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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 Short Communications 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 ShortCommunications [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. January 1984] ShortCommunications 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-