The Blackpoll Warbler in California

THE BLACKPOLL WARBLER IN CALIFORNIA

Guy McCaskie

INTRODUCTION

The AmericanOrnithologists' Union (1957) indicatesthe Blackpoll Warbler(Dendroica striata) migrates from its extensivebreeding grounds in Alaskaand Canadato its winter rangein SouthAmerica via the West Indies, and returns by the sameroute. The only record cited for the southwestern United States and northern Mexico is an accidental occur- rence in New Mexico (Fort Webster). In recentyears the Blackpoll Warblerhas been found regularly in smallnumbers in California.A total of 238 had been reportedby the end of 1969, andall but one of these werereported during the last eightyears. The vastmajority (93%) oc- curredduring the fall (late Augustto mid November)withonly 17 being reportedin the late springand summer(mid May to earlyAugust). The peak occurredbetween 24 Septemberand 8 October,with 50% of the autumnrecords falling within thesetwo weeks(fig. 1). An analysisof the records is therefore in order.

SPRING RECORDS AND DISCUSSION

Themajority of the few springrecords are from the SoutheastFarallon Island,San FranciscoCounty (9), and Point Reyes,Marin County(6), but there is one record for Point Loma, San DiegoCounty (21 June 1966), and anotherfor ImperialDam, ImperialCounty (15 May 1955). All the springbirds have been identified as to sex(most by plumage), and all were malesexcept three from the FatalIons(22 June 1965, 3 June 1969, and 1 August1969), onefrom PointReyes (21 June1969), andthe onefrom PointLoma. Thishigh percentage (70%) of malescan- noteasily be explained, but it maybe theresult of dealingwith a sample of: insufficientsize. Tenaza(1967) reportson the conditionof the testesof the malescollected on the Farallons,which indicated the birds were in breedingcondition. Thisspecies is a late springmigrant in the east. Lowery(1960) states it occursin Louisianabetween 13 April and 13 May; Bull (1964) re- portsit passingthrough the New York areabetween early May andmid Calif.Birds 1: 95-104,1970 95 THE BLACKPOLL WARBLER IN CALIFORNIA

7ø1

I0-

FIGURE 1. Seasonalpattern of Black- FIGURE 2. Distributionby counties pollWarbler records in California. of fallBlackpoll Warblers in California.

June,with the maximum numbers present in midMay; Gabrielson and Lincoln(1959) indicate it arrives in Alaskaduring the last week of May, andreport eggs found on 10June. In allprobability birds should be on theirbreeding grounds by June if theyare to nestsuccessfully. The late- ness(70% in Juneand later) of theCalifornia spring records indicates these birds cannot reach the breedinggrounds in time to nest success- fully,if indeed,they ever do reach the nesting grounds. In springBlackpoll Warblers appear to be muchmore numerous alongthe central coast of Californiathan farther south (88% of there- cordsare from the FatalIonsand Point Reyes). This would suggest the birdsare not followingthe coast northward from the wintering grounds in SouthAmerica. They are conceivably lost individuals wandering west- wardafter having turned westward from theirnormal migration route at a latitudetoo far southto put theminto theirregular breeding range. 96 THE BLACKPOLL WARBLER IN CALIFORNIA

