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J. Field Ornithol., 64(1):32-41

THE NORTH AMERICAN INVASION PATTERN OF THE SHINY

WILLIAM POST • AND ALEXANDER CRUZ Departmentof EPO Biology Universityof Colorado Boulder, Colorado 80309 USA

DOUGLAS B. MCNAIR 303 Robinson Street Rockingham,North Carolina 28379 USA

Abstract.--Continuing a 90-yr trend of northward rangeexpansion through the , the brood parasiticShiny Cowbird (Molothrusbonariensis) appeared in the Florida Keys in 1985, and was first reportedon the North Americanmainland in 1987. By 1991 the cowbird had been reported as far north as Maine, a displacementof 2300 km from its putative arrival point on the mainland. Shiny are permanentresidents south of Tampa, Florida (28øN). Their numbersin southernFlorida are augmentedin the spring,apparently from the Greater Antilles. The cowbirdsreinvade areas north of Tampa eachspring in an explosivepattern. Most sightingshave been along the Gulf and Atlantic coasts.The cowbirds disappearfrom the northern part of their range in the late autumn. The fat reservesof cowbirdscollected in Florida in late summerwere sufficientfor flightsgreater than 400 km, and it is possiblethat cowbirds return to the Greater Antilles in the winter. Long-range movementsof Shiny Cowbirds in North America support the hypothesisthat the cowbird spreadfrom unaidedby human introductions.

EL PATR•)N DE INVASION A NORTE AMP, RICA POR PARTE DE MOLOTHRUS BONARIENSIS Sinopsis.--E1tordo lustroso(Molothrus bonarienszs) apareci6 en los Cayosde la Florida en 1985, y fue informadoen tierra firme continentalen 1987. Todo estocomo continuaci6n de su expansi6nterritorial hacia el norte, que ha venidoexhibiendo durante los filtimos 90 aftos, a trav6s de las Indias Occidentales. Para el 1991 el tordo habla sido informado tan lejos como en Maine, un desplazamientode 2300 kin de su punto original de partida, en tierra firme. El tordo lustrosoes un residentepermanente del sur de Tampa (28øN). Sus nfimerosen la parte sur de Florida aumentanen la primavera,aparentemente con la 11egada de avesde las Antillas Mayores. Cada primaverael ave reinvadefireas al norte de Tampa en un patr6n explosivo.La mayoria de los avistamientoshan sido a lo largo de las costas del Golfo y del AtlSntico.No obstante,tarde en el otofio desaparecende la parte norte de su extensi6nterritorial. Las reservasde grasa de tordoscoleccionados en Florida tarde en el verano han demostradoser suficientespara viajar distanciasmayores a los 400 km. Es posible que durante el invierno las aves regresenalas Antillas Mayores. Los movimientos de largas distanciasde estasaves en los EstadosUnidos apoyan la hip6tesisde que el tordo se extendi6desde Am6rica del Sur sin la ayuda del hombre. Since about 1900 the Shiny Cowbird (Molothrusbonariensis), a with generalizedchoice of hosts,has beenexpanding its breeding range northward through the Antilles from the regionaround the Guianas and Trinidad (Bond 1976, Postand Wiley 1977a). Its populationgrowth and movement through the initially was gradual. But in

• Current address:Charleston Museum, 360 Meeting Street,Charleston, South Carolina 29d03 USA.

32 Vol.64, No. I ShinyCowbird in NorthAmerica [33 mid-century, as the cowbird reachedthe larger islandsof the Greater Antilles, its population size and range increasedrapidly (Arendt and Vargas Mora 1984, Cruz et al. 1989, Post et al. 1990, Post and Wiley 1977a,b). By 1971 the cowbirdhad reachedMona Island, and by 1972, (Post and Wiley 1977a). It spreadquickly throughthe Do- minican Republic (Arendt and Vargas Mora 1984), appearedin north central in 1982 (Garrido 1984), and was first sightedin the Florida Keys in 1985 (Smith and Sprunt 1987). The purposeof this paper is to 1) documentthe early phaseof the Shiny Cowbird'sinvasion of the North American mainland; 2) examine seasonalpatterns of occurrence;and 3) estimatethe flight rangesof individual cowbirds,to addressthe question of whether Shiny Cowbirds undertake periodic long-rangemovements between the and the North American mainland.

