Abundance and Distribution of the Common Raven and American ...

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Bird Counts, and breeding bird atlases yielded similar distribution patterns. Breeding ... Abundances of the Common Raven (Corvus corax) and American Crow.
ABUNDANCE AND OF THE COMMON

DISTRIBUTION RAVEN AND AMERICAN

IN THE SAN FRANCISCO

CROW

BAY AREA, CALIFORNIA

JOHN P. KELLY and KATHERINE L. ETIENNE, CypressGrove ResearchCenter, AudubonCanyonRanch,Marshall,California94940 JENNIFER E. ROTH, Point Reyes Bird Observatory,4990 ShorelineHighway, StinsonBeach,California94970

ABSTRACT:Duringthe breedingseasonof 1999, we surveyed fromroadsin the San FranciscoBay areato determinethe regionalabundance anddistribution of the Common Raven (Corvus corax) and American Crow (C. brachyrhynchos).Ravens

concentrated alongthe outercoastand occurredin relativelylow numbersin some interiorareas,whereasthe numberof crowsincreasedsignificantly from the outer coastto interiorandbayshore locations. Bothspecies occurred in significantly greater densities alongurbanandsuburban surveyroutesthanalongruralroutes,butdramatic exceptions wereevidentin someareas.Data fromBreedingBirdSurveys,Christmas BirdCounts,andbreedingbirdatlasesyieldedsimilardistribution patterns.Breeding BirdSurveysand ChristmasBirdCountsrevealedstrongregionalincreases in both species,butannualtrendsvariedsubstantially at localscales.Thisvariation,aswellas significant abundance variationamongsurveyroutes,suggested considerable local differences in eitherhabitatsuitability or capacityfor furtherpopulationgrowth.Our resultssuggested that ravenand crowpopulations may increasein both ruraland developedareasundergoing rapidurbanization andthatlocalconditions, ratherthan whetherthe habitatis ruralor urban,may influenceregionalpatterns.

Abundancesof the Common Raven (Corvuscorax) and American Crow

(C. brachyrhynchos) have increasedsubstantially over much of North America (Marzluffet al. 1994, Boarman and Heinrich 1999, Sauer et al. 2001). The Common RavenoccursthroughoutCaliforniabut is scarcein muchof the CentralValley,the centralcoastfrom the SantaLuciaMoun-

tainssouthto northwestern VenturaCounty,and irrigatedportionsof the Coachella,ImperialandlowerColoradoRivervalleysin the southeast corner of the state.The AmericanCrow is widespreadbut absentfrom somedrier westernpartsof theSanJoaquinValley,interiorfoothills,andthesoutheastern deserts(Small 1994). Throughouttheir range, ravensappear to be invadingagricultural areasto a greaterextentthan urbanareas(butare common in many urban areas), whereascrows appear to be invading urbanizedareasmore rapidlythan agricultural areas(Marzluffet al. 1994, Marzluffand Restani1999). Both speciesare residentthroughmostof the San FranciscoBay area, a highlyurbanizedregionwith largesectionsthat remainundeveloped or usedfor agriculture.Both specieshaveincreasedin numberrecenfiyin the southernportionof the region(Coston1998). The overallstatusand distributionof thesecorvidswithin the San FranciscoBay area, however,have not been addressed. To determine the abundance and distribution of ravens and crows in the

SanFrancisco Bayarea,we surveyed the regionduringthe springof 1999 (Figure1). Becausefieldobservations werelimitedto repeatedroadsurveys alongparticularrouteswithina singleseason,we comparedthe resultswith existingBreedingBirdSurvey(BBS;Saueret al. 2001), AudubonChrisb•nas 202

WesternBirds33:202-217, 2002

COMMON

RAVEN AND AMERICAN

CROW IN THE SAN FRANCISCO

BAY AREA

11

/• Mt. Diablo

San

12

Pacific Ocean San Jose 0 km

North

50 km

Figure1. Routesusedin road surveysof CommonRavensand AmericanCrowsin the San FranciscoBay area. Labels,route-identification numbers;thin lines,county boundaries(seeFigures5 and 6 for countynames).

