Breeding and Natal Dispersal in the Puerto Rican Vireo

JOURNAL OF FIELD ORNITHOLOGY Published by Associationof Field Ornithologists

VOL. 69, No. 1 Winter 1998 PAGES 1--148

J. Field Ornithol., 69(1):1-7

BREEDING AND NATAL DISPERSAL IN THE PUERTO RICAN VIREO

BETHANY L. WOODWORTH 1 Departmentof Ecology,Evolution and Behavior Universit)'of Minnesota St. Paul, Minnesota 55108 USA

JOHN FAABORG Division of BiologicalSciences Universit)'of Missouri Columbia, Missouri 65211 USA

WAYNEJ. ARENDT International Institute of TropicalForestry P O. Box B Palm• Puerto Rico 00721 USA

Abstract.--Information on dispersalis critical for understandingthe population dynamicsof birds.We estimatedbreeding and natal dispersalin two studiesof a populationof the Puerto Rican Vireo (Vireo latimeri) that is in danger of local extirpation due to low reproductive success.From 7.1-29% of adult malesand 12.5-25% of adult femaleschanged territories betweenbreeding seasons.The median breeding dispersaldistance of 12 birds (7 males,2 females,and 3 of unknownsex) was370 m (range 110-560 m). Four fledglingsdispersed a median distanceof 490 m from their natal neststo breeding territories (range 300-2030 m). The implications of these data for population trends of the vireo in Gufinica Forest are discussed.

DISPERSION REPRODUCTIVA Y NATAL EN V/REO LATIMERI Sinopsis.--La informaci6n sobre el patr6n de dispersi6nde un ave es critica para entender la dinfimicapoblacional de la especie.Estimamos la dispersi6nreproductiva y natal del Bien- teveo (Vireo latimeri) en dos estudiospoblaci6nales que se 11evarona cabo en el Bosquede Gufinica, Puerto Rico. Esta poblaci6n estfi en peligro de extinci6n debido al bajo •xito re- productivode la misma. E1 7.1-29% de los machosadultos y el 12.5-25% de las hembras adultascambiaron de territorio entre fipocasde reproducci6n. La dispersi6nreproductiva promedio de 12 individuos(7 machos,2 hembrasy 3 de sexo desconocido)fue de 370 m (alcance 110-560 m). Cuatro volantones tuvieron una dispersi6n promedio, del lugar en donde nacieron, de 490 m (alcance300-2030 m). Se discuteen el trabajo las implicaciones de estos resultados.

• Currentaddress: Pacific Island EcosystemsResearch Cent• BiologicalResources Div., U.S.G.S., P.O. Box 44, Hawaii National Park, Hawaii 96718 USA. 2] B. L. Woodworthet al. J.Field Ornithol. Winter 1998

The Puerto Rican Vireo (Vireo latimeri) is a small (11-12 g) foliage- gleaning insectivore(Cruz and Delannoy 1984, Wetmore 1916) endemic to the islandof Puerto Rico (Bond 1956). Long-termmonitoring in Gufin- ica Forest, Puerto Rico's largestdry forest reserve,has showna gradual decline in numbersduring 1973-1989, with a more precipitousdecline during 1989 to the present (Faaborget al. 1997). Additionally,reproductive successof this vireo in Gu/tnica in recent yearshas been low, due to high rates of brood parasitismby the introduced Shiny Cowbird (Molo- thrusbonariensis) and high ratesof nestpredation (Woodworth1997). A population dynamicsmodel for the speciesindicates that, as a result of cowbird parasitism,the vireo population in GufinicaForest has become a "sink" population (Pulliam 1988, Woodworth, in press). The prospects for continued existenceof this population depend in part on the ability of the Puerto RicanVireo to disperselong distancesfrom areasof greater reproductivesuccess ("source" populations). The couplingof two studies, one a long-term mist-netstudy (1973-present) and the other a demographic study of color-markedindividuals (1990-93), providesthe first published information availableon the frequency and distancesof natal and breeding dispersalin the Puerto Rican Vireo.