METHODS We obtainedinformation on ShinyCowbirds by reviewingthe following journals publishedin 1985-1991: Alabama Birdlife, American , Chat, Florida Field Naturalist, Migrant, Mississippi Kite, Oriole and Raven. We verified most extralimital reportsby correspondingdirectly with observers.As a result of the similarity betweenfemale Shiny Cow- birds and female Brown-headed Cowbirds (M. ater), we include only reportsof malesat extralimital localities.For estimatesof flock sizes,we include birds identified as males, females or juveniles of unknown sex. We calculatedflight rangesby using Pennycuick's(1989) computer program #1. Calculationswere basedon a wing span of 124 mm for male Shiny Cowbirds (Post, unpubl. data). The empty body massand payloadmass (fat) for a givenwing sizewas estimatedfrom a regression of weight on wing length (Mascher 1966). Weights and wing lengths (chordsfrom wrist to wing tip) were from 80 male Shiny Cowbirds capturedin mist netsduring May-June in centraland southernSt. Lucia (map in Postet al. 1990). Twelve of 15 randomly-chosenSt. Lucia males had no visible fat in their furcula (fat class= 0; Helms and Drury 1960); three had a trace of fat (fat class-- 1). The relationship between weight (mean ñ SD = 39.6 ñ 2.8 g) and wing length (95.5 ñ 2.8 mm) was estimatedby: weight = 0.435 wing length- 1.884.There wasa significant positivecorrelation between weight and wing length(r = 0.48;P • 0.001). North American Shiny Cowbirdswere weighedimmediately after being shot. We estimatedthe expectedweight of each individual by inserting its wing length in the aboveequation. Fat reserveswere assumedto be the differencebetween expectedand observedweights.

RESULTS Shiny Cowbirds appearedin the central Florida Keys in 1985, but were not seen on the Florida mainland until 1987 (Smith and Sprunt 1987). The cowbird was first reportedoutside southern Florida in 1988, when one male appearedat Black Hammock Island, near Jacksonville, Florida (Table 1; Fig. 1). The latter recordwas 350 km from Fort DeSoto, 34] W. Postet al. j. FieldOrnithol. Winter 1993 Vol.64, No. 1 ShinyCowbird inNorth America [35

1991

1989 g

ß 1990e 1989f ß 1989c

ß 1990c ß 1990d 1991 b 28ø N

1990 a 1989 a

1989 d 1989b 1987 600 .j km20019•.,•_•890• I e• I I 1988

FIGURE1. Localitiesof maleShiny Cowbirds in northcentral Cuba and United States, 1982-1991.Except for Flamingo,Florida, the firstmainland locality, only range ex- tensionsof over 100 km are shown.The dashedline designates28øN. For yearswith multiplelocalities, the lowest letter indicates the earliest record. Key to localities:1982: Cardenas,Cuba; 1985: Lower MatecumbeKey, Florida (FL); 1987: Flamingo,FL; 1988a:Dry Tortugas,FL; 1988b:Homestead, FL; 1988c:Fort DeSoto,FL; 1988d: BlackHammock Island, FL; 1989a:Cape San Bias,FL; 1989b:Delray Beach,FL; 1989c:Warner Robbins,Georgia; 1989d: Port Fourchon,Louisiana; 1989e: Cameron, Louisiana;1989f: Sullivan's Island, South Carolina; 1989g: Aurora, North Carolina; 1990a:Bon Secour,Alabama; 1990b: Levy County,FL; 1990c:Fort Hood,Texas; 1990d:Tallahassee, FL; 1990e:Winborn Spring, Oklahoma; 1991a: Monhegan Island, Maine; 1991b: Waveland, Mississippi. 36] w. Postet al. j. FieldOrnithol. Winter 1993