Bird Count (CBC; Butcher 1990), and breedingbird atlas(BBA; Robbins 1990) datato providea more thoroughperspectiveon regionaldistribution and to evaluatelocaland regionaltrendsin abundanceof thesespecies. STUDY

AREA

The studyarea coveredall nine countiesadjacentto San FranciscoBay: Marin, Sonoma, Napa, Solano, Contra Costa,Alameda, Santa Clara, San Mateo,andSan Francisco(Figure1). The studyareadidnot extendintothe deltaeastof the confluenceof the Sacramentoand San Joaquinrivers.We did not conductroad surveysalongthe outer coastof Sonoma Countyor alongthe EastBay shoreline,althoughtheseareasare includedin analyses of BBS, CBC, and BBA data. Analysisof BBS data includedsomeroutes that extendedslightlybeyondthe bay area counties,into coastalSantaCruz Countyto the south,coastalMendocinoCountyto the north, San Joaquin Countyto the east, and Yolo Countyto the north (Figure2). 203

COMMON

RAVEN AND AMERICAN CROW IN THE SAN FRANCISCO BAY AREA

187

Napa TB

Point

Pacific Ocean

0krnNorth 50 krn

319

9

Figure2. BreedingBirdSurveyroutes(boldlinesand associated numbercodes)and ChristmasBirdCountcircles(lettercodes)in the San FranciscoBayarea.Thin lines, countyboundaries (seeFigures5 and 6 for countynames).See Tables2, 3, and4 for details on each route or circle.

Abovethe coastalterracesand alluvialshorelines,the area is characterized

by rollinghillsand mountainsof the Coast Range. Moist coastredwood (Sequoiasempervirens),Douglasfir (Pseudotsugamenziesii),and mixed broad-leaved evergreen forestsdominatethe outercoastaldrainages, giving way to wider expansesof grassland,chaparral,and oak woodlandto the east.Nonnativeeucalyptus (predominantly bluegum,Eucalyptusglobulus) occursthroughoutthe regionin patchesmostlysmallerthan one hectareor as narrow windbreaks.

Urbanandsuburban habitatsare concentrated in centralSonomaCounty near Santa Rosa, in southern Napa County near Napa, and south of Sonoma,NapaandSolanocountiesalongbayshorelines. The shorelineand terracesalongthe outercoastand northernSan PabloandSuisunbaysare generallyruralandopen.Ruralareasare primarilyusedfor agriculture, with range cattle productiondominatingmost areas, dairy ranchinglocally dominantin the PointReyesarea, and farmsandvineyardscharacterizing much of the northernportionof the region. 204

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RAVEN AND AMERICAN

CROW IN THE SAN FRANCISCO

BAY AREA

METHODS

From 26 Marchthrough21 June1999 we countedCommonRavensand AmericanCrowsalong 18 surveyroutesaveraging49 + 14.0 (standard deviation)km in length, establishedalong roads throughoutthe region (Figure1). We recruited40 experienced birders,capableof distinguishing crowsandravensin thefield,asvolunteer observers to conduct mid-morning roadsurveys alongeachroutetwicemonthlyon standardized dates.Because eachobserverconsistently surveyedthe sameroute, ratherthan rotating among routes,observervariabilitycouldhave influencedthe resultsto an unknownextent.Surveyteamsconsistingof one driverand one observer traveledat speedsof 56 to 72 km/h. Eachsurveybeganfrom a standard startingpoint. For each ravenor crow observation,observersrecordedthe distancealong the surveyroute, speciesname, group size (numberof individuals within 100 m of each other),and perpendiculardistance(< 100 m, 100-200 m, or >200 m) from the road.

