to clarify our
associations between Great Horned
Owls and Comunderstandingof the coexistence
mechanismsof owls.
For critical comments and important suggestions
mon Barn-Owls,lending supportto the recentsymparry explanation.Perhapsthe high diet overlap de- we are most grateful to Fabian M. Jaksi•, SusanK.
tectedin northern California and Washingtonis not Knight, William J. Mader, Carl D. Marti, Gordon H.
the result of recent sympatrybut is insteada char- Orians, Thomas W. Sherry, Karen Steenhof, Chrisacteristicof Bubo-Tyto
pairs in temperateshrub-steppe topher H. Stinson,and StanleyA. Temple. We thank
areas.Likewise, low diet overlap may be character- Sievert A. Rohwer (Thomas Burke Memorial Muistic of owls in mediterranean ecosystems.Central seum,Univ. Washington), and Gordon D. Alcorn and
California is the climatic/physiognomic analog of Ellen B. Kritzman (PugetSoundMuseumof Natural
central Chile, both of which are mediterranean-type History, Univ. Puget Sound) for allowing us to execosystems
(ThrowerandBradbury1977).Herreraand amine museum specimens. J. Trumand and T. W.
Hiraldo (1976) found high dietary overlap in owl Pietsch assisted in invertebrate and fish identificacommunitiesin middle and northern Europe and tion, respectively.Herb Camp kindly allowed us acto one-quarter as great (Table 2). Thus, high foodniche overlap values are not always characteristicof
minimal overlapin a mediterraneanowl community.
cess to his land.
These differences were associatedwith a tight clus-
teringof owl speciesfeedingon microtineswith high
populationlevelsin middle and northern Europeand
fewer owl species with wider niche breadths depending on less abundant small mammals in the
mediterranean community.
Evidence presentedin Table 2 for coexistingGreat
Horned
Owls and Common
Barn-Owls
in North
and
LITERATURE CITED
BUREAU OF LAND MANAGEMENT.
1979.
Snake
River
birds of prey specialresearchreport. Boise,Idaho, BLM.
FITCH,H. $. 1947. Predation by owls in the Sierran
foothills
of California.
Condor
49: 137-151.
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HERRERA, C. M., & F. HIRALDO.
1976.
Food-niche
and trophic relationshipsamong Europeanowls.
Ornis
Scandinavica
7: 29-41.
-, &:F. M. JAKSIQ.1980. Feeding ecologyof the
Barn Owl in central Chile and southern Spain: a
comparativestudy. Auk 97: 760-767.
JAKSI•,F. M., & J. L. YXI•IEZ. 1980. Differential utilization of prey resourcesby Great Horned Owls
and
Barn Owls
in central
Chile.
Auk
97: 895-
896.
ßR. L. SEIB,&: C. g. HERRERA. 1982. Predation
by the Barn Owl (Tyto alba) in mediterranean
habitats of Chileß Spain and California: a comparative approach.Amer. Midi. Natur. 107: 151162.
179
MARTI, C. D. 1974. Feeding ecology of four sympatric owls. Condor 76: 45-61.
IVIANKA,E. R. 1974. Niche overlap and diffuse competition. Proc. Natl. Acad. Sci. USA 71: 21412145.
RUDOL?H,
S. G. 1978. Predation ecologyof coexisting Great Horned and Barn owls. Wilson Bull.
90: 134-137.
SMITH, D. G., & R. L. KN•CHT. 1981. Winter population trends of raptors in Washington from
Christmas bird counts. Olympia, Washingtonß
Washington Dept. Game.
STEWART,
I2. g. 1980. Population trends of Barn Owls
in North
America.
Amer.
Birds 34: 698-700.
THROWER,N.J. w., &: D. E. BRADBURY(Eds.). 1977.
KNIGHT,R. L., &: D. G. SMITH. 1982. Summer raptor
populations of a Washington coulee. Northwest
Sci. 56: 303-309.
Chile-California
mediterranean
nia, Dowden, Hutchinson,
KRITZMAN, E. B. 1977. Little mammals of the Pacific
Northwest. Seattle, Washington, Pacific Search
scrub
atlas:
a
comparative analysis. Stroudsburg, Pennsylva& Ross.
WILSON,D.S. 1975. The adequacyof body size as a
niche
difference.
Amer.
Natur.
109: 769-784.
Press.
LEVINS,R. 1968. Evolution in changing environments: some theoretical explorations. Monogr.
Pop. Biol. No. 2, Princeton, New Jersey,Princeton Univ.
Received8 February1983, accepted6 July 1983.
Press.
Broodedness
in Bobolinks
THOMAS A. GAVIN
Departmentof Natural Resources,
CornellUniversity,Ithaca,New York 14853USA
Extensive quantitative information on the number
of broods raised per female bird per seasonis scarce
(Cody 1971).This knowledge is necessaryfor an accurate analysis(or modelling) of population dynamics or reproductive "strategies" of individuals. In
particular, a comparisonof the reproductive fitness
of monogamousmaleswith that of polygynousmales
depends on knowing the reproductive successof
paired femalesßwhich is pertinent also to questions
dealing with sexual selection, female choice, and
polygyny threshold models. To obtain complete data
on reproduction, it is necessarythat individual females be marked and monitored throughout the
breeding season.
Replacementof lost clutchesof eggsis common in
birds (Lack 1968), but Lack (1968: 302) generalized
that "most speciesof birds raiseonly one brood in a
year, becausethe time required for courtship, nestbuildingßincubationand raising the young to independence is so long that a second brood could not
normally be completed before the ecological conditionswhich permit breedinghaveendedfor the year."
Great Tits (Parusmajor),however, were found to raise
two broodsper year more frequentlyin habitatwhere
food was more abundant (Perrins 1965), and older
femaleswere more likely to be double-broodedthan
younger females in this species(Kluijver 1951). The
latitude at which a bird breeds apparently is related
to broodednessin two opposing ways. Lack (1968:
196) statedthat "... most passerinebirds of high latitudes raisemore than one brood eachyear .... "presumably becausethe longer daylength reducesthe
time to fledging relative to that at lower latitudes.
Ecologicalconditions,however, may be suitable for
nesting for a longer period of time at lower latitudes,
and, therefore, more broodsper female may be raised
in a year than at higher latitudes (e.g. doves, many
passerines).
The Bobolink (Dolichonyxoryzivorus)
is a polygynous, ground-nesting icterid that winters from November
to March
in South America
between
latitudes
8øS and 32øS (Engels 1969) and breeds in North
American hayfieldsand meadowsfrom May to July
between
latitudes
40øN and 50øN. Recent
studies
of
populations of marked birds in Wisconsin (Martin
1971), Oregon (Wittenberger 1978), and New York
(R. L. Kalinoski pers. comm.) reaffirmed the earlier
conclusion of Bent (1958) that Bobolinks can renest
after nest failure but do not attempt a secondnesting
after fledging young from the first nest. An un-