Florida, the previousnorthernmost locality (Fig. 1). The Jacksonville sightingwas not precededby any substantialincrease in populationin southern Florida. We estimatethat only 35 males were seen in Florida in 1988, and sevenwas the largestnumber seenat one place (Table 1). Outsidethe winter periodShiny Cowbirdsincreased throughout south- ern Florida in 1989, and cowbirdsappeared at sevenlocations north of Tampa, one as far north as Aurora, North Carolina (Fig. 1). The north- ward movementof the cowbird was accompaniedby an increasein total numbersseen, and in the maximum number seenat one place (Table 1). The trend toward populationincrease and range expansioncontinued in 1990, and the cowbird was seenat 21 localitiesnorth of Tampa, the northernmostbeing New Bern, North Carolina (Table 1; Fig. 1). But in 1990 fewer cowbirdswere reportedthan in 1989 (Table 1). Possibly, this decreaseis a result of uneven observercoverage, as the cowbird's novelty as a rare waned. In 1991 male cowbirdscontinued to make erratic, long-rangenorthward movements,which were not precededby intermediaterange consolidation. The northernmostrecord was on Monhegan Island, off central Maine, 1200 km from the previousnorthernmost locality, Aurora, North Carolina (Fig. 1). No ShinyCowbirds were reportedfrom the interveningAtlantic coastalregion during the 18 mo betweenthe two sightings,although this zonecontains some of the mostintensively monitored bird-watching sites in North America. Monhegan Island, Maine is 2340 km from Flamingo, Florida, wherethe specieswas first seenon the North Americanmainland (Fig. 1). Most sightingsof Shiny Cowbirdshave been of one (n -- 30) or two (n = 15) individuals.The largestgroups have been in southernFlorida (52 in the Dry Tortugas on 25 May 1989) and on the northern coastof the Gulf of Mexico (28 on Dauphin Island, 11 May 1990). The mean (+SD) size of the 10 largestflocks seen during April-July was 24.7 + 15.6. In contrast, only individuals or small groups of cowbirds were reportedafter July (3.5 + 3.3; n -- 10). By examining seasonaldistribution during 1989-1991, the following pattern is discernible:Shiny Cowbirds are most commonduring April- July. In the fall, at the sametime cowbirdsdisappear from areasnorth of Tampa, Florida, there is an overall reduction in cowbird numbers throughout their North American range, including even the southern Florida Keys (Fig. 2). Eight male Shiny Cowbirds have been collectedin North America as of February 1992. Fresh weightsare available for four of the specimens (Table 2). A third-year male collected30 Apr. 1991 on Sullivan'sIsland, South Carolina, had a slight amount of subcutaneousfat (fat class= 1; Helms and Drury 1960), and had a potential flight range of 231 km (Table 2). Two males collectedat Ft. Pierce,Florida on 25 Jul. 1991 were extremely fat (fat class= 5). Their estimatedflight ranges were 337-444 km (Table 2). No femaleShiny Cowbirds have been collected in North America.Two Vol.64, No. 1 ShinyCowbird in NorthAmerica [37

225 IßI'-I SouthNorth of Tampa, Florida

175

o rr 125

o • 75 E

25

0 Jan. - Feb. Mar. - Apr. May - Jun. Jul. - Aug. Sept. - Oct. Nov. - Dec. FIGURE2. Seasonalpattern of occurrenceof Shiny Cowbirdsin North America. The open histogramsshow the number of Shiny Cowbirdsrecorded in each two-monthperiod from January 1985 to December1990 north of Tampa, Florida (28øN; seeFig. 1). The shadedhistograms indicate the total numberof cowbirdsrecorded south of Tampa during the same intervals. of the males, one collected at Sullivan's Island, South Carolina, and the other at Fort Pierce, Florida, had adult nuptial (secondalternate) plum- ages (Friedmann 1929), and thus were in their third year. Six of the specimenshad first nuptial (first alternate) ,which are similar to adult nuptial, except in the retention of varying amountsof brown feathers. Little is known about the behaviorof Shiny Cowbirds in North Amer-

