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Home , Abies balsamea, Crossbill, Red crossbill

SCIENCE been uncommon on Newfoundland for the last Io,ooo yearssince the glacialretreat (Delcourt and Del- courtI987). In general,White-winged are more efficient than THEEVOLUTION, Red Crossbillswhen foragingon sprucecones, whereas the converse is true with cones(Benkman ECOLOGY,_AND•987b). Giventhat percnais adaptedfor survivingyear-round on Newfound- DECLINEOF THE RED land (GriscomI937), two questions mustbe addressed. First, why has the Red differentiated into a CROSSBILLOF distinctsubspecies on Newfound- land?Second, since percna is a res•- dent in the spruceand Tamarack NEWFOUNDLANDforests of Newfoundland,why is per- cna,or a similarlysized crossbill, not foundthroughout similar forests on the mainland? No other crossbill byCraig W. Benkman speciesor subspeciesin North Amer- icais restrictedto a smallpart of the rangeof a coniferspecies or groupof specieson which they forage. Eachof thesequestions will bead- dressedin turn. Backgroundinfor- The correlation between bill sizes of White-wingedCrossbills also occur mationis given first, and then meth- differentspecies of Eurasiancross- on Newfoundland, but have not dif- ods,results, and discussion are briefly bills (Loxia) and the sizesof the ferentiatedinto a distinctsubspecies; combined. conifercones they exploitis well they apparentlymove on and off known(Lack I944 , NewtonI967). In Newfoundlanddepending on the Why has a large-billedcrossbill evolved , thereis a similar cor- sizeof the conecrop. (Picea) on an island with small cones? respondencebetween bill sizesof has been common on Newfoundland Answeringthis question requires in- White-wingedCrossbills (Loxia I. for at leastthe last8000 years (Del- formationon coniferripening pat- leucoptera)and Red Crossbills(L. courtand Delcourt•987), and Black ternsand crossbill ecology (Benkman curvirostrabendire•) and the sizesof Spruce(P. mariana),White Spruce I987a, I987b, I99zb, I993a). Cone coneson whichthey feed; this cor- (P.glauca), Tamarack (Larix larici- cropsfluctuate tremendously from respondenceextends to mostsub- ha), and BalsamFir (Abiesbalsamea) year to year (Fowells•965, Smith speciesofthe (Griscom arepresently the predominant coni- I97o,Janzen I97I ), andduring some i937, BenkmanI987a, I987b, Benk- fers(Damman I983). The relatively yearsthere are few, if any, avail- man I993a). However,the resident small- and slender-billed White- ableover large areas. On the main- Red Crossbill on Newfoundland, L. wingedCrossbill (Fig. Ia) spends land,crossbills are rarely present in a c percna(Griscom I937, Dickerman mostof itstime foraging on the rela- localregion for more than a year, and I986) , isan exceptionto thisgeneral tivelysmall cones of BlackSpruce theyoften remain absent from exten- pattern(Benkman I989, I99za ). (Fig.Ie), WhiteSpruce (Fig. If), and siveareas for evengreater lengths of The subspecies?ercna has a more Tamarack on the mainland of North time (BenkmanI987a; also Lawrence massive bill than is characteristic of America (Benkman I987a, i987b, I949, Baileyeta/. I953). Although White-wingedCrossbills and a rela- I99zb). In theNortheast, Red Cross- crossbillsmay be foundin a given uvelydeep bill comparedto other bills(L. c.bendirei; Fig. •b) are typical- areafor severalyears in succession, Red Crossbills in North America ly associatedwith the large-conedthey are probably just transientsm (e.g.,L. c.bendirei; Fig. I, TableI), yet EasternWhite and Red (Pinus someyears. Crossbills escape local or it dependson conifersnormally ex- strobusand P. resinosa; Fig. Ig,h) (Benk- regionalcone failures by moving out ploitedby White-winged Crossbills. man i987a, I987b), yet pineshave (Newton I97z, Smith and Balda

