J. Field Ornithol., 67(1):119-140 TIMING OF MIGRATION AND STATUS OF VIREOS (VIREONIDAE) IN LOUISIANA J. V. P•MSEN,JR., STEWN W. C•qDIVV,•X. ND DONN^ L. DITr• Museum of Natural Science Louisiana State University Baton Rouge,Louisiana 70803 USA Abstract.--Data are presentedon the statusof the vireos (Vireonidae) that occur in Louisi- ana. Basedprimarily on year-round surveysat coastalsites in southwesternLouisiana and censusesat an inland site in central Louisiana,data on timing of migration are presented for White-eyed (Vireo g'riseus),Solitary ( V. solitarius),Yellow-throated ( V. flavifrons), Phila- delphia (V. philadelphicus),and Red-eyed(V. olivaceus)vireos. In general,migrant vireosin spring are much more common on the coastthan inland, whereasthe reverseis true in fall. Bell's Vireo (V. belli•) has been recorded 12 times in southern Louisianabetween 4 November and 22January; this representsa substantialportion of all late fall/early winter recordsfrom eastern North America. No documented recordsexist of Yellow-throatedVireo from early November to early March for Louisiana,or probablyelsewhere in the Gulf Coast region, despitenumerous published sight records.Warbling Vireo (V. gilvus) has declined dramat- ically as a breeding speciesin Louisianafor unknown reasons;there have been almostno reportsof breeding birdsfor three decades.Two specimensof the subspeciesV. g. swainsonii from western North America have been collected in Louisiana, one of which is the first winter specimen of the speciesfor eastern North America. One specimen of White-eyed Vireo from Louisianais V. g'riseusmicrus;, this representsthe first record of this taxon north of southern Texas. One of the three Louisiana specimen records for Bell's Vireo is of a subspecies(V. b. medius)from the southwesternUSA and north-centralMexico. One speci- men record of SolitaryVireo from Louisianais a distinctivesubspecies (V s. plumbeus)from western North America. A specimenof Yellow-greenVireo (V flavoviridis) from Louisiana evidently is just the seconddocumented record for the eastern United States.Two of 12 Louisianaspecimens of Black-whiskeredVireos (V. altiloquus)are of the nominate subspecies (from the Greater Antilles), for which there was only one previousrecord for the United States. FECHADE MIGRACION Y ESTADODE LOS VIREOS (VIREONIDAE)EN LOUISIANA Sinopsis.--Sepresentan datos sobre el estatusde los vireosen Louisianae informaci6nsobre el per5odo de partida migratorio para estasaves. El trabajo se fundament6 en la bfisqueda de estosp•tjaros, a trav6sdel afio, en 1ocalidadesde la costaen el suroestede Louisianay censosen la parte central del estado.Los datos se tomaron para Vireogriseus, V. solitarius, V. flavifrons, V. philadelphicusy V. olivaceus.Por lo general, durante la primaveralos men- cionadosresultaron m• comunesen la costa queen las partes interiores del estadoy 1o contrario se encontr6 durante el otofio. V bellii fue encontrado en 12 ocasiones en el sur de Louisianaentre el 4 de noviembrey el 22 de enero. Esto representauna porci6n sustancial de todoslos registrosde esta ave tarde en el otofio y temprano en el invierno, en la parte este de Norte Am6rica. No existen registrosdocumentados de V.flavifrons desde principios de noviembre a principios de marzo en Louisianay probablemente1o mismo aplique a la costadel golfo. V. gilvusha descendidodr•ticamente como especieresidente de Louisiana. Dos individuosde V. g. swainsonii,del oeste de Norte Am6rica, han sido coleccionadosen Louisiana,uno de los cualeses el primer informe de la especiepara el estede Norte Am6rica, durante el invierno. Un individuode V. griseusmicrus representa el primer registrode esta raza al norte del sur de Texas. Uno de los tres informes de V. belliipara el estado,corres- ponde al de la subespecieV. b. medius.Adem•, uno de los individuosde V. solitariuscor- respondea la subespeciede V. s. plumbeus.Por su parte un individuo de V.flavoviridis es el segundoinforme documentado para el este de los EstadosUnidos. Dos de los 12 registros de V. altiloquuspertenecen a la subespeciede las Antillas Mayores,del cual habia un solo informe para los EstadosUnidos. 