FALL RECORDS

The fall recordsare primarilyfrom alongthe coast,with single interiorrecords from Litchfield,Lassen County (23 September1961), MahoganyFlat in thePanamint Mountains, Inyo County (15 September 1961),Morongo Valley, San Bernardino County (31 October1965), and nearBard, Imperial County (27 October'1968). Alongthe coast mostof the records(82%) are from the well-worked areas of SanDiego, Montereyand Marin Counties, and the FarallonIslands, but thereare enoughadditional records to indicatethat individualsoccur along the entirecoastline (fig. 2). Whenthe San Diego area is compared with the otherwell-worked areas, it is evidentthat the BlackpollWarbler is more numerousalong the southerncoast of Californiathan farther north. Nineof the 12fall specimenshave been sexed, and both sexes appear to be equallycommon (five males and four females). Of the 58 birds aged(10 asspecimens and 48 by checkingthe skullossification of live birds)55 (95%)wereimmatures. The three exceptions were two banded on the FatalIons(31 August1968 and 23 October1969), and one bandedon PointReyes (27 September1968). Thirteen individuals from the coasthave been weighed, and the meanweight is foundto be 11.8 grams(range 9.5 to 14.7 grams,with a standarddeviation of 1.44). Forty-twoindividuals from the Farallonshave been weighed, and the meanweight is foundto be 11.4grams (range 9.3 to 14.0grams, with a standarddeviation of 0.95). The numberof recordshas steadily increased each fall sincethe 1961 bird was discovered,but there is still somevariance in the numberspre- senteach year (fig. 3). Thesteady increase is due to a numberof factors. Morepeople are aware of thefield marks of thefall plumagedBlackpoll Warblerand are specificallylooking for thesebirds at likelyconcentra- tion pointsalong the coast. The SoutheastFarallon Island has been mannedas an observatory and banding station since 1967 and permanent- ly mannedas such since 1968; the vast majority of thebirds landing there are seen,if not caught,and lost land birdsare no doubtattracted to it fromappreciable distances. There are also recently established permanent bandingstations on PointReyes (Point Reyes Bird Observatory)and PointLoma (home banding stations of AlanCraig and Virginia Coughran); numbersof BlackpollWarblers have been seen and/or captured in both of these areas. Bearingin mind the steadypost-1961 increase in the numberof ob- serverslooking for BlackpollWarblers, it is apparentin figure 3 that exceptionalnumbers were recorded in the fall of 1964 and 1969 (author 97 THE BLACKPOLL WARBLER IN CALIFORNIA

saw 19 and 14 individuals,respectively) while in the fall of 1963 this specieswas quite scarce(author saw only 3 individuals). Numbers occurringeach fall mayreflect breeding successes of the precedingsum- mer; in years when many young are raisedwe may have larger than normal numbersoccurring on the Californiacoast. Weatherconditions betweenCalifornia and the northernpart of the BlackpollWarbler's usual migratorypath may also influencethe number of individuals which reach the state. There are a numberof activeobservers concentrated along the coast, especiallyduring the fall, and this no doubthas a definitebearing on the number of records for that area. But this alone does not account for the high percentage(98%) from that area if BlackpollWarblers were evenly distributedthroughout California. The coastalways has a concentratingeffect on night migrants,including the BlackpollWarbler, sincethose individuals located over the oceanat dawnattempt to return to the coast,thus putting all the birdspresent in a ratherwide belt along the coastalwaters into a narrowcoastal land strip. However,aside from thisconcentrating effect, there is enoughfield work beingdone at inland localitiesto demonstratethat the speciesis truly extremelyrare away from the immediatevicinity of the coast.

DISCUSSION

BlackpollWarblers avoid most of the United Statesin fall (fig. 4). Birds from the westernportion of the breedingrange apparently head eastward,arriving on the Atlantic in the northeasternUnited States. Nisbet,Drury, and Baird(1963) presentedvery convincingevidence to supportthe theory that thesebirds stop to build up theirfat resources in New England,then fly non-stopover the water to South America. Murray (1965) arguedagainst this theory,and proposedthat the birds followed the coastal area southwardto the southeasternUnited States, then turnedtoward South America. It maywell be that someindividuals do fly non-stopfrom New Englandto South America,while others follow the coastsouthward for somedistance before headingout over the oceanto South America. Either way, the birdshave to make a trans- oceanicflight at somepoint alongthe route. It wouldappear that adultsare quite prevalent in samplesfrom New England,but the percentageof adultspresent diminishes farther south along the coast. A samplefrom Bermudahad a high percentageof adults (an indication these birds are not lost), but a samplefrom Michiganhas an unprecedentedhigh percentage of immatures.Nisbet 98 THE BLACKPOLL WARBLER IN CALIFORNIA