METHODS Gu/tnica Commonwealth Forest Reserve (17ø55'N, 67ø05'W) is a 4015- ha dry subtropicalforest on the southwesterncoast of Puerto Rico. In the eastern tract where these studiestook place, the upland forest includes scrub, deciduous,and semi-evergreenassociations. A detailed description of the vegetation and bird communitiescan be found in Terborgh and Faaborg (1973) and Woodworth (1995). Rainfall is variable,with an av- erage of 860 mm falling annually in a bimodal pattern: a short rainy seasonApril-May, and heavy rains from August-November,with almost no rainfall from December-February (Murphy and Lugo 1986). Puerto Rican Vireos generally breed from April-July, depending upon rainfall (Woodworth 1997). We collected data during a long-term mist-net studyand a 4-yr demographic study of color-markedindividuals. The long-term mist-net study (Faaborgand Arendt 1995) hasbeen conductedcontinuously since 1973, exceptin 1977 and 1979. One net line wasoperated from 1973 to present, with 7-8 additionalnet lines in operationsince 1989. At eachsite, 16 mist nets (36-mm mesh, 12-mmlong, and 2.6-m high) were placedalong trails end-to-end. The length of each line was approximatelythe same as the averagePuerto Rican Vireo territory (200 m; B. L. Woodworth, unpubl. data). The net in which each bird wascaptured was recorded so that each bird's capture site was known to within 12 m. Net lines were operated from dawnto duskfor three consecutivedays in Januaryor Februaryeach year. The demographicstudy (Woodworth 1997) wasconducted from 1990- 1993 on four 50-ha studyareas. Study areasencompassed the locationsof four of the long-term net lines, and were within 500 m of four of the Vol.69, No. 1 Dispersalin thePuerto Rican Vireo [3 other lines. Birds were captured and color-bandedby luring territorial malesor pairs into nets usingrecorded vireo song.Territorial boundaries were determined by recording all sightingsof color-markedbirds, nests, songperches, boundary disputes,and territorial responseto playbackon 1:2500-scalemaps. Observationsof color-marked birds outside of their defended area, or in areas known to be defended by other birds, were excluded.From 10-42 vireo territorieswere under observationeach year. Twice each seasonwe surveyedall habitat within 300 m of the studyarea boundariesusing playback,to searchfor dispersedbirds. We defined breeding dispersalas movementof individualsbetween suc- cessivebreeding sites(Greenwood and Harvey 1982). BecausePuerto Ri- can Vireos build severalnests in a singleseason within a singleterritory, we consideredthe breeding site to be the territory (ClassA territories, Nice 1941). Becauseno caseof within-seasonbreeding dispersalwas recorded during the demographicstudy, we consideronly between-season breeding dispersalhere. We counted the number of opportunitiesfor a bird to disperseas the number of years elapsingbetween capture and recapture. If a bird was present on the study area for two consecutive years,it was consideredto have had one opportunity to disperse.If present on the study area for three years, it wasjudged to have had two opportunities, and so on. Using data from the two studies,we estimatedbreeding dispersalin three ways: Mist-netting study.--Capture-recapturedata from the mist-net study were examined for evidenceof movement.A bird recaptured in the net line in which it wasoriginally banded wasassumed to have remained on its original territory, and if captured in a different net line was assumed to havemoved. Dispersal distances were measuredas the distancebetween captureand recapturesites. Dispersing birds are unlikelyto be recaptured in the net lines, and so this estimateis expected to underestimatedis- persal. Mist-netting and relocationon territory.roWe examined movementsof birds originallycaptured in the mist-netstudy and recapturedon territory during the demographicstudy. For this analysis,we considereda bird to have remained on its original territory if its territory boundariesat the time of relocation included the site where it was originally banded. Dis- persaldistances were measuredas the distancebetween the original banding site and the geometric center of its new territory. In this analysis, birds originally captured during extra-territorial forays will have been wronglyjudged asdispersing, and so this samplewill tend to overestimate breeding dispersal. Demographicstudy.--We examined territorial boundariesof color-banded birds presenton the studyarea for two or more breeding seasons.We considereda male to have remained on his previousterritory if his territory of one year overlappedhis territory of the previousbreeding season by at least 80% (followingNolan 1978). Becausemales are more active in territory maintenance than are females (B. L. Woodworth, unpubl. 4] B. L. Woodworthet al. j. FieldOrnithol. Winter 1998 data), a female's territory was considered to coincide with that of her mate. In the demographic study,dispersal distances were measuredas the distance between the centers of the territories. We defined natal dispersalas the movement from birth site to first breeding or potential breeding site (Greenwood and Harvey 1982). Natal dispersalis usually measured as the distance between the natal nest and the first breeding nest. We relocated birds marked as nestlingsand, be- causethe location of a bird's first breeding nest was often not known, measured the distance from the natal nest to the center of the bird's first breeding (or potential breeding) territory. Becauseof the extremely low rate of reproductive successin this population, data were available on only four birds marked as fledglings. In studiescovering a finite study area, bias existsin the distribution of recorded dispersaldistances because birds dispersinglong distancesare lesslikely to be detected by the investigator(Barrowclough 1978). It is possible to correct for this bias, but sample size was too low to do so in this study.