We distributed surveyroutessystematically to sampletwo land-usetypes (>75% ruralvs. >75% urban/suburban) andthreesubregions (outercoast, bay shore,and inland).We apportionedthe routesto approximatethe extent of each land-usetype and subregionwithin the studyarea and maximizedthe distancesbetweenroutesto ensureindependenceof counts (Figure1). To keep surveyspeeddown, we avoidedfreewaysand major highways.In urbanizedareas,we selectedroutesthat allowedobserversto maintainnormalsurveyspeeds,althoughtemporawslowingwasnecessary on someoccasions. Giventheseconstraints, suitablesurveyroutesfor each land-usetypeandsubregion weresometimes determinedbyavailableroads. Open/ruralhabitatin the interiorof the easternportionof the studyarea andsaltpondsin the southernpartof the baywereundersampled relativeto their extents.

We usedthe numberof birdsobservedper kilometerof surveyrouteto index corvid densities. We then examined differences between rural and

urban/suburban habitats,and amongcoastal,interior,and bayshorelocationsusinganalysis ofvariance.Beforeanalysis, we (natural) log-transformed the data so that densitiesdid not differ significantly from normality(P < 0.05). Post hoc testsfor differencesamong groupsincludedBonferroni adjustments forexperimentwise error.Because routeswereselected systematicallyon the basisof land use, subregion,and availabilityof roads,we modeledvariationamongroutesnestedwithinlanduseandlocationtypesas fixedeffects.However,becauseroutescovereda substantial portionof the studyareaandwereselected to achievea representative sampleof regional populations,we alsotested(whereverindicatedin Results) the effectsof land useandsubregion against"random"variationamongroutesas a proxyfor other,unknowninfluences on generalregionalpatterns(Bennington and Thayne 1994). Indicationsof suchinfluencesshouldbe interpretedcautiouslybecauseselection of surveyrouteswassystematic ratherthanrandom. We alsoexaminedBBSandCBC data,to determineregionaltrendsin the numbersof ravensand crowsand to compareabundances with patterns suggested byourroad-survey data.The BBSisalsoa roadsidesurvey,based on 50 three-minutepoint countsconductedalongthousandsof 39.4-km 205

COMMON

RAVEN AND AMERICAN

CROW IN THE SAN FRANCISCO

BAY AREA

routes over much of North America (Sauer et al. 2001). Because BBS

surveysdatebackasfar as 1966, they providea perspectiveon changesin regionalbreeding-season abundance.We examinedlong-termtrendsin winter abundancewith CBC data, which are based on total countscon-

ductedannuallywithinstandard circlesof 24.1-kmdiameter(458 km2; Butcher1990). We analyzedBBS and CBC trendsby linearrouteregression,withregionaltrendsexpressed in percentchangeperyearweightedby meanabundanceand numberof yearscoveredat eachBBS routeor CBC location(Butcheret al. 1990, Geisslerand Sauer 1990). We estimatedthe mean and variance of regional trends from 400 random bootstrap subsamples drawn with replacementfrom the originalsampleof trends (Geisslerand Sauer 1990, Thomas and Martin 1996). CBC countswere standardizedto reflect expectedabundancesand trendsfor the average effort (175 party-hours)in the studyarea over all countcirclesand years, usingthe proceduredescribed by Butcherand McCulloch(1990; survey efforthad no significant effecton ravennumbers,P > 0.90). We modified calculation of the exponentrelatingsurveyeffortto numberof birdscounted byincluding termsin themodel(dummyvariables) to accountforunbalanced effects of CBC locations on the overall relation between effort and abun-

dance.We did not adjustfurtherfor differencesin effort, which include annualdifferencesin distancecoveredper party-hourand distributionof effortamonghabitattypesof varyingsuitability (e.g., ranchlandvs. open water), althoughwe consideredhabitatdifferencesamong count circles when interpretingthe results.We emphasizethat trendsand abundances based on CBC data should be evaluated with considerable caution and

confirmedby additionalstudybecauseof theseandotherpotentialsources of error,includingproblemswith identification,counting,weather,access, and habitatcoverage(Butcher1990). We furtherevaluatedregionalbreedingdistributions by compilingbreeding bird atlas(BBA) data within the studyarea. BBAs use standardized criteriato determinethe breedingstatus(possible, probable,confirmed)of eachbirdspecies within5-km2blocks (Robbins 1990)andareorganized and developedseparatelyby county(Marin County:Shuford1993; Sonoma County:Burridge1995; NapaCounty:R. LeongandB. Grummer,unpubl.; Contra CostaCounty:S. Glover,unpubl.;AlamedaCounty:B. Richmond andH. Cogswell,unpubl.;SantaClaraCounty:SantaClaraAtlasComm., unpubl.;San Mateo County:R. Johnson,unpubl.;San FranciscoCounty: M. Eaton,unpubl.).WhereatlasblocksspannedBBA (county)boundaries, we usedthe statuscodefrom the BBA that indicatedthe greatestlikelihood of breedingfor that location. RESULTS