'70

FIGURE 3. Annual pattern of Black- FIGURE 4. Range of the Blackpoll poll Warbler occurrencein California Warblerwith the breedingrange shown with the springand fall recordsshown in black, the winter range shownshad- separately. ed, and the normalfall migrationroute indicated by heavy arrows and cross hatching.

et al. (1963) reported61% of 1,832Blackpoll Warblers at RoundHill in eastern Massachusettswere adults in 1962; 42% of 123 at Drumlin Farm in easternMassachusetts were adultsin 1961, and 64% of 54 were adultsin 1962; 18%of 55 in coastalNew Englandwere adults between 1959 and 1961; 19% of 78 at IslandBeach, New Jersey,were adultsin 1962;and 58% of 136 at Bermudawere adults in 1962. Murray(1966) reported9.6% of 658 at IslandBeach were adults in 1963,and 5.7% of 87 in Michiganwere adults. Murray(1966) indicatesthere is little variancein the weightsof BlackpollWarblers from the northeastern portion of theUnited States at a giventime in fall;butthat thereis a trendtoward an increase in weight in the latter half of the migrationperiod. He indicatesthe mean weight of a sample(85) of iramaturesfrom Michigan to be 12.7grams (range 99 THE BLACKPOLL WARBLER IN CALIFORNIA

11.2 to 17.9), a sample(552) of immaturesfrom New Jerseyto be 11.4 grams(range 8.8 to 21.9), and a sample(707) of immaturesfrom Massachusettsto be 12.1 grams(range 9.3 to 21.5). None of these birds shouldhave recentlycompleted long non-stopover-water flights. In CaliforniaBlackpoll Warblers are settinga regularpattern in their occurrences,and they are appearingin largeenough numbers to suggest that this regularoccurrence may be normal. If they are followinga definite migration route they would have to remain on it from the breedingrange to the winter range,and there shouldbe a scatteringof recordsalong its entire lengthto documentthis. It is then worth investi- gatingsome of the proposedways by which BlackpollWarblers may reach California.

RANDOM SCATTERING

If a small percentageof the populationof BlackpollWarblers scatter randomly from the breedinggrounds rather than follow the normal migrationroute, we couldexpect individuals to occuralmost anywhere. The numberspresent in any givenarea shoulddecrease functionally to the distancefrom the breedingrange due to a fanningout effect. The coastwill, however,always have a concentratingeffect, resultingin an increasednumber of recordsfrom that-area. The BlackpollWarbler must be one of the most numerous of the Parulidae. If the birds are reachingCalifornia by randomscattering we could reasonablyexpect a few individuals to occur anywhere outside their normal range and migrationroute, especiallywhen we considerthe numbersfound asfar from the normal rangeas San Diego. So far, the BlackpollWarbler has remained unrecordedin Washington,Oregon, and the southwestern states,except California, during the fall.

WEST COAST MIGRATION ROUTE

If someof the BlackpollWarblers from the northwesternportion of the breedingrange were to followa migrationroute south along the west coast,we would expecta high percentageof the recordsto comefrom thecoastal areas. The species would have to occurin thecoastal regions to the northof Californiain numbersequal to, or exceeding,those found on the Californiacoast, but, to date,there are no fall recordsfor Oregon or Washington.The birdswould also be expectedto proceedsouthward alongthe coast,but DeSante(pers. comm.) was unable to find a single individualduring his four months'stay in the CapeRegion of Baja 100 i-

Fall plumaged Blackpoll Warbler Dendroica striata, 19 September 1965 (left), and (right) an adult female, 21 June 1966. Both birds mist netted. Point Loma, San Diego. Photos b), Alan M. Craig

California, Mexico, in the fall of 1968, even thoughhe waslooking for vagrants. The speciesshould be more numerousin the Point Reyes- Farallon lsland area than in the San Diego area, sinceit is farther north, but the reverseappears to be true (52% of the recordsfrom thesethree localities are from SanDiego). Also, one would expectto encounteradults along any normal migration route, but most of the birds agedare iramatures, and there is a strongpossibility the three individualsreported as adults were also iramatures(ageing live birdsis subjectto error,especially in the late fall).