119 120] J.V. Reinsen,Jr. et al. J.Field Ornithol. Winter 1996 Timing of migration is a critical componentof a bird species'biology. Although concentrationsof migrating birds in the Gulf Coastregion of the southeasternUnited Statesare perhapsgreater than anywhereelse in North America (Lowery 1974: 73), few quantitativedata have been pub- lished on migration phenologyof any bird speciesin this region. Species accountsof migratory passerinesin the Birds of North America series published so far (Poole et al. 1991-1994) present few such data. Given the escalatingattention given to conservationof Nearctic migrants that winter in the Neotropics (e.g., Hagan and Johnston 1992), suchdata take on increasedsignificance. For example, comparisonof migration phe- nologiesover a range of latitudesand longitudeswould allow determi- nation of whether a specieshas "stagingareas" or whether it passesquick- ly through its migratory range. Also, migration phenologiesallow docu- mentation of interseasonaland interspecific differences in migration route (e.g., Rappole et al. 1979) Most accountsof regional avifaunaspresent some information on mi- gration phenology.These are usuallyprose statements,however, concern- ing peak and extreme dates (e.g., Oberholserand Kincaid 1974,James and Neal 1986) or bar graphsthat usea semi-quantitativescale for relative abundance(e.g., Lowery1974, Toupsand Jackson 1987). In the absence of censusdata from defined areas throughout the annual cycle,these regionalworks are unable to presentmore quantitativedata, therebyhin- dering inter-regional comparisons.We here present such data for the vireos (Vireonidae) that occur in Louisiana. We also include information for certain specieson statusoutside migration periodsand occurrenceof subspecies. METHODS Our data on seasonal status of vireos in Louisiana are from two sources. First, for coastal southwesternLouisiana, we compiled our field notes from 329 day-long field trips to coastalCameron Parish from October 1978 through May 1995. Each field trip began near sunriseand lasted through at leastmid afternoon,with a substantialproportion of effort (at least 4 h) spent surveyingcoastal woodlands ("cheniers"). We also used suchday-long field trip listsprovided by Kenneth V. Rosenberg(n -- 29), Curtis A. Marantz (n = 15), and Andrew W. Kratter (n = 8), for a total of 381 trips. The sitessurveyed are primarilypatches and stripsof scrubby deciduouswoods (canopy height usually4-10 m) dominatedby hackber- ry (Celtislaevigata); adjacent thickets dominated by acacia(Acacia smallz) received disproportionate attention because of the abundance of mi- grants there. Most woodlandsare within 2 km of the Gulf of Mexico, and all are within 200 m of fresh or brackishmarsh. They are separatedfrom the forestsof interior Louisianaby a zone of largelytreeless marsh rough- ly 25-30 km wide; this zone concentratesmigrating birds that prefer woodedvegetation (Lowery 1974: 77-78). The coverageof datesis reasonablycomplete (as can be seen from the year-rounddistribution of surveysin Fig. 1), exceptfor gapsof more than Vol.67, No. 1 Vireosin Louisiana [ 12 1 W ite-eyed ireo: coast 40 '> 30 '' x• 20 ß ß 10 ß ] 1 I ß '"1 / ß oø o 4-**-.•-=,•h•' . '•,,-,,-,/•-•_.4-,•"•,-•:.."•-.._•.e..-,...<•_ J F M A M J J A S O N D W ite-eyed ireo: inland • 10 ß ß '13 ß ß ß II Oil Oil ß-- ß ß INII ß ß ß ß I1{ ß ß It II I tt I I ß ß loll J F M A M J J A S O N D F•cum•1. (Top)Number of individualWhite-eyed Vireos recorded per dayon surveysof coastalwoodlands in CameronParish, Louisiana. On thisand other figures, single-letter codesfor monthsare givenon the x-axis,and countsof zero individualsare indicated by hollowcircles on the appropriatedate on the x-axis.(Bottom) Number of individuals recordedper censusof a 2.5-hainland sitein IbervilleParish, Louisiana. 122] J. V. Remsen,Jr. etal. J.Field Ornithol. Winter 1996 7 d in late February,late July, and from late Decemberto earlyJanuary. As spring migrantson the Louisianacoast typically sing infrequently if at all, we do not believe that there are seasonaldifferences in detectability that would influence seasonalcomparisons. Clearly, timed censusesof a proscribedarea would providebetter seasonaldata, but the field trip lists, with the number of individualsrecorded per day for each species,provide at least a first approximation of seasonalpatterns. Our data-basehas one advantageover single-siteor single-yearstudies in that the erratic year-to- year and among-sitevariations in speciesabundance generated by the unpredictability of weather are minimized. Whether patterns of occur- rence from extreme southwestern Louisiana reflect those of the whole coastis unknown. We are aware of some qualitative differencesdiscussed herein; also, Philadelphia Vireo is rarer in spring and Warbling Vireo is rarer at any seasonin southeasternLouisiana (D. P. Muth, pers. comm.). Second, for an inland locality,Remsen conducted spot-mapcensuses of all birds seenor heard on an approximately2.5-ha plot in south-central Louisiana about 15 km south of Baton Rouge at 545 Pecan Drive, 3.7 miles north of St. Gabriel, Iberville Parish, at least once per week from 18 Dec. 1989 through3Jun. 1995.The plot is approximately90% mature, lowland bottomland hardwoods(of which approximately75% is undis- turbed and 25% had its original undergrowthremoved in 1988 and is in the processof regenerating) and 10% house, lawn and