Tt•I•E 2. Estimated flight rangesof male Shiny Cowbirds collectedin North America. The payloadmass (fat weight) is the differencebetween expected and observedweights. Proceduresfor calculatingexpected weights and flight rangesare in methods. Wingchord Weight(g) Flight Date Locality length (mm) Expected Observed range (km) 30 Apr. 1991 Sullivan's I., SC 95.5 40.1 44.9 231 18 Jun. 1991 Sullivan's I., SC 100.0 41.6 32.0 0 25 Jul. 1991 Ft. Pierce, FL 95.0 39.4 46.5 337 25 Jul. 1991 Ft. Pierce, FL 93.0 38.6 48.0 444 3 8 ] W.Post et al. J.Field Ornithol. Winter 1993 ica. Males appear to be reproductivelyactive at least during the period 30 April-25 July. Seven of the eight males collectedat this time had enlarged testes.In Florida and South Carolina, we often heard male Shiny Cowbirds singing ("twitter song" of Friedmann 1929), usually when they were flying. Post also saw severalmales displaying to female Shiny Cowbirds on Sullivan's Island, South Carolina in 1991. On 26 May, one male in post-juvenalplumage and two femaleShiny Cowbirds were looselyassociated as they perchednear a bird feeder. The male Shiny Cowbirds sangand made a low, swoopingflight toward the two females,and attemptedto land next to them. One of the femalesgave a rattle call (Friedmann 1929), and the two femalesmoved away together. Brown-headedCowbirds were alsoin the vicinity,but the ShinyCowbirds did not approachthem while feeding,nor when they movedto and from the feedingstation. The male Shiny Cowbird oncelanded within 1 m of a male Brown-headedCowbird. The two gavebill-ups (Friedmann 1929) while perchednext to each other. The male also chaseda juvenile Eu- ropean Starling (Sturnusvulgaris) about 25 m. During the chase,the cowbirdsang and the starlingemitted distress calls. Interspecificchasing by Shiny Cowbirdshas been seenin (Post, unpubl. data). Again, on 2 June, Post watchedtwo male Shiny Cowbirdsapproach a female, which gavea rattle call, and movedaway. Sixteenreports mentioned habitats or feedingsites of Shiny Cowbirds; 10 of thesewere of individualsor groupsat bird feeders,three on lawns, and two at feedlots.Only oneindividual was reported in a natural habitat: a male near Mahogany Hammock in EvergladesNational Park. Of the eight males collected,four were at bird feeders,two were capturedin cowbird-controldecoy traps and two were in a roost.The stomachsof the roostingbirds were full of millet seeds,which were likely obtained at a bird feeder. Thus, all cowbirds collected in North America have been associatedwith provisionedfood. ShinyCowbirds in North Americajoin mixed-speciesroosts, a behavior that has been reported in the Caribbean (Post and Post 1988). On the eveningof 25 Jul. 1991 Post watched two males in a roost in a small cattail marsh near Ft. Pierce, Florida (Post 1992). The two, onein second- year plumageand the other in full adult ,arrived at the roost about 1600 hours EST, after other blackbirdshad already arrived. The otherswere Brown-headedCowbird (170), CommonGrackle (Q•uiscalus quiscula)(50) and Boat-tailedGrackle ((•. major)(30). Beforethey were collected,the male Shiny Cowbirdssat 2-3 m apart about 1 m abovethe water on cattail stems.Neither bird perchedcloser than 2 m to individuals of other roostingspecies. A third male Shiny Cowbird was in the roost at 0520 on 26 Jul. 1991. It also sat apart from other birds. It preened, sang,and left the roost at 0530 with a flock of other blackbirds. A secondroost was observedby McNair near City, Florida on 3-4 May 1990. Two male Shiny Cowbirdssettled with about 10 Red- wingedBlackbirds (Agelaius phoeniceus) and 25 Brown-headedCowbirds on a small shrub-coveredisland in a brackishpond. The Shiny Cowbirds Vol.64, No. 1 ShinyCowbird in NorthAmerica [39 roostedin cattailson the edgeof the island, and about 1.5 m abovewater. It was not determinedhow closelythey associatedwith the other roosting species.

