The Condor93:864-868 0 The CooperOmithological Society 1991

FLOCKING BEHAVIOR OF MIGRATORY WARBLERS IN WINTER IN THE VIRGIN ISLANDS

DAVID N. EWERT The Nature Conservancy,2840 East Grand River Ave., Suite 5, East Lansing, MI 48823

ROBERT A. ASKINS Department of Zoology, ConnecticutCollege, New London, CT 06320

Abstract. We assessedthe flocking behavior of birds on St. John and St. Thomas, U.S. Virgin Islands, with systematicsurveys along trails in moist forests.Winter residents(species breedingin North America and that winter in the Virgin Islands), all of which were warblers, comprised 9 1% of the individuals found in 28 flocks but only 49% of solitary individuals. The average flock size was 4.0 individuals of 3.1 species,and did not differ between St. John and St. Thomas even though the averageforest tract on St. John (1,000 ha) was much larger than on St. Thomas (62 ha). Northern Parula (Pa&a americana) and Black-and- white Warbler (Mniotilta varia), the most common speciesin flocks, occurred in 76% and 79% of the flocks, respectively. Northern Panda flocked significantlymore frequently on St. Thomas than on St. John, but no other species showed a difference in flocking behavior between the two islands. Each flock typically included one individual of each species. Key words: Parulinae; winter residents;Virgin Islands; mixed foraging flocks; habitat fragmentation.

INTRODUCTION dominate lower slopeswhile moist forest occurs There are few descriptions of mixed foraging along high ridges,in ravines and in coastalbasins flocks of winter-resident and permanent-resident (Woodbury and Weaver 1987). Virtually all for- species of songbirds in the West Indies (Eaton estshave become re-established since the 1800s. 1953, Lack and Lack 1972, Willis 1973, Post Moist forestshave two to three strata and a can- 1978, Staicer, in press).As part of a study of the opy height of 10-30 m. More than 70% of the ecology of wintering birds (speciesthat only oc- trees are evergreen. In contrast, dry forests are cur during the winter and migration) in the U.S. dominated by deciduous, thorny, small-leaved Virgin Islands, we investigated their flocking be- trees that are typically 5-10 m high (Woodbury havior on St. John (50 km*), which has extensive and Weaver 1987). tracts of forest and is 62% forested, and on St. We recorded the social status (single or in a Thomas (71 km2), which is largely developed flock) of permanent-resident and winter-resident (only 12% forested)and has many small forested insectivorous, granivorous and nectarivorous tracts that are isolated from one another (Askins passerinesfrom 12 November-4 December 1987 et al., in press).Only 3 km apart, the two islands on St. John and from 5-15 December 1987 on are topographically and floristically similar. St. Thomas. Surveys were completed in moist Considering the differences in forest configura- forest or ecotone between moist forest and dry tion on the two islands, the goals of our study woodland because most species of winter resi- were (1) to describethe composition and dynam- dents were largely restricted to these habitats ics of mixed foraging flocks and (2) to determine (Robertson 1962, Askins et al., in press).Surveys if the occurrence or composition of mixed for- on St. John were conducted in three extensive aging flocks is associatedwith different patterns tracts of forest (average tract size = 1,000 ha; of habitat fragmentation. range 82-2,238 ha) while those on St. Thomas were completed in three forest fragments (aver- METHODS age tract size = 62 ha; range 3-l 5 1 ha). We spent Steep slopes,cut by narrow ravines, characterize 31.2 hr censusing 14.9 km of trails on St. John St. John and St. Thomas. Dry forests and scrub and 3.2 hr censusing 3.0 km of trails on St. Thomas. Surveys were completed between 07: ’ ’ Received20 November 1990. Final acceptance2 39 and 16:45 Atlantic Standard Time. July 1991. We searched for both permanent and winter