Differencesin corvidnumbersamongsurveyroutes,nestedwithinlandusetype or subregion, weregreaterthan expectedby the variabilitywithin routes(F -- 40.65; df -- 10, 110; P < 0.0001), indicatingthat variation amongsurveyroutesaccounted for significant regionalvariability in corvid densities(Table1, Figure1). On average,89% + 2.3 (standarderror)of ravensand 92% + 1.8 of crowswere observedwithin 200 m of survey 206

COMMON

Table

1

RAVEN AND AMERICAN CROW IN THE SAN FRANCISCO BAY AREA

Number of Common

Ravens and American Crows Observed

DuringRoadSurveysin the San FranciscoBay Area Duringthe Breeding Season of 1999

Birdsperkilometer

Route LandUse 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Subregion Length(km) N

Rural Coast Rural Coast Rural Coast Rural Coast Rural Coast Rural Bay Rural Interior Rural Interior Rural Interior Rural Interior Rural Interior Rural Interior Urban/Suburban Coast Urban/Suburban Bay Urban/Suburban Bay Urban/Suburban Bay Urban/Suburban Interior Urban/Suburban Interior

19.4 59.4 33.0 41.6 50.8 41.0 50.7 79.8 65.2 40.9 51.0 41.8 39.6 46.2 50.4 41.8 63.1 65.4

12 18 7 7 7 7 2 6 6 4 8 3 7 7 6 6 7 5

Common Raven

American Crow

Mean

(SE) a

Mean

(SE)

0.584 0.040 0.241 0.169 0.128 0.038 0.089 0.029 0.070 0.006 0.007 0.016 0.375 0.012 0.205 0.134 0.238 0.003

(0.290) (0.005) (0.081) (0.024 (0.018 (0.019 (0.010 (0.013 (0.036 (0.006 (0.005 (0.016) (0.086) (0.009) (0.016) (0.040) (0.036) (0.003)

0.014 0.222 0.322 o.o34 0.000 0.056 o.335 0.090 0.116 0.000 2.457 0.318 0.007 0.444 0.251 0.017 0.253 0.281

(o.010) (0.030) (0.078) (0.014) (0.000) (0.025) (0.177) (0.023) (0.039) (0.000) (0.567) (0.076) (0.005) (0.079) (0.054) (0.017) (0.044) (0.075)

aSE, standarderror.

routes, with 69% + 4.4 of ravens and 70% + 7.1 of crows observedwithin

100 m. The proportionof crowsor ravensthatoccurred in pairs,suggesting possible breeding status,didnotdiffersignificantly bylanduse,subregion, or route (P > 0.05). American

Crow

In general,densitiesof AmericanCrowsalongruralandurban/suburban surveyroutesdidnot differsignificantly (F = 1.76; df = 1,110; P = 0.19). If route 11, an open agricultural areawith unusually highnumbersof crows (Table1) wasexcluded,however,crowsweresignificantly moreabundant alongurban/suburban surveyroutesthanalongthe remainingruralroutes (F = 10.9; dr= 1, 103; P < 0.01; Figure3). Whentestedagainstrandom route effects (see Methods), the number of crows in rural and urban/

suburban areasdidnot differsignificantly (F = 0.39; df = 1, 15; P = 0.54). Densities of crowsalongsurveyroutesdidvarysignificantly bysubregion