RESULTS

From 1973-1996, 135 AHY Puerto RicanVireos were banded aspart of the mist-netstudy, and 42 (31%) were recapturedat leastonce in a later year. We also studied two birds banded in a separatestudy in the early 1980s (J. Colon, pers. comm.). Twenty-fiveof thesevireos were recaptured on their territories during the demographic study. Finally, from 1990- 1993, 78 vireoswere color-bandedas part of the demographicstudy, and 25 were followed in two or more breeding seasons. Between-yearbreeding dispersal of malesranged from 10-29% (5-11% of between-yearopportunities) and of femalesranged from 13-25% (9- 10% of between-yearopportunities) (Table 1). Medianbreeding dispersal distance of 12 birds (7 males, 2 females, and 3 of unknown sex) was 370 m (range 110-560 m). Four malesrecaptured 8-12 yr after initial banding were still resident within 500 m of their original banding sites. In the demographic study,none of 5 males present in the study area for 2 yr changed territories; 2 of 15 birds present for 3 yr changed territories; and 1 bird remained on territory for 4 yr. In both instancesof dispersal,males moved to an adjacent territory upon death or disappear- ance of a neighbor, and so dispersaldistances were small (Table 1). Of four females color-banded in 1990-1991, one was resident on her territory for 3 yr, and two remained for 4 yr. One female movedto an adjacent territory after her first breeding season.On four occasions,a marked female's mate disappeared;in each case, the female remained and sub- sequentlypaired with the new territory owner. In no casedid a male with a mated female move, so we could not examine the relative importance of mate versussite fidelity in females. Ten vireos were color-bandedas fledglings during the demographic study.Of these,four were relocatedin the breeding seasonfollowing their Vol.69, No. 1 Dispersalin thePuerto Rican Vireo [5

TABLE1. Breeding dispersalfrequency and distanceof Puerto Rican Vireos in Gu•nica Forest, Puerto Rico, as determined from three sampling techniques.Sampling techniques are describedin detail in the text.

Samplingtechnique and sex Mist-netting and Mist-netting recapture study• on territoryb Demographic study• Parameter Both Males Females Males Females

No. birds recaptured 42 17 8 21 4 Median time to recap (yr) 1.0 1.6 0.6 0.75 0.75 No. opportunities to disperse 71 44 11 38 10 No. birds dispersed 3 5 1 2 1 Percent dispersal(per bird) 7.1 29 12.5 9.5 25 Percent dispersal(per opportunity) 4.2 11 9.1 5.3 10 Median distance (m) 380 460 240 120 340 (Range) (360-480) (130-560) (110-130)

a Birdswere captured and recapturedin the long-term mist-netstudy. bBirds were originallycaptured in the long-termmist-net study, recaptured on their territories using playback,and their territories mapped. c Territories of color-bandedbirds were spot-mapped. birth. Their median natal dispersaldistance was 490 m (300 m and 2040 m for two females; 410 m and 490 m for two males).