W41 WINTER FLOCKS IN THE VIRGIN ISLANDS 865 residents by walking slowly along trails as de- winter residents, but permanent residents only scribed by Hutto (1987). Each trail was surveyed occurred in 17% of the flocks. Of the 116 indi- only once to avoid double-counting birds during viduals detected in flocks on both islands, 9 1% and between observation periods. Birds found were winter residents. Winter residents com- 25 m or more from any other passerine birds prised 49% of the 97 individuals recorded as were classifiedas single individuals. Pairs of per- being alone. manent-resident birds were considered non- The mean number of individuals per flock and flocking individuals. Birds within 25 m of each mean number of speciesper flock were not sig- other constituted a flock (Post 1978) which is nificantly different between St. John and St. equivalent to an aggregation (Lack and Lack Thomas (Wilcoxon two-sample test, t = 0.27, P 1972). Birds within 25 m of each other were > 0.05 and t = 0.69, P > 0.05, respectively). consideredto be part of a flock becausewe found When data from the two islands were pooled, the that birds that close together frequently traveled mean number of individuals per flock was 4.0 together. Becauseit takes an average of approx- and there was an average of 3.1 speciesper flock. imately 1 min to locate each member of a flock Of those flocks observed for at least 20 min, (Gibb 1960, Hutto 1987) solitary birds were ob- six remained in a tree or group of trees while served for at least 2 min to determine their social nine moving flocks traveled through the vege- status. Plocks were followed until lost from view tation column at an average rate of 1.9 m/min or for a period of at least 30 min. We recorded (range: 0.4 m/min-4.8 m/min; standard error = the composition of the flock, agonistic interac- 0.4). All flocks were seen between 08:02 and 16: tions between birds, and the distance the flock 40. moved (the linear distance between the first and Of the 28 flocks we located on both islands, last points of observations of flock members) 76% included Northern Parulas and 79% includ- during the observation period. We classifiedun- ed Black-and-white Warblers. A significantly identified warblers as winter residents since only larger proportion of Northern Parulas partici- 2.0% of 15 1 observations of identified warblers pated in flocks on St. Thomas compared to St. were of Yellow Warblers (scientific names are John (x2 = 4.7, P c 0.05; Table l), but the pro- given in Table l), the only speciesof warbler in portion of Black-and-white Warblers in flocks the Virgin Islands that is a permanent resident was not significantly different between the is- (Pashley 1988). lands (x2 = 0.1, P > 0.05). Northern Waterthrushes and Ovenbirds oc- RESULTS curred singly, accounting for 2 1 of the 48 obser- Yellow Warbler, Bananaquit, Black-faced Grass- vations of solitary winter residents. Only one in- quit and 14 speciesof wintering warblers were dividual of each species occurred with a flock. noted during the 34.4 hr of timed survey work; Both of thesespecies are reported to be territorial all except one species (Hooded Warbler) were elsewherein their winter ranges (Schwartz 1964, found in at least one of the 28 flocks we observed Rappole and Warner 1980). (Table 1). Northern Parula and Black-and-white Seventeen of the 28 flocks (60%) had one in- Warbler were the most common winter resi- dividual of each species.Of-1 1 flocks having two dents, representing 29% and 21% of wintering or more conspecifics,only three flocks contained warblers seen on the surveys, respectively, and individuals of the same sex. Two flocks had the only four other species were commonly seen. maximum number of three Northern Parulas While conducting point counts of birds during (each with one male and two females) and one the same two month study period (Askins et al., flock had the maximum number of three Black- in press),density of Northern Parulas and Black- and-white Warblers (two males and one female). and-white Warblers in moist forests on St. John We noted 1.0 agonistic interactiomhr (based was estimated to be 4/ha and l-2/ha, respec- on 8.9 hours when flocks were under observa- tively, and on St. Thomas, the estimated den- tion) among winter residents identified to spe- sities were l-2/ha and < l/ha, respectively. cies; six of nine observations were interspecific More of the individuals in flocks were winter chases between members of a flock and three residents than permanent residents on both St. were chasesinvolving conspecifics(Northern Pa- John (x2 = 31.2, P < 0.005) and on St. Thomas nda, Cape May Warbler and American Red- (x2 = 13.7, P < 0.005). Every flock seen had start). In these chases,males supplanted females 866 DAVID N. EWERT AND ROBERT A. ASKINS