Volume 47, Number 2 ß •979,Benkman •987a, I992b). These data were obtained from g Based on data from the captivecrossbills foraging on mainland(Benkman I987a, BlackSpruce cones gathered •992b),Whiteand Black spruce fromMaine (Benkman •987b). andTamarack are presumably Alsoshown are the predicted themost important intakerates of percna,based for crossbills on Newfound- on the sizeof its bill (Table0- land.White Spruce and Tama- It should be noted that the rackcone crops fluctuate great- preciselocation of the intake ly betweenyears (Fowells •965, h ratesof percnacannot be as- Benkman•987a), Furthermore, signed,but basedon thecon- the conecrops of thesethree sistentpatterns for the two conifersoften cyclein syn- mainlandspecies and among chrony(Smith and Balda I979; differentsubspecies of Red pets.obs.). Thus, in someyears Crossbills, certain relations virtuallyno seeds are available seemreasonable. For example, over extensiveareas. For ex- Fig.1. Head profiles offour subspecies ofcrossbills (a-d), drawn from as bill depthand width in- crease,seeds in closed cones ample,in •987, Tamarack pro_ ofa photograph (e-h). ofmuseum Crossbills: skins,a=North and open American conifer White-winged co,es offour species duccdfew if any seedsin Crossbill(L.L/eucoptera), b= North American RedCrossbill (L.c. becomemore accessible (Benk- Newfoundland(pcrs. obs.), bendirei),c=Newfoundland RedCrossbill (L.c. percna), d=European man I993a).The deep-billed RedCrossbill (L. c. cuntirostra);conifer cones: e=Black Spruce (Picea althoughWhite Spruce did mariana),f=White Spruce (P.glauca), g=Eastern WhitePine (Pinus percna is predicted to bemore producesome seedsalong stobus),h=Red Pine (P. resinosa). Scale indicates I cm. efficient on closed cones than Newfoundland'swest coast (B. May- cone-cropproduction (Fowells x965), areWhite-winged Crossbills, which bank,pets. comm.). Black Spruce oftenin synchronywith spruce and havetheir highest intake rates just af- conecrops, in contrast,rarely fail, Tamarack(pets. obs.). terthe cones open. Although scales of andduring most years at leastsome Conefailures on islandscannot thesecones are only slightly separat- seedsare available(Fowells •965). be escapedby nomadicmovements. ed, White-wingedCrossbills can Moreover,atleast some Black Spruce Therefore, crossbills confined to is- slide their slender bills between the seedsare heldyear-round in the landsshould have bills that provide scaleseasily. Red Crossbills,with cones(Chai and Hansen•95z). In accessto seedsover the complete widerbills (Table I), havetheir high- yearsof White Spruce and Tamarack cone-ripening phenology. Large-billed est intake rates after the cone scales conefailure, a residentcrossbillwould crossbills can extract seedsmore havespread farther apart, even though haveto subsist on Black Spruce seeds rapidly from a widerrange of cone some seedshave been shed. Percna is throughoutthe year. Surveys ofbirds sizesand, most importantly, from hypothesizedto be less efficient than in Newfoundlandfrom •957 to •963 morestages of coneripening than theother crossbills on open cones be- characterizedthe Red Crossbill as can small-billedcrossbills (Benkman causeof its wider bill, and its peak in- preferring"the old growth,black •987b).Figure z showsthe intake takerate shouldbe on fully open spruceforest" (H. Deichmann,pets. ratesof mainlandRed and White- cones (shifted even farther to the comm.).On the continent,White- wingedcrossbills foraging on Black rightin Fig.z). wingedCrossbills usually forage on Sprucecones atdifferent