(F = 50.5; df = 2, 110; P < 0.0001),withnumbers increasing significantly fromthe outercoastto the bayshoreto the interior(P < 0.01). If route11, an interior route with unusuallyhigh numbersof crows (Table 1) was excluded,densitiesin bayshoreand interior locationsdid not differ (P > 207

COMMON

RAVEN AND AMERICAN

CROW IN THE SAN FRANCISCO

BAY AREA

0.2

ß

CommonRaven

[•] AmericanCrow 0.1

0.0

Rural

Urban/Suburban

Figure 3. Mean numberof Common Ravensand AmericanCrows observedper kilometerof surveyroute by ruralor urban/suburban land usein the San Francisco Bay area. Valuesfor the AmericanCrow excluderoute 11 (seetext). Error bars, standard errors.

0.99; Figure4). Despitesignificantlylowerdensitiesof crowsalong outercoastroutes,differencesby locationwere not significantly greaterthan the estimatedrandom variationacrossthe region (randomeffectsF -- 1.54; dr=2, 15; P -- 0.25). Land use and subregioninfluencedthe numberof crowson each routeindependently(F = 0.13; df = 2, 112; P > 0.87). Common

Raven

Densitiesof Common Ravenswere significantlygreater along survey routesin urban/suburbanthan in rural areas (F -- 5.75; df -- 1, 107; P =

0.02; Figure3). In contrast,the highestnumbersof ravensoccurredin the pastoralzoneof PointReyesNationalSeashore(route1; Table 1, Figure1). Land-useeffectsdidnot differsignificantly from estimatedrandomvariation in the region(randomrouteeffectsF = 0.70; df -- 1, 16; P -- 0.41). Ravendensitiesvariedsignificantly by subregion(F -- 18.66; df = 2,107; P < 0.0001), with numbersincreasingsignificantly frominteriorto bayshore to outer-coastsurveylocations(P < 0.05; Figure4). The test for random route effectsrevealeda significantcontrastbetweenthe outer-coastroutes and the bay shoreor interior(F = 5.12; df -- 1, 15; P -- 0.04). Therefore, higherconcentrations of ravenscharacterize the outercoast.Landuseand subregioninfluencedthe numberof ravenson eachrouteindependently (F = 0.003; df = 2, 108; P > 0.95). Other SurveyData Numbersof ravensand crowsobservedalong 15 BBS routesin the San FranciscoBayarearevealdistribution patternssimilarto thosesuggested by ourroadsurvey,includingdramaticabundance variationfromrouteto route (Table2). BBS data suggestsignificantoverallannual increasesin both ravens(5.16% + 0.15; P < 0.001) and crows(3.21% + 0.13; P < 0.001). 208

COMMON

RAVEN AND AMERICAN

CROW IN THE SAN FRANCISCO

BAY AREA

0.2

ß

CommonRaven

[•] AmericanCrow (1)

13. 0.1

0.0

Outer

Bay

Coast

Shore

Interior

Figure4. Mean numberof CommonRavensand AmericanCrowsobservedper kilometerof surveyroute in outer coast,San FranciscoBay shore, and interior subregions in the San FranciscoBay area. Valuesfor the AmericanCrow exclude route 11 (see text). Error bars, standarderrors.

BBS trendsvary greatlyacrossthe region,however,suggesting effectsat smallerscales(Table2). BBS dataimplystrongincreasesin the numberof ravensalongthe southernoutercoastof the studyarea and in centralNapa County.They suggestsignificantincreasesin crowsnear Santa Rosa and alongpart of the outerSonomacoast.Crowsapparentlyincreasedalong route 203 in the eastern interior of Contra Costa and Alameda counties

