NOTES

THE WESTERN BLUEBIRD AS HOST FOR THE BROWN-HEADED COWBIRD: A NEW RECORD FROM CALIFORNIA

MELANIE ALLEN TRUAN, Departmentof Wildlife,Fish & ConservationBiology, Universityof California, 1 ShieldsAvenue, Davis, California95616

Nest recordsfor North Americasuggest that broodparasitism by Brown-headed Cowbirds(Nlolothrus ater) on cavity-nestingspecies is relativelyrare. Indeed, a summaryby Friedmannand Kiff (1985) indicatedthat cavitynesters constitute only 8% of known hosts of the cowbird. Of these, only the ProthonotaryWarbler (?rotonotaria citrea) was consideredto be a major host. In bluebirds(Sialia spp.), all cavity nesters,documented incidences of cowbird parasitismare extremelyrare (Gowatyand Plissner1998). Most involvethe Eastern Bluebird(S. sialis).Power and Lombardo(1996) reportedonly four cases of cowbird parasitismof the Mountain Bluebird(S. currucoides),with no accountsof cowbirds raisedto fiedging.Campbell et al. (1997) foundthat onlytwo of 767 WesternBluebird (Sialia mexicana)nests in BritishColumbia contained cowbird eggs, and neither producedfledgling cowbirds. Bendire (1893) and Friedmannand Kiff (1985) alluded to occasionalcowbird parasitism of the WesternBluebird but did not indicatethat the WesternBluebird ever fledged cowbirds. Recently, however, I discoveredthree cases of parasitismof the WesternBluebird, one of whichresulted in a cowbirdfiedging and associatedbluebird mortality. The observationsoccurred within the contextof a studyof artificialnest boxes conductedalong lower Putah Creek, Yolo and Solano counties,in California's SacramentoValley. This fragmentedriparian habitat, surrounded by agriculturaland grazinglands, is frequentlyinvaded by cowbirdsduring the breedingseason (Gaines 1980). On 2 June2001, ! discovereda cowbirdegg with threeWestern Bluebird eggs in an artificialnest box (labeledE3.4). The pair of bluebirdshad alreadyfledged five chicksfrom a nearbynest box (E3.0). The parasitizednest was in a standardNorth AmericanBluebird Society side-opening box with dimensionsof 26 x 15 x 15 cm, with an entrancehole 4 cm in diameter.This box had been retrofitted to hangfrom a treebranch via a wirehook (Purvis 2000) andwas hung at a heightof approximately 3 m abovethe ground,facing southeast, in a matureInterior Live Oak (Quercus wislizenii) treelocated in a 1-hectarefragment of oaksavanna adjacent to the riparian corridor. On 11 June, I discoveredthat the female bluebirdhad laid anotheregg in her clutch--suggestingthat she had not been deterredby the presenceof the cowbird egg--and had begunincubation. The malebluebird remained nearby, tending to the juvenilesfrom the firstbrood. None of the eggsin the parasitizedclutch hatched-- despitedbeing incubated to term--and sowere deposited with the nestin the Museum of Wildlifeand FisheriesBiology, University of California,Davis (museum number WFB-4968). The samepair of bluebirdsthen initiateda thirdclutch in anothernearby nestbox (E3.7). This clutch also failed to hatch.There was no embryonicdevelopment in any of the eggsfrom either clutch. In 2002, afteragain successfully fiedging a firstclutch from boxE3.0, the female bluebirdagain laid a secondclutch in E3.4 that was againparasitized by a cowbird. Sincecowbirds typically have strongsite fidelity(Dorst 1971, Lowther 1993), it is possiblethat the nestwas parasitized by the samecowbird as in the previousyear. I do not know whetherthe cowbirdremoved any hosteggs at the time of parasitism,a commonpractice among cowbirds (Lowther 1993), but one bluebirdegg did disap- pearafter incubationhad begun,resulting in a clutchof two bluebirdeggs and one

WesternBirds 34:111-113, 2003 111 NOTES cowbirdegg. This time, two of the eggshatched: the cowbirdegg on or about16 June,and one of thebluebird eggs on or about17 June.The otherbluebird egg failed to hatchand containedno embryonicdevelopment. The cowbirdwas about one-third larger than the bluebirdchick at hatchingand quicklyoutpaced it in growthand development.On 26 June,the cowbirdfledged whilethe bluebirdremained behind in the nestbox. Its eyeshad still not opened-- thoughbluebirds' eyes typically open at aroundseven to eightdays of age(Guinan et al. 2000)--and it weighedonly 18.0 grams,compared to an averageweight of 22.4 gramsfor bluebirdnestlings of comparableage from the same location (unpubl. data). Oncefledged, the cowbirdreceived the majorityof the hostpair's attention. In a timedobservational study of parentalprovisioning, the cowbirdfledgling was fed five timesin 22 minuteswhile the bluebirdnestling was fed only once.This observational periodwas cut short by attemptedpredation of the cowbirdby a WesternScrub Jay (Aphelocomacalifornica). The jay anda companion(probably its mate) were driven off by the mobbingbluebird pair, which then remainedwith the groundedcowbird fledgling,neither making any attemptto visitthe nestlingbluebird. Twodays later, the fledglingcowbird had disappeared and the nestling bluebird was dead.I observedthe malebluebird looking into the boxcontaining the deadnesfiing on one occasion.As in the previousyear, the bluebirdsinitiated a third clutchin box E3.7. Thisclutch was also parasitized with a cowbirdegg but was abandoned when the eggsfailed to hatch.No embryonicdevelopment was detected in anyof the eggs. Of all NorthAmerican bird species, few haveexperienced a greaterexpansion in distributionthan hasthe Brown-headedCowbird (Campbell et al. 2001). Further- more, there is evidencethat cowbirdscolonizing parts of westernNorth America parasitizedmany species that had littlerecent experience with them (Friedmanand Kiff 1985). Indeed,since the 1920s,the numberof speciesthat have been known to serve as hosts for Brown-headed Cowbirds has increased from 157 to over 220 (Campbellet al. 2001). Brown-headedCowbirds have been very successfulin colonizingCalifornia's CentralValley, most likely because of conversionof nativelandscapes to agriculture (Laymon1987). As their populationsincrease, cowbirds may continueto expand their suiteof preferredhosts to includeformerly underexploited species such as the Western Bluebird. My sincerethanks to JudithA. Guinan,who locatedpublished accounts of brood parasitismin WesternBluebirds, and to AndrewEngilis, Jr., JohnM. Eadie,and Alex Cruz for technicalsupport and helpfulreview of the manuscript.I acknowledge especiallythe excellentwork of my field assistantsMichael T. Atamian, Andrew R. Grant, Ian T. Taylor,and JonathanH. Widdicombe.