DISCUSSION Adult Puerto Rican Vireos in Gu•tnicaForest exhibited a relativelylow frequencyof between-seasonbreeding dispersal.Vireos that did disperse typicallyremained within two territory widths of their original banding site. Puerto Rican Vireo pairs are resident on, and defensive of, their territoriesthroughout the year (B. L. Woodworth,unpubl. data). Tropical birds that are residentyear-round on their breeding territory may be expected to exhibit higher site-fidelitythan breeding migrants, because permanent residents do not have to re-establish territorial boundaries each spring. In the migrant Prairie Warbler (Dendroicadiscolor), Nolan (1978)found that 22% of returningmales, and 90% of returningfemales, changedterritories between breeding seasons.Likewise, 60% of male Yel- low-headedBlackbirds (Xanthocephalus xanthocephalus, Beletsky and Or- ians 1994) and 94% of male Great Reed Warblers(Acrocephalus arundi- naceus,Bensch and Hasselquist1991) dispersedbetween breeding seasons.However, some migrants exhibit low ratesof breeding dispersal(e.g., 4% of male EasternKingbirds, Tyrannus tyrannus, Murphy 1996;5% of ASY male Indigo Buntings,Passerina cyanea, Payne and Payne 1993). A high proportion of banded nestlingswas relocated on the studyarea in subsequentyears, reflecting high natal philopatry or high juvenile sur- vival. Residentpasserines generally show high natal philopatry as com- 6] B. L. Woodworthet al. J.Field Ornithol. Winter 1998 pared to migratory ones (reviewedin Weatherhead and Forbes 1994). Of 11 studiesof resident populations,only the isolatedpopulation of song sparrowson Mandarte Island had natal philopatry approachingthat ob- servedin this study(39.7%, n -- 463; Arcese1989, Smith 1988). In pas- serines,natal dispersaldistance is frequentlyfemale-biased, and natal dispersal distancesare usually greater than breeding dispersaldistances (Greenwood and Harvey 1982). The longest dispersaldistance recorded in this studywas for a female, banded as a nestling,who wasrelocated in her secondyear over 2 km from her natal nest site. However,most young in this study settled within one to three territory widths of their natal nest. We caution that our estimatesof dispersalfrequency and dispersaldistances were underestimates of the true values. This was because it was impossibleto exhaustivelysearch all suitablehabitat in Gu/tnicaForest for marked birds, and dispersaloutside of the searchedareas could not be distinguishedfrom mortality.This limitation is presentwith all studiesof open populations (Pollock et. al 1990). However,adult mortality rates recorded for the Puerto Rican Vireo in Gu/tnicaForest have historically been low (32%, Faaborgand Arendt 1995; 26%, Woodworth1995), and this suggeststhat few dispersingbirds were missed. The data suggestthat this non-migratorybird does not routinely disperse over long distances,at leastwithin Gu/tnicaForest. This may reflect the fact that a declining population createsavailable territories in high qualitybreeding habitat closeto current breedingterritories or natal sites. In a more saturatedpopulation, dispersaldistances might be larger.How- ever,if dispersalfrequency and distancesfor Puerto RicanVireos are typ- ically small, this behavior may accentuate the isolation of the Gu/tnica Forestvireo population. The eastern tract of Gu/tnica Forestis bordered on two sidesby ocean,with agriculturalfields and residentialareas sep- arating it from other possiblevireo habitat to the north and east. If the Gu•tnicapopulation is isolated,it will surviveonly by reproducingenough to replace mortality, which in recent yearshas not occurred (Woodworth, in press).The data presentedhere suggestthat immigration from distant sourcepopulations is not likely to sustainthe populationin Gu/tnicaFor- est. The locationand extent of potential "source"habitats, and the spe- cies' dispersalfrequency and distance in saturated habitats, should be priorities for further research.

ACKNOWLEDGMENTS

We thafikJ. Colonfor kindlyproviding details on originalcapture dates and locations of twovireos banded by him. The PuertoRico Departamentode RecursosNaturales and Miguel Canalsgave permission to work in Gufinicaand logisticalsupport. The demographicstudy was supportedby the International Council for Bird Preservation--U. S. Section;Frank M. ChapmanFund of the American Museum of Natural History; SigmaXi Grant-in-Aidof Re- search;Dayton Natural History Fund and Wilkie Fund for Natural History Research,Bell Museum of Natural History; Grants for ResearchAbroad, Universityof MinnesotaGraduate School;Eastern Bird BandingAssociation; Paul A. StewartAward of the WilsonOrnitholog- ical Society;and a DoctoralDissertation Fellowship from the Universityof Minnesota.Finan- Vol.69, No. 1 Dispersalin the Puerto Rican Vireo [7 cial support for the mist net studywas provided by the Frank M. Chapman Fund of the American Museum of Natural History; the ResearchCouncil of the Graduate School, Uni- versityof Missouri-Columbia;the U.S. Agencyfor International Development;the U.S. Forest Service, International Institute of Tropical Forestry;and the U.S. Fish and Wildlife Service. Many thanks to the numerousfield assistantswho participatedin both field studiesover the years.Comments by J. Brawn and K. Klimkiewicz improved the manuscript.

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