from1972 to 1991 butdeclinedalongnearbyBBSroute303 from 1992 to 1999 (Table2, Figure2). Inspectionof scatterplots for all individualroutes, however,suggested that suchdifferences may dependon the numberof yearssurveyed,becauselinearslopesmeasuredoverperiodsof lessthan 810 yearsmay contrastwith longer-termtrends. CBC data reveal significantoverall annual increasessince 1950 in numbersof both ravens(6.57% + 0.12; P < 0.001) and crows(3.31% + 0.05; P < 0.001), increasesthat accelerated duringthe 1980s and 1990s (ravens:7.23% + 0.16, P < 0.001; crows: 5.02% + 0.08, P < 0.001). Patternsof variationin abundanceby CBC circleare generallyconsistent with road-survey and BBS resultsand suggestsubstantial localvariationin numbersof both ravensand crows(Tables3 and 4). In neitherBBS or CBC resultsdidwe find significantcorrelations,positive or negative,betweenthe two species'abundancesor trends(P < 0.05), suggesting theirdistributions are independent.In contrast,our roadsurveys implya significantinverserelationshipbetweenravenand crowabundances (r -- -0.55, P < 0.05), suggesting theirdistributions are complementary. For example, raven and crow numbersvaried dramaticallyamong urbanized routes,with high numbersof ravensin San Franciscobut relativelyfew crows,and surprisingly low numbersof ravensalongthe urbanizedcorridor 209

COMMON

RAVEN AND AMERICAN CROW IN THE SAN FRANCISCO BAY AREA

Talkie 2 BreedingBird SurveyTrendsand Mean Numberof Birdsper Survey Route for the Common Raven and American Crow in the San FranciscoBay Area American Crow BBS

route a 14 15 16 71 83 172 186 187 189 193 194 202 203 303 319

Years

Period 1968-2000 1969-2000 1968-2000 1971-2000 1975-2000 1972-2000 1972-2000 1972-1988 1972-2000 1972-2000 1972-1995 1972-1997 1972-1991 1992-1999 1992-1996

Trend

surveyed(%peryear) b 31 27 18 24 25 19 28 14 20 25 23 26 20 7 5

-3.27 -11.12 6.80 1.82 1.34 -10.92 9.07** 12.30'* 3.04 19.89' -1.40 0.12 13.07'* -8.13'* 0.00

Mean birds

Common Raven Trend

perroute (%peryear) b 0.55 2.52 6.39 20.75 17.18 4.05 52.50 24.78 25.30 1.12 0.07 29.54 13.30 124.43 0.00

2.33** 10.00 19.92 6.45 0.36 0.00 0.32 0.00 9.42 -5.40** 18.10' 10.93'* -6.83 6.75 37.02*

Mean birds

perroute 20.48 1.81 1.17 7.83 7.65 0.05 1.86 0.21 0.45 8.88 0.52 4.81 0.15 6.43 18.20

aSeeFigure2 for location.

bLinear trendsignificant at *P < 0.05, **P < 0.01.

of easternMarin Countywhere crowswere relativelyabundant(routes13 and 14, Table 1). Compositebreedingbirdatlases fortheregionreflectdistributions thatare generallyconsistent with the road surveysand BBS (Figures5 and 6). We emphasizethat atlas data for the bay area representa wide range of samplingperiodsdatingbackasfar asthe late 1970s andshouldtherefore be interpretedwithconsiderable cautionandcarefulcomparisons withother availabledata. BBAs indicateCommon Raven concentrations along the outercoast,particularly in the PointReyesareaandalongthe SanFrancisco and San Mateo coast,in urbanizedareasalongthe southernSan Francisco Bayshoreline,in somehigherelevations of easternContraCosta,Alameda, and Santa Clara counties,and in parts of Napa county.Such "concentra-

tions"are basedon a greaterfrequencyof 5-km2 blockswith breeding ravens,ratherthangreaterravenabundances per se. However,broadareas with breedingconfirmedover many adjacentblocksare likelyto support more ravensthan areaswherebreedingis confirmedin only a few blocks. Breedingravensappear to be relativelyscarcein large areas of central Contra Costa, Alameda, and Santa Clara countiesand in relativelylow numbersin centralSonomaCounty.BBAsrevealthat the AmericanCrow's breedingdistribution is relativelycontinuousin mostbayshoreand interior areas(althoughhabitatis not necessarily saturated) butsparseor brokenin manypartsof the outercoastaldrainage(Figure6). In additionto resembling 210

COMMON RAVEN AND AMERICAN CROW IN THE SAN FRANCISCO BAY AREA

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