LITERATURE CITED

Bendire,C. 1893. The Cowbirds.U.S. NationalMuseum, Annual Report of the Boardof Regents,Washington, D.C. Campbell,R. W., Dawe, N. K., McTaggart-Cowan,I., Cooper,J. M., Kaiser,G. W., Stewart,A. C., and McNall,M. C. E. 1997. The Birdsof BritishColumbia, vol. III. Univ. of BritishColumbia Press, Vancouver. Campbell,R. W., Dawe,N. K., McTaggart-Cowan,I., Cooper,J. M., Kaiser,G. W., Stewart,A. C., and McNall, M. C. E. 2001. The Birdsof BritishColumbia, vol. IV. Univ. of BritishColumbia Press, Vancouver. Dorst, J. 1971. The Life of Birds.Weidenfeld and Nicolson,London. Friedman,H., andKill, L. F. 1985. The parasiticcowbirds and their hosts. Proc. W. Found. Vert. Zool. 2:226-304.

112 NOTES

Gaines,D. 1980. The valleyriparian forests of California:Their importanceto bird populations,in RiparianForests in California:Their Ecologyand Conservation (A. Sands,ed.), pp. 57-73. Agric.Sci. Publ.4101, Univ. of Calif., Berkeley. Gowaty,P. A., and Plissner,J. H. 1998. EasternBluebird ($ialia sialis),in The Birds of North America (A. Poole and F. Gill, eds.), no. 381. Acad. Nat. Sci., Philadelphia. Guinan, J. A., Gowaty, P. A., and Eltzroth, E. K. 2000. Western Bluebird($ialia rnexicana),in The Birdsof North America (A. Poole and E Gill, eds.),no. 510. BirdsN. Am., Philadelphia. Laymon,S. A. 1987. Brown-headedCowbirds in California:Historical perspectives and managementopportunities in riparianhabitats. W. Birds18:63-70. Lowther,P. E. 1993. Brown-headedCowbird (Molothru$ ater), in The Birdsof North America (A. Poole and E Gill, eds.),no. 47. Acad. Nat. Sci., Philadelphia. Power,H. W., and Lombardo,M.P. 1996. MountainBluebird ($ialia currucoide$), in The Birds of North America (A. Poole and F. Gill, eds.), no. 222. Acad. Nat. Sci., Philadelphia. Purvis,D. 2000. Purvis'hanging nestbox and box lifter.Bluebirds Fly! 6(1 & 2):12. Calif. BluebirdRecovery Program, P.O. Box 39, Somerset,CA 95684.

Accepted $ May 2003

Western Field Ornithologists' Field Trip Birding San Bias, Mexico, January 17-24, 2004 SanBias is a premierebirding spot on Mexico'swest coast. It becamefamous in the 1960s and '70s followingthe publicationof PeterAlden's guide and a Christmasbird countthat flirtedregularly with the 300-speciesmark. It remainsa prime destination for birdersbecause of the town's charm as a smallfishing village combined with its proximityto habitatsranging from ocean to mangroveswamp, dry tropical forest, dry scrub,tropical deciduous forest, and pine-oak forestin the nearbymountains. This range of habitatssupports many endemic speciessuch as the Rufous-bellied Chachalaca,Elegant Quail, Mexican Parrofiet, Eared Poorwill, Golden-crowned Emerald, Mexican Woodnymph, BumblebeeHummingbird, Citreoline Trogon, Golden-cheekedWoodpecker, White-striped Woodcreeper, Black-throated Magpie Jay, San BlasJay, Purplish-backedJay, SinaloaCrow, SpottedWren, SinaloaWren, Happy Wren, Red Warbler,Red-headed Tanager, Rusty-crowned Ground-Sparrow. Another specialtygroup are mangrovespecies such as the Boat-billedHeron, Snail Kite, Limpkin,Mangrove Cuckoo, Mangrove Vireo, MangroveSwallow, and Man- grove Warbler. The group sizewill be limitedto eight. The trip is open to a wide range of bird- watchingskill levels, and the pace will be moderate.There will be eveningpresenta- tions by the leadersand local biologistson the natural historyof the area and ornithologicalresearch that is taking place in west Mexico. Costswill range from $1550 to $2150 from PuertoVallarta. For moreinformation, please write to Tom Ryanat [email protected]. This fieldtrip is open to both membersand nonmembers of WFO.

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