DIRECT LINE FLIGHT

Another possibilityis that someof the Alaskanbirds are making a direct over-waterflight from the breedinggrounds southeastward to the coastof California,and then proceedingonward to South America.This would explainthe lack of recordsfrom the areanorth of California. We would expect many of the California birds to be low on fat resources after the long flight. However, the samples(13 and 42) of Blackpoll Warblersfrom the coastof Californiahave mean weights falling between that of the Massachusettssample and the New Jerseysample, which would indicatethey had not madeany longnon-stop flights. 101 THE BLACKPOLL WARBLER IN CALIFORNIA

In addition,Blackpoll Warblers would be expectedto continueon the direct line course,thus crossing Mexico. Eisenmann(1955) refers to the only fall recordfor Mexico(Tehuantepec City, Oaxaca,on 19 October1896).

DRIFT

Paxton(1967) pointedout that manysuch vagrants as the Blackpoll Warblerappear on the coastwhen there are high pressureareas, with their accompanyingeast winds, centered over the northernGreat Basin. The northernGreat Basinis outsidethe normalrange of the Blackpoll Warbler,and there are few if any recordsfor that regionin fall. Any BlackpollWarbler being drifted toward Californiaunder theseweather conditionsis alreadyoutside its normalrange and off course.Drift no doubt helps someindividuals reach the west coast,but it is doubtful whetherthis is the solefactor involvedin placingthese birds there.

"MIRROR IMAGE" REVERSED MIGRATION

A numberof authoritieshave indicated a smallportion of the populationof somespecies heads in a direction180 ø to the appropriate directionfor the time of year. Lack(1963), Drury andKeith (1962), and Drury andNisbet (1964) haveall reporteddetecting reverse move- mentswith the aid of radar.Dolnik and Shumakov (1967) reportedtwo species(Scarlet Grosbeak Carpodacus erythrinus and BarredWarbler Sylvianisoria) they testedin Kramercages had a strongtendency to reorient in the reversedirection, aswell as the correct direction. Nisbet (1962) suggestedreversed migration was the means by whichsome of the southeastEuropean species reach Fair Isle (situatedoff the north coast of Scotland)in the fall. Baird,Bagg, Nisbet, and Robbins (1959) and Nisbet(1962) associatereversed migration with high temperatures in the fall, but Evans(1968) detectedthis type of movementfrom three birds beingoriented in Kramercages in normalBritish fall weather.A typical exampleof a speciesreaching California by reversedmigration would be the appearanceof TropicalKingbirds (Tyrannus melancholicus) in fall. Rabol (1969) discussedfour Old Worldwarblers, (Phylloscopus inornatus,P. proregulus,P. trochiloides,and P. borealis),which breed in northeasternEurope and northern Asia and migrate first eastward then southwardin fall (their migrationroute is analogousto thatof the BlackpollWarbler). He discussedthe appearanceof thesespecies in GreatBritain during the fall andconcluded that theirappearances were 102 THE BLACKPOLL WARBLER IN CALIFORNIA dueto reversedmigration in a westwarddirection by a part of the populationof eachspecies. If thissame phenomenon occurred in theBlack- poll Warblerpopulation we would expecta concentrationof records from British Columbia. Normally,Blackpoll Warblers migrate eastward (primarily ESE) and then southward(primarily SSE). If someindividuals were to migrateon a mirror imageof thisroute, that is, first westwardand then southward, they wouldconceivably reach the PacificCoast in California.(The plane of reflectionof thisimage would be on a north-southaxis). Continuing this mirror imagereversed migration route, the birdswould fly southward, with many followingthe coastfor somedistance before heading out overthe Pacific. This type of movementcould well accountfor the presenceof BlackpollWarblers on the coastof Californiaduring the fall, and could account for the fact that there are no records from the areas to the north or south of the state. This also accounts for the concentrationof recordsfrom the coastalareas and could be used to explain the variancein numbersbetween localities along the coast. Iramatures would be expectedto be predominantsince none would be expectedto surviveto repeatthe feat the followingyear.