DISCUSSION In addition to North America, Shiny Cowbirds are colonizingnew areasof SouthAmerica, aided by introductionswest of the (Johnson 1967, Long 1981) and by deforestationin the Amazon Basin (Ridgley and Tudor 1989). Their rapid increasein the Western Hemispherepar- allelsthat of the Cattle Egret (Bubulcusibis) and Brown-headedCowbird, specieswhich also have respondedto clearing of forestsfor hus- bandry (Arendt 1988, Davis 1960, Friedmann 1929, May field 1965, Rothsteinet al. 1980, Siegfried 1978). Similarly, the Shiny Cowbird's successin spreadingnorth through the Caribbean has dependedon the availabilityof suitablefeeding areas for adultsas much as on opportunities for nesting(Post et al. 1990). Many of the Shiny Cowbirdsseen in the United Stateshave been at bird feeders.The abundanceof feedingstations in North America may aid the cowbird's spread. The Shiny Cowbird's invasionpattern of North America is character- ized by erratic, long-rangemovements. The annual distributionpattern that has developedhas the following features. 1) Although the cowbirds are permanent residentsin Florida south of Tampa, their numbers are augmentedeach spring, possibly by movementsfrom the Greater Antilles. 2) Cowbirds then appear to reinvade the northern part of their range, possiblyfrom the Florida populationcenter. 3) The annual reinvasion occursin an explosivepattern, althoughmost propagules are concentrated alongthe Atlantic and Gulf coasts.The cowbirdshave penetrated farthest north alongthe Atlantic coast,although they are recordedmore frequently along the northern Gulf of Mexico. 4) Shiny Cowbirds disappearfrom the northern part of their range in late fall, and few are seen north of Tampa in the winter. 5) It is not yet possibleto relate the movement patternsof the Shiny Cowbird to extrinsicfactors such as food or host availability. The Shiny Cowbird differs from speciesof passerinesthat have been introducedto Florida and the West Indies (reviewedin Long 1981) in its tendencyto make repeatedlong-range movements. The physiological conditionof the two Shiny Cowbirds collectedat Ft. Pierce, Florida in late July indicatesthat they were capable of flights of up to 444 km, which, had they departedfrom the Miami area, would have put them in range of Cuba. The limited data that we have suggestthat, compared with Shiny Cowbirdsexamined in St. Lucia, the fat reservesof cowbirds collectedin Florida are adaptationsfor long-range dispersal. All Shiny Cowbirdsthat have been collectedin the West Indies and North America belongto the smallestsubspecies, M. b. minimus.This form was initially confinedto the tropics(Hellmayr 1937). Tropical, continentalpopulations of minimusare not knownto migrate(Friedmann 1929). South Temperate Zone Shiny Cowbirds are at least partially 40] W. Postet al. j. FieldOrnithol. Winter 1993 migratory, however (Friedmann 1929, Hudson 1920). Thus it is possible that the long-rangedispersal behavior of Shiny Cowbirdsin North Amer- ica will developinto a pattern of periodicmigration. Large gaps in the Caribbean distributionof the Shiny Cowbird led Bond (1976) to suggestthat its spreadto the Greater Antilles was abetted by human introductions.This idea was supportedby information that the specieshad been popular as a cagebird on someislands (Bond 1976). Others,working in Puerto Rico and Mona Island (Postand Wiley 1977a), agreedwith Bond'sinterpretation. But the pattern of the Shiny Cowbird's invasionof North America lendssupport to the alternativeview, that the speciesspread unaided through the Caribbean. The distancebetween southernMartinique, the probablenorthern limit of its breedingdistri- bution in the Lesser Antilles (Post et al. 1990) and eastern Puerto Rico is 675 km, well within the range of the species'movements in North America. Further, informationfrom the LesserAntilles may help explain why the cowbird has not been seenon many islands,although suitable hostsare available. The cowbird'sabundance in St. Lucia, and scarcity on Martinique, may be related to availability of suitable feeding sites suchas stockyards, rather than to nestingopportunities (Post et al. 1990). Although cowbirdsmay have landed on the small islandsof the Lesser Antilles north of Martinique, they may not have stayedbecause provi- sioned food was not available. The arrival of Shiny Cowbirds in North America presentsa unique opportunityto studythe colonizationof a continentby a broodparasite. The invasionprocess is made especiallyinteresting by the presenceof another cowbird that is also a host generalist. A potential problem for studiesof Shiny Cowbird populationgrowth is that of differentiatingtheir eggs,fledglings and femalesfrom thoseof Brown-headedCowbirds. This will make it difficult to study hostchoice in areaswhere the two cowbirds co-occur.As of February 1992, we know of no substantiatedreport of parasitismby Shiny Cowbirds in North America. We believe that con- firmation cannotbe basedon sight records,but will require collectionof Shiny Cowbird eggsor fledglings.

ACKNOWLEDMENTS

For their assistancewe thank P. Butler, D. Howell, W. H. Hutcheson, R. Johnson,W. Mathurin, S. A. Miller, T. K. Nakamura, R. Ondrejko, E. Phoebus,C. A. Post, K. W. Post,V. Remsen,C. A. Sealsand W. P. Smith. We appreciatethe cooperationof the Florida Game and Fresh Water Commission.We are grateful to the Ministries of , Lands,Fisheries, Cooperatives and Labour,Castties, St. Lucia for permissionto conduct research in St. Lucia. We thank Bob and Ann Walters, Phoenix Estate, Vanard, St. Lucia, for allowingus to capturecowbirds on their property.Research in St. Lucia wassupported by NSF grantPRM-8112194 to theUniversity of Colorado,A. Cruz, principalinvestigator. Researchin Florida was supportedby the Drum Island Fund of the CharlestonMuseum. The manuscriptbenefitted greatly from the critiquesof Wayne Arendtand SteveRothstein.

LITERATURE CITED

ARENDT,W.J. 1988. Range expansionof the Cattle Egret (Bubulcusibis) in the Greater Caribbean basin. Colonial Waterbirds 11:252-262. Vol.64, No. 1 ShinyCowbird in NorthAmerica [41

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