cone stages. Thesize and shape ofpercna's bill BlackSpruce only from is similar to that of the Red Table I. Bill dimensionsIx + SE) of four crossbillsubspecies (H=10 winterto carlysummer, individualsforeach). Measurementswere taken as follows: uppermandible Crossbill in (L. c. whenthe conesarc open lengthwasmeasuredfromthetipofuppermandibletotheanteriorendofthecurvirostra;Fig. x, Table 0- and seedsarc most acccssi- right nares; lower mandible length was measured from the tip of lower mandibleto the basewhere raml meet;bill depthwas measured at the nares; Thissimilarity in billshape blc(Bcnkman •987 a, •987b, andupper mandible width was measured atanterior edge of nares. maybe a resultof the im- I992b).Eastern White Pine Crossbill Mandible Lengths Total bill (Upper Mandible) portanceof seedsin closed is an importantseed (locality) upper(mm) Iower(mm) depth(mm) width(mm) conesfor bothsubspecies. for crossbills,but is un- L.t•eucop•e•a13.8_+0.21 9.3_+0.16 7.8_+0A0 5.6_+0.09 Seedsin closedpine cones commonon Newfound- (NorthAmerica) areprobably more critical, L.c. benditel 15.3_+0.33 12.0_+0.29 9.3_+0.09 7.4_+0.08 landand by the early t9oos (NorthAmerica) on average,to Old World hadbeen mosdy removed L e.•,a 14.9_+0.18 11.0_+0.16 10.6_+010 8.1_+0.09 Red Crossbills becauseOld by logging(Montevecchi (•4ewfound•and) World pinesusually hold 15.3_+0.18 11.7_+0.11 10.7_+0.14 8.2_+0.06 mature seeds in closed and Tuck •987). Further-(Eurasia)t ...... irostra more, it also fluctuates in conesfrom fall to early

226 - American , Summer 1993 spnng(Newton •972), whereas most in spruceforests on the mmnland New World pineshave cones that rangefrom 2.o to 2.5individuals per openin thefall (Fowells•965). Pre- hectare(six studiessummarized in sumably,this is why Old World pop- Ruschand Reeder [I978]). These val- ulationsof Red Crossbills that forage uesprobably overestimate Red Squir- onpine have much deeper bills than rel densitiesin BlackSpruce habitat thosepopulations in the New World on the mainland,because they in- (Griscom•937). Alternatively, percna CONE STAGE dudeRed Squirrels in speciesother Fig.2. Kernelintake rates (X _+SE) for two may be a recentcolonist from the subspeciesof crossbillsforaglug ou conesof thanBlack Spruce and Black Spruce Old World(J. Groth,pers. comm.), Mack spruce.The data are for L. I. leucoptera isnot a preferredconifer (Rusch and and • c. benditelforaging ou a total of 59 cones andthe similarityof itsbill sizeand (973 seeds)aud 87 cones(1417 seeds), Reeder•978). Red Squirrelswere shapeto thatof L. c.curvirostra may respectively.Predicted iutake rates for percna foundto losebody mass when given resultfrom lack of divergence.This are indicatedby the dotted liue. onlycones from Black Spruce from cannotbe confirmed without genetic five scalesfrom the lower half of each the mainland(Alaska), but not when analysis.Nevertheless, percna's per- of five to ten cones,from five or six givenonly conesof White Spruce sistence on Newfoundland and its trees from each of the above four lo- (Brink and Dean •966). In sum, absencefrom the boreal forests of the cations(Table z). Scalethickness was squirreldensities in BlackSpruce mainlandneeds explanation. measured because it is the one scale forestsare probablyconsiderably characteristicof sprucein eastern morethan twice as high on New- Why is percna confinedto Newfound- NorthAmerica that is uniformly cor- foundlandas in comparableforests land?The absenceof RedSquirrels relatedwith the time requiredby onthe mainland. The higher squirrel (•miasciurushudsonicus) from New- crossbillsto remove seedsfrom closed densityon Newfoundlandmay be foundlandmay have been