SUMMARY

All the informationavailable on the 238 BlackpollWarblers recorded in Californiathrough 1969 is summarized.The majority of the birdsare iramaturesoccurring in the fall at coastallocalities. The normalfall migrationroute, and the conditionof the birdsfound along this route, are briefly discussed.Five proposedways by whichBlackpoll Warblers may reachCalifornia are indicated,and the theory that somefollow a route that is a mirror imageof the normalfall migrationroute is pro- posed.

ACKNOWLEDGMENTS

I wishto thankall thosepeople, too numerousto mentionindividual- ly, whohave submitted records of BlackpollWarblers to meas the South- ern PacificCoast Regional editor of AudubonField Notes. Theodore Chandik,one of the two MiddlePacific Coast Regional editors of. AudubonField Notes, helped in evaluatingthe records from that areaof California.Fred Sibley,Henry Robert, and Jon Winter,all of Point ReyesBird Observatory,permitted use of datataken by them. Waldo Abbottof theSanta Barbara Museum of NaturalHistory, and Eugene A. 103 THE BLACKPOLL WARBLER IN CALIFORNIA

Cardiff of the San BernardinoCounty Museum of Natural History suppliedinformation on specimensdeposited in theirmuseums. I am particularlygrateful to JamesBaird, Alan Craig,Pierre Devillers, and JosephR. Jehl,Jr. whoread drafts of thispaper and offered valuable commentsand suggestions.

LITERATURE CITED

American Ornithologists'Union. 1957. Check-listof North Americanbi•ds. Fifth ed. Amer. Ornithol. Union, Baltimore. Baird, J., A.M. Bagg,I. C. T. Nisbet, and C. S. Robbins. 1959. Operationre- covery- report on mist-nettingalong the Atlanticcoast in 1958. Bird-Banding 30:143-171. Bull, J. 1964. Birds of the New York area. Harper & Row, New York, Evanston, and London. Dolnik, V. R. and T. I. Blyumental. 1964. Bioenergeticsof bird migration. Us- pekhi SovremennoiBiologii 58: 280-301 (in Russian). Drury, W. H., Jr. and J. A. Keith. 1962. Radar studiesof songbirdmigration in coastalNew England. Ibis 104:449-489. Drury, W. H., Jr. and I. C. T. Nisbet. 1964. Radar studiesof orientationof song- bird migrantsin southeasternNew England.Bi•d-Banding 35:69-119. Eisenmann,E. 1955. Status of the Blackpoll, Bay-breasted,and Connecticut Warblers in Middle America. Auk 72:206~207. Evans,P. R. 1968. Reorientationof passefinenight migrants after displacementby the wind. British Birds 61:281-303. Gabrielson, I. N. and F. C. Lincoln. 1959. Bi•ds of Alaska. The StackpoleCom- pany, Harrisburg,Pennsylvania and the Wildlife ManagementInstitute, Wash- ington, D.C. Lack, D. 1963. Migration acrossthe North Sea studiedby radar: Part IV. Autumn Ibis 105:1-54. Lowery, G. H., Jr. 1960. Louisiana birds. Louisiana State University Press. Murray, B. G., Jr. 1965. On the autumnmigration of the BlackpollWarbler. Wil- son Bull. 77:122-133. Murray, B. G., Jr., 1966. BlackpollWarbler migration in Michigan. The Jack- pine Warbler 44: 23-29. Nisbet, I. C. T. 1962. South-eastrarities at Fair Isle. British Birds 55:74-86. Nisbet,I. C. T., W. H. Drury, Jr., and J. Baird. 1963. Weight4ossduring migration. Bird-Banding 34:107-159. Paxton, R. O. 1967. Oasesfor migrantson outer Point Reyes, 1963-66. Point Reyes Bird Observatory Newsletter 8:24-25. Rabol, J. 1969. Reversedmigration as the causeof westwardvagrancy by four Phylloscopus warblers. British Birds 62:89-92. Tenaza, R. R. 1967. Recent records of land birds from South Farallon Island, California. Condor 69:579-585.

San Diego Natural History Museum, Balboa Park, San Diego, California 92112. 104