TABLE 1. Number of individuals of each speciesrecorded during the transect surveys in the Virgin Islands during 1987. Each individual was classifiedas being alone (>25 m from any other passerinebird other than a presumed mate) or in a flock (~25 m from any other passerinebird).

St. John St. Thomas Species Alone In flock Alone In Bock

Blue-winged Warbler (Vermivoru pinus) 0 1 0 0 Northern Panda (Parr& americana) 11 23 Yellow Warbler (Dendroicupetechiu) 1 i 101 Chestnut-sided Warbler (Dendroicapensylvanica) : 1 Magnolia Warbler (Dendroica magnolia) 1 : : Cape May Warbler (Dendroica tigrina) :, 1 0 5 Black-throated Blue Warbler (Dendroica caerutescens) 1 2 Black-throated Green Warbler (Dendroica virens) z : Prairie Warbler (Dendroica discolor) 0 5 : 31 Black-and-white Warbler (Mniotilta varia) 4 21 1 7 American Redstart (Setophaguruticitta) 4 11 8 3 Worm-eating Warbler (Helmitheros vermivorus) 2 3 Ovenbird (Seiurusaurocapillus) 7 1 3 : Northern Waterthrush (Seiurusnoveborucensis) 10 1 1 0 Hooded Warbler ( Wilsoniu citrina) 0 warbler (unidentified) : 4 0 : Bananaquit (Coerebajlaveola) 38 5 : 3 Black-faced Grassauit (Tiaris bicolor) 0 0 2 1