essential cones (Benkman •987b). becausethere are fewerpredators. for the evolutionof percna.Red Scalethickness was significantly However, food supply,and not Squirrelshave been a majorselective less for cones from Newfoundland predators,generally limits densities forceon Cønestructure, particularly than for those from the other sites ofsquirrels in thegenus Tamiasciurus onconifers that produce regular cone (Tablez; LSD [leastsignificant dif- (Smith•968, •97o; Kemp and Keith cropsand hold their seeds for extend- ference]multiple range test, P_< o.os), •97o; Ruschand Reeder•978; Sulli- edperiods (Smith •97o, Elliott •974). as predicted.Cones from Riding vanand Sullivan •982). In westernNorth America, Lodge- Mountainhad significantly thinner Thepresence ofRed Squirrels may pole Pine (P. contorta)has thicker scales than did those from the two haveprecluded a residentpercna- conescales where cone crops are Ontario sites,and scalethickness de- typecrossbill on themainland. Even moreregular and Red Squirrel popu- creasedto the west in the three main- thoughpercnaispresumably more ef- lationsare more stable (Smith •97o). land samples.The significanceof ficienton closed Black Spruce cones Consequently,the presence of squir- these last two results is unknown. than are other subspeciesof Red relsas a selectiveagent can have indi- RedSquirrels were introduced to Crossbillsin NorthAmerica, it may rect effects on other seed consumers Newfoundlandin •963 and •964 not be able to survive for extended (Benkmanetal. •984). If BlackSpruce (Dodds•983). The latter introduc- periodson mainlandBlack Spruce cone scales are thinner on New= tion,by the Newfoundland Wildlife which have thicker cone scales.There foundland because of the absence of Service,was apparently done to pro- are no records of Red Crossbills on Red Squirrels,then seeds in dosed videprey for the Pine Marten (Martes the LabradorPeninsula adjacent to conesmay be more accessibleto americana),which had been over- Newfoundland,where black spruce crossbillson Newfoundland than on trapped.Red squirrels are now quite is abundant(Todd •963). There is no themainland (Benkman •987b, Benk- commonon Newfoundland(Dodds indicationthat percna moves regular- man•993a). •983;pers. obs.). For example, the av- ly betweenNewfoundland and the Todetermine whether Black Spruce eragespring density for four 3.o to mainland(contra Dickerman •987), cones differ between the mainland 6.z5 hectare plots in northwestNew- noris there evidence that percna has andNewfoundland, I gathered Black foundlandin •985was 3.5 individuals persistedon the mainlandfor ex- Sprucecones from Riding Mountain per hectare(West •989). If Black tendedperiods. National Park,, from west- Spruceon the mainland has more de- Alternatively,White-winged Cross- ern and easternOntario, and from fensesagainst vertebrate seed preda- billsmay competitively exclude perc- Newfoundland. Cone-scale thickness tors, then lower densities of Red na from the mainland. This alone, wasmeasured with digital calipers at Squirrelsare to be expectedthere. however,is nota sufficientexplana- theanterior edge of theseed scars of Thespring densities of RedSquirrels tion, becauseWhite-winged Cross-

