north and another two birds were foraging 75 m to the south. We infer that the .... Latest news on rare and interesting birds in the Netherlands and the Western ...
NOTES LATE NESTING GNATCATCHER
OF THE
CALIFORNIA
MICHAEL A. PATTEN, Tierra Madre Consultants,1159 Iowa Avenue, Suite E, Riverside,California 92507 KURT F. CAMPBELL, 600 Central Avenue #118, Riverside,California 92507
The nominatesubspecies of the CaliforniaGnatcatcher(Polioptila californica) has recenfiy been listed as a threatenedspeciesunder the federal Endangered SpeciesAct (Salata 1993). Atwood (1992) estimatedthe subspecies' populationin the United States, as of i991, at between 1811 and 2291 pairs. Critical to management andprotectionof the CaliforniaGnatcatcher isa clearunderstanding of its nestingphenology. On 30 July 1991, at Reef Point, CrystalCove State Beach, Orange County, California,we discovered a CaliforniaGnatcatchernestcontainingfour naked,blind nestlingsthat had hatchedabouttwo daysearlierand were gapingactively.On 15 August1991, the nestwasempty,stillin goodshape,butslowlybeingplunderedby ants.A pair (i.e., adultmale and female)and a juvenilewere seenabout50 m to the north and anothertwo birdswere foraging75 m to the south.We infer that the youngprobablyfledgedon 12 or 13 August.In addition,RayVizgirdas (pers.comm.) reporteda fledgingdate of 25 Augustfrom a nest at Oak CanyonNature Center, OrangeCounty. Theseobservations extendthe lateextremedatefor nestingin thisspeciesto midto lateAugust,a fullmonthto sixweeksafterWoods'(1949) latestdateof 12 July(a nestcontaining"fourwell-grownyoung")and later than the vague"throughJuly" reportedby Atwood(1990). The latestof Roach's(1989) 34 nestswasin the first week of July,while Bontrager(1991) foundthat nestingactivityin southernOrange Countywasconcludedby mid-July. We examined 101 data cards for museumegg sets, mostly at the Western Foundationof VertebrateZoologyandthe San BernardinoCountyMuseum,with a few from the Museum of Vertebrate Zoology, the San Diego Natural History Museum,and the Santa BarbaraMuseumof Natural History.The latestestimated dutch completiondate was 10 July (for a nest in coastalSan Diego County, California),implyinga fiedgingdateof about7 August. The mean estimatedclutchcompletiondate of the sets was 5 May (standard deviation-- 21.7 days),figuredby subtracting 2 daysfromthe collection datefor sets in whichincubationwasslight,5 daysfor setsin whichembryosweretakingshape, 9 daysfor setsabouthalf incubated,and 12 daysfor setsin whichincubationwas advanced.The majorityof eggsetswerecollectedwithina few daysof the finalegg beinglaid,with 47 (43%) labeled"fresh"and 29 (27%)labeledasincubation"slight." Historiceggcollections, however,do not constitutea randomsample.Whereasearly datesare well represented,late datestend to be representedvery poorly (McNair 1987, L. F. Kiff in litt.). We suspectthat CaliforniaGnatcatchers nestinto Augustmorefrequentlyalong the immediatecoast, where daytimetemperaturesin summeraveragecoolerthan they do fartherinland. We thankEugeneA. Cardiff(SanBernardinoCountyMuseum),TamarDanufsky (SantaBarbaraMuseumof NaturalHistory),Ned K. Johnson(Museumof Vertebrate Zoology),LloydF. Kiff (•VesternFoundationof VertebrateZoology),andPhilipUnitt (SanDiegoNaturalHistoryMuseum)for allowingusaccess to CaliforniaGnatcatcher 110
Western Birds 25:110-111, 1994
NOTES
eggset data in their care. RichardA. Erickson(LSA Associates), Stephend. Myers (TierraMadreConsultants), andRayVizgirdas(U. S. FishandWildlifeService)kindly providednestinginformationthey havegatheredin Orangeand westernRiverside counties.RichardA. Erickson,LloydF. Kiff, andPhilipUnitt revieweda draftof this noteandgreatlyimproveditscontent. LITERATURE CITED
Atwood, J. L. 1990. Statusreviewof the CaliforniaGnatcatcher(F'olioptilacalifornica). ManometBird Observatory,P.O. Box 1770, Manomet,MA 02345.. Atwood,J. L. 1992. A maximumestimateof the CaliforniaGnatcatcher'spopulation size in the United States. W. Birds 23:1-9.
Bontrager,D. R. 1991. Habitatrequirements, homerange,and breedingbiologyof the California Gnatcatcher(Polioptila californica)in south Orange County, California.Unpublishedreport;copyin Van Tyne Library,Universityof Michigan, Ann Arbor,MI 48109. McNair,D. B. 1987. Eggdata slips--arethey usefulfor informationon eggølaying dates and clutch size? Condor 89:369-376.
Roach,J. D. 1989. The influenceof vegetationstructureand arthropodabundance on the reproductive success of CaliforniaBlack-tailed Gnatcatchers Polioptila rnelanuracalifornica.M. S. thesis,San Diego State Univ. Salata,L. 1993. Endangeredand threatenedwildlifeand plants;determinationof threatened status for the coastal California Gnatcatcher.Federal Register 58:16742-16757.
Woods,R. S. 1949. Polioptila rnelanuracalifornicaBrewster:Black-tailed Gnatcatcher,in Life historiesof NorthAmericanthrushes,kinglets,andtheirallies(A. C. Bent, ed.), pp. 374-381. U.S. Natl. Mus. Bull. 196. Accepted 7 February 1994
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