in six casesbut no females displaced males. This Powell 1985 for comparative data in the neo- is a conservative estimate of agonistic behavior tropics), and usually include only one individual becausewe observed three chasesin which nei- of each species.These flocks consist almost en- ther participant could be identified. tirely of migratory warblers, unlike most flocks The only avian predatorswe noted in the moist on mainland Mexico, Central America, and South forests of the Virgin Islands were one Peregrine America, where migrants typically join mixed- Falcon (Falcoperegrinus) and one Sharp-shinned speciesflocks of residents(Johnson 1980, Powell Hawk (Accipiter striatus). The Sharp-shinned 1980, Powell 1985, Hutto 1987, but see Chipley Hawk was the first record of an accipter in the 1976). On other West Indian islands, associa- Virgin Islands (Norton 1988). We did not ob- tions or flocks of winter residents do not appear serve a Merlin (Falco columbarius), an uncom- to be cohesive (Lack and Lack 1972, Post 1978) mon winter resident in the Virgin Islands (Wauer except on Cuba (Eaton 1953), and may include 1988, Raffaele 1989) but Merlins have been seen resident species (Eaton 1953, Lack and Lack chasing Pearly-eyed Thrashers (Margarops jius- 1972). Relatively few warblers participate in catus) on St. John (Robert Norton, pers. comm.). flocks on Puerto Rico (Willis 1973), at least in The introduced mongoose (Herpestes auropunc- cohesiveflocks (Post 1978, Faaborg, pets. comm., tutus), though common on the forest floor, prob- Staicer, in press). At Cabo Rojo, Puerto Rico, ably takes few, if any, birds in the canopy or Staicer (in press) reported that although 60% of sub-canopy where most flock members foraged Northern Parulas, Prairie Warblers, and Cape (Seaman 1952, Pimentel 1955, Seaman and May Warblers observedwere in associations(i.e., Randall 1962). within 10 m of another individual), these were not necessarily cohesive flocks. These descrip- DISCUSSION tions of flocksmust be interpreted cautiously since In winter, interspecific flocks of the same size researchershave neither defined flocks consis- and similar speciescomposition occur in small tently nor usedthe same methods to study flocks. forest fragments on St. Thomas, where the den- Flocks in the Virgin Islands moved at approx- sity and diversity ofwinter residentsare low, and imately the same rate as mixed foraging flocks in large tracts of forest on St. John, where mi- composed of permanent residents and migrants grants are more abundant. Flocks are found pri- in open forests of Costa Rica, (1.6 m/min and marily in the canopy, are relatively small (see 1.3 m/min, respectively), which is less than half WINTER FLOCKS IN THE VIRGIN ISLANDS 867 the rate of movement of flocks of resident un- David Nellis of the U.S. Virgin Islands Division of derstory speciesin the neotropics (Powell 1980). Fish and Wildlife for their invaluable assistancein find- Mixed foraging flocks are seen during most of ing study siteson St. Thomas. Kirsten Canoy provided facilities for us at the Virgin Islands Ecological Re- the daylight period in Central and South America searchStation, and she and Michael Canoy helped us (Powell 1985) and in the Virgin Islands. Ago- find study sites on St. Thomas. Gary Owen of the Ca- nistic encounters between winter residents in ribbean Research Institute and Edward Towle of the flocks or aggregationswere uncommon, as in Co- Island ResourcesFoundation provided referencesand helpful advice. We thank John Faaborg, Richard Hut- lombia (Chipley 1976), Costa Rica (Tramer and to, George Powell, Cynthia Staicer and an anonymous Kemp 1980), and Puerto Rico (Staicer, in press). reviewer for commenting on drafts of this manuscript. Social behavior and dispersion of the two most common winter residents in forestsin the Virgin LITERATURE CITED Islands, Northern Parula and Black-and-white ASIUNS, R. A., D. N. EWERT,AND R. L. NORTON. In Warbler, is quite variable during the non-breed- press. Abundance of wintering migrants in frag- ing season.At Cabo Rojo, Puerto Rico, Northern mented and continuous forests in the U.S. Virgin Islands. In J. M. Hagan and D. W. Johnston[eds.], Parulas exhibit winter site fidelity, have restrict- Ecology and conservation of neotropical migrant ed home ranges, and frequently associate with landbirds. Smithsonian Institution Press, Wash- other Parulinae, including conspecifics(Staicer, ington, DC. in press). However, at Guanica, Puerto Rico, CHIPLEY,R. M. 1976. The impact of wintering mi- grantwood warblerson residentinsectivorous pas- winter site fidelity was rarely observed in an area serines in a subtropical Colombian oak woods. where density of migrants was low and return Living Bird 15:119-141. rates were determined by mist netting (Faaborg EATON,S. W. 1953. Wood warblerswintering in Cuba. and Arendt 1984). Eaton (1953) found that Wilson Bull. 65:169-174. Northern Parulas occur in mixed-species flocks FAABORG,J., AND W. J. ARENDT. 1984. Population sizesand philopatry of winter resident warblers in on Cuba. They were more likely to join flocks Puerto Rico. J. Field Omithol. 55:376-378. on St. Thomas than on St. John, but this result GIBB, J. A. 1960. Populations of tits and goldcrests is difficult to interpret with our small sample size. and their food supplyin pine plantations. Ibis 102: Black-and-white Warblers display winter site fi- 163-208. HUTTO, R. L. 1987. A description of mixed-species delity in the West Indies (Faaborg and Arendt insectivorousbird flocks in western Mexico. Con- 1984, Robbins et al. 1987). They are frequently dor 89:282-292. found alone on Puerto Rico, as in the Virgin JOHNSON,T. B. 1980. Resident and North American Islands, yet mixed-species flocks often have sin- migrant bird interactionsin the Santa Marta high- gle individuals of this species (Staicer, pers. lands,northern Colombia, p. 239-247. In A. Keast and E. S. Morton teds.], Migrant birds in the neo- comm.). tropics: ecology, behavior, distribution and con- Although the factorsthat determine why a bird servation. Smithsonian Institution Press, Wash- joins a flock are based on complex ecologicaland ington, DC. behavioral interactions, our results suggestthat LACK,D., AND P. LACK. 1972. Wintering warblers in flocks did not differ significantly in size or species Jamaica. Living Bird 11:129-153. NORTON,R. L. 1988. West Indies region. Am. Birds composition between small, fragmented forest 42:142-144. patches on St. Thomas and relatively large and PASHLEY,D. N. 1988. Warblers of the West Indies, continuous forest tracts on St. John. This indi- I. The Virgin Islands. Caribb. J. Sci. 24:l l-22. cates that differences in habitat fragmentation PIMENTEL.D. 1955. Bioloav of the Indian Monaoose in Puerto Rico. J. Mammal. 36:62-68. - did not obviously influence flocking behavior of POST,W. 1978. Social and foraging behavior of war- wintering warblers. blers wintering in Puerto Rican coastalscrub. Wil- son Bull. 90:197-214. ACKNOWLEDGMENTS POWELL,G.V.N. 1980. Migrant participation in Neo- tropical mixed speciesflocks, p. 477-483. In A. This project was supportedby the National Geographic Keast and E. S. Morton [eds.], Migrant birds in Society, the World Nature Association and the Na- the neotropics:ecology, behavior, distribution and tional Park Service. John Miller, Caroline Rogers and conservation. Smithsonian Institution Press, Jennifer Bjork, staff on the Virgin Islands National Washington, DC. Park, helped us in many ways. Robert Norton provided POWELL,G.V.N. 1985. Sociobiology and adaptive us with information on the distribution of birds in the significanceof interspecific foraging flocks in the Virgin Islands and much logistical support. William neotropics, p. 713-732. In P. A. Buckley, M. S. Robertson generouslyshared his knowledge of Virgin Foster, E. S. Morton, R. S. Ridgley, and F. G. Island birds with us. We thank Ann Swanbeck and Buckley [eds.], Neotropical ornithology. Omithol. 868 DAVID N. EWERT AND ROBERT A. ASKINS