Volume 47, Number 2 221 billsdo not seemany less common Table 2. BlackSpruce cone scale thickness for four ThisIs but one example of the•m- on Newfoundlandthan in compara-sites in Canada.There were significantdifferences pactsof manon crossbills.There are betweenall samples(P< 0.05), except between ble forestson the mainland(but see the two Ontariosamples more, someof which are more subtle. below). Site Scale thickness (mm)' A subspecies(species?) of crossbill Presumably,Red Squirrels are now x SE Nb Longitude moreappropriately considered awest- exertinga strongselective force on Manitoba 1.04 0.02 5 110øW ern form (œ.c. bendirei;Type 2 of the structureof BlackSpruce cones Western Ontario 1.11 0.02 5 91øW Grothx988) associated mostly with on Newfoundland. If cone-scale Eastern Ontario 1.15 0.02 6 82øW Newfoundland 0.96 0.02 6 58øW PonderosaPine (P. ponderosa) (Benk- man x987a,x993a) has increasedin thicknessincreases among Black Measured at the middle of the scale at the anterior Spruce,then intake rates for percna end of the seed scars the Northeast(Dickerman x987). I N equalsthe numberof trees. Foreach tree, five attribute this increase to the increase will decline. This, in combination scales were measrued from each of 5-10 cones. with coneremoval by squirrels,a in mature Red Pine, which has rela- competitiveeffect, may cause the de- seed resources so that few White- tivelytough cones similar to (although dine,and possible extinction, of the wingedCrossbills can now survive smaller) than those of Ponderosa onceplentifulpercna (Austin x968 for most winters on Newfoundland. Pine.Red pine used to be foundin commentson percna's former abun- On an encouragingnote, percna scatteredsmall stands(Cook et al. dance).In fact,there are tremendous maysurvive on smallerislands near x952)and may not have been plenti- differencesin the percentage of Black the main island of Newfoundland, fulenough to support aRed Crossbill Sprucecones removed by Red Squir- whereBlack Spruce is presentand population.Now Red Pine is planted rels between Newfoundland and the squirrelsare absent.Indeed, percna moreextensively, particularly in the mainland.For example, during years has been collected on Anticosti Is- Great Lakesregion. Indeed, th•s of poorcone crops, when Red Squir- land (NationalMuseum of Canada) "western"subspecies is most com- rels most intenselyharvest Black andI havereceived photos of a pair of mon in the Great Lakes area. More- Spruceand the seedsupply is most percnataken by Philip Burseyon over,there may have been a decrease limiting, Red Squirrelsat several Random Island, Newfoundland, in in bill size in these Red Crossbills in Newfoundlandsites removed 64-96% May, i988. AlthoughI hold little the East relativeto thosein the West, of thecones of BlackSpruce by early hopefor percna where Red Squirrels asone would expect considering the October(West I989), but removed persist,I am optimisticthat islands relativelysmall cones of Red Pine. onlyx9% of the BlackSpruce cones such as Anticosti and those sur- The meanbill depthof Type2s in on the mainland(Ontario) (Pr•vost roundingthe mainisland of New- theAppalachians is9.48 mm (Groth etal. x988). foundlandmay providerefuge for •988;I averagedthe mean of the males Thereappears to havebeen a large percna.Nevertheless, it is particularly [n = 4x]and females[n = 23]),where- decline in Red Crossbills on New- discouragingthat sucha distinct as those I measured in the Northwest foundlandsince x957-x963, when it population,and most likely a distinct had a meanbill depthof 9.7o mm wascharacterized as a "fairlyregu- species(Groth I988), has such a pre- (SE= o.x4,n = 9). lar resident"(H. Deichmann,pers. carious existence. Other impactson crossbillsare comm.).Such a decline'isconfirmed morepronounced and negative (Benk- by datafrom annual Christmas manx993b). Crossbills have declined Counts in , 4.0 i/• becauseof forestloss and fragmen- Newfoundland(Fig. 3). It is ironic tation in Finland(Helle x985)and that this introductionto help one 2.0 oo • Dickerman(x987) suggests that the "northeastern"subspecies of Red species,the Pine Marten, has the po- 1,0 i 10 tential to causethe demise of another. Crossbill declined because of exten- White-wingedCrossbills have also 0.o, , , •-Oese•A~O•,. A•O__o sivelogging near the beginningof declined on Newfoundland Christ- YEAR thiscentury. Moreover, the tendency masBird Counts since the late x96os Fig.3. Thenumber of RedCressbills observed for crossbillsto concentrateduring duringthe annualTerra NovaNational Park, (W. A. Montevecchi,pers. comm.). Newfoundland,Christmas Bird Count from 1968 anygiven year in areasof largecone White-wingedCrossbills still move ta 1988 (data from Terra NovaNational Park cropswill potentiallyresult in much onto Newfoundland in the summer records).This trend has continued.For example,in 1991 no RedCrossbills were greaterdeclines ofcrossbills than those andfall when there are large Tama- observed on the Terra Nova Christmas Bird justproportional to habitatlost. In Count.A "par•-hour" is onehourof rackand White Sprucecone crops observationby an independentgroup of the fall and winter of •988-x989, (Benkman,pers. obs.). However, the observers.The numberof par•-hours ranged mostof theWhite-winged Crossbills from 12 (in 1971) to 88 (in 1984). The census intensecone harvests by RedSquir- area encompassesextensive areas of in North Americanwere probably relsin the fall presumablydepletes undisturbed Black Spruce forest (pars. obs.). concentrated in the Maritime Prov-

228 American B•rds, Summer 1993 incesof Canada(pers. obs.). Thus, if sourcesand the evoluuonof crossbill(Lox- LACK, D 1944 Correlationbetween ia)diversity. Ecol. Monogr., in press. and food in the crossbill(L. curvirostra) 50%of theconifer forests in theMar- Ibis86:552-553. ltimeswere to be logged,we might ---. 1993b.Logging, conifers, and the conservation of crossbills. Conservation LAWRENCE,L. DE K. 1949.The RedCross- expectnearly a 50% declinein the Biology,in press. billsat PimisiBay, Ontario. Can. Field- Natur. 63:147-16o. White-wingedCrossbill population, BENKMAN, C. W, R. P. BALDA, and C. C. eventhough the decrease in the total SMI WH. 1984.Adaptations for seed disper- MONTEVECCHI, W A., and L. M. TUCK areaof conifersused by thisspecies saland the compromises due to seedpreda- 1987.Newfoundland birds: exploitation, representsa much smaller fraction tionin limberpine. Ecology 65:632-642. study,conservation. Publ. Nutt. Ornithol ClubNo. 21.Cambridge, Ma. than50%. Indeed, nomadic behavior BRINK, C. H., andE C. DEAN. 1966.Spruce makescrossbill populations both seedas a foodof redsquirrels and flying NEWTON, I. 1967.The adaptiveradiation squirrelsin interiorAlaska. J. Wildl.Man- andfeeding ecology of someBritish - more vulnerable to habitat loss and ag. 3o:5o3-512. es.Ibis •o9:33-98. more difficult to assess. Because CHAI, T. S.,and H. L. HANSEN.I952. Char- --. 1972. .Collins, London, crossbillabundance is difficult to as- acteristicsof black spruce seed from cones U.K. sess,the decline in the Newfound- of differentages. Minn. Forest.Notes 2, PRI}VOST, Y. H., J. E. LAING, and V. F Univ. Minn., St. Paul, Minn. landcrossbill remained unappreciat- HAAWSTO.1988. Seasonal damage by in- ed until now. COOK, D. B., R. H. SMITH, and E. L. sectsand squirrels to femalereproductive STONE.1952. The naturaldistribution of ßstructures of blackspruce, redpine in NewYork. Ecology 33:500-9 2. (Mill.) B.S.P.Can. Ent. 12o:1113--112L Acknowledsmen•s P R. Grant,A. Knox,D. Schluter,C. C. Smith, DAMMAN,A. W. H. 1983.An ecologicalsub- RUSCH, D. A., and W G. REEDER. 1978 and two anonymousreviewers made com- division of the island of Newfoundland, Populationecology of Albertared squir- mentsthat improvedan earlyform of this pp. 163-2oo.In G. R. South(ed.), Bio- rels.Ecology 59:400-42o. manuscript.B. Maybankgathered and sent geographyand ecologyof the islandof SMITH, C. C. 1968.The adaptivenature of NewfoundlandßJunk, The Hague, Neth. me the conesfrom Newfoundland,for which socialorganization in the genusof tree I amgrateful. I am also grateful to H. Deich- DELCOURT, P. A., and H. R. DELCOURT. squirrels,•miasciurus. Ecol. Monogr mannfor sending me an unpublished manu- 1987.Long-term forest dynamics of the 38:31-63. script.The conifercones and crossbills were temperatezone. Springer-Verlag, N.Y. graciouslydrawn by E Kingand D. Nash,re- ß197o. The of pinesquir- spectively.Support during this project was DICKERMAN,R. W. 1986.A reviewof the rels (Tamiasciurus)and conifers.Ecol providedby an NSF PostdoctoralFellow- Red Crossbill in New York state, Part z. Monogr.4o:349-371. Identificationof specimensfrom New shipin EnvironmentalBiology and an NSF andR. P.BALDA. 1979. Competition York.Kingbird 36:127-134. NATO Fellowshipin Scienceand Engineer- amonginsects, birds and mammalsfor ---. 1987.The "old northeastern"sub- coniferseeds. Amer. Zool. 19:1o65-1o83. speciesof Red Crossbill.Amer. Birds SULLIVAN,T. P.and D. S. SULLIVAN.1982 41:188-:194. Literature Cited Populationdynamics and regulation of the AUSTIN,O. L. Jr., (Ed.)1968. Life histories DODDS,D. 1983.Terrestrial mammals, pp. Douglassquirrel ( Tamiasciurusdouglasit) of North Americancardinals, grosbeaks, 509-550.In G. R. South(ed.), Biogeogra- with supplementalfood. Oecologia buntings,towhees, finches, sparrows and phy and ecologyof the islandof New- 53:264-270. alliesßPart I. U.S. Natl. Mus.Bull. 237. foundland.Junk, The Hague, Neth. TODD, W. E. C. 1963.Birds of the Labrador BAILEY,A.M., R. J. NIEDRACH,and A. L. ELLIOT,E E 1974.Evolutionary response of Peninsulaand adjacent areas. Univ. Toron- BALLY.1953 . The RedCrossbills of Col- plantsto seed-eaters:pine squirrel preda- to Press,Toronto, Canada. orado. Publ. Denver Mus. Nat. Hist. tion on lodgepolepine. Evolution WEST,R. J. 1989.Cone depredations by the 9:1-64. 28:221-231. redsquirrel in blackspruce stands in New- BENKMANC. W. 1987a.Food profitability FowEL L S, H.A. 1965.Silvics of foresttrees in foundland:implications for commercial andthe foraging ecology of crossbills.Ecol. theUnited States. U.S.D.A. Agric. Handb. cone collection. Can. J. For. Res Monogr.57:251-267. 271. 19:12o7-121o. --.. 1987b.Crossbill foraging behavior, GRI sc o M, L. 1937.A monographicstudy of bill structure,and patterns of foodprof- the Red Crossbill. Proc. Boston Soc. Nat. itability.Wilson Bull. 99:351-368. Hist. 41:77-21o. ß1989. On theevolution and ecology GROTH,J. G. 1988.Resolution of cryptic of islandpopulations of crossbills.Evolu- speciesin AppalachianRed Crossbills. tion 43a324-133o. Condor9o:745-76o. ß1992a. A crossbill'stwist of fate.Nat- HELLE,P. 1985. Effects of forestfragmenta- --Departmentof Zoology uralHistory lO1(12) :39-42ß tion on bird densities in northern boreal UniversityofBritish Columbia --.. 1992b.White-winged Crossbillß In forests.Ornis Fennica 62:35-41. 6270 UniversityBlvd. Vancouver,B. C. V6T2A9 The Birdsof North, No. 27 (A. Poole,E JANZEN,D. H. I971.Seed predation by ani- Canada Stettenheim,and E Gill, Eds.). Philadel- mals.Ann. Rev. Ecol. Syst. 2:465-492. phia:The Academyof NaturalSciences; Washington,DC: The AmericanOr- KEMP,G. A. and L. B. KEITH. 197o.Dy- --Departmentof Biology, Box30001, New MexicoState nithologists'Union. namicsand regulationof red squirrel ( Tamiasciurushudsonicus) populations. Universi•Las Cruces, NM ß 1993a.Adaptation to singlere- Ecology5I:763-779 . 88003. (currentaddress)

Volume 47, Number 2 2•