Monogr. No. 36. American Ornithologists’ Union, STAICER,C. A. In press. Socialbehavior of the North- Washington, DC. em Panda, Cape May Warbler and Prairie War- RAFFAELE, Ff. A. 1989. A guide to the birds of Puerto bler wintering in second-growth forest in south- Rico and the Virain Islands.Princeton Univ. Press. western Puerto Rico. In J. M. Hagan and D. W. Princeton, NJ. ” Johnston [eds.], Ecologyand conservationof neo- RAPPOLE,J. H., ANDD. W. WARNER. 1980. Ecological tropical migrant landbirds. Smithsonian Institu- aspects of migrant bird behavior in Veracruz, tion Press, Washington, DC. Mexico, p. 353-393. In A. Keast and E. S. Morton TRAMER,E. J., AND T. R. KEMP. 1980. Foragingecol- [eds.], Migrant birds in the neotropics: ecology, ogy of migrant and resident warblers and vireos behavior, distribution and conservation. Smith- in the highlandsof Costa Rica, p. 285-296. In A. sonian Institution Press, Washington, DC. Keast and E. S. Morton [eds.], Migrant birds in ROBBINS,C. S., B.A. DOWELL,D. K. DAWSON,J. COLON, the neotropics:ecology, behavior, distribution and F. ESPINOZA,J. RODRIGUEZ,R. SUTTON,AND T. conservation. Smithsonian Institution Press, VARGAS. 1987. Comparison of Neotropical win- Washington, DC. ter bird populations inisolated patchesversus ex- WAUER,R. H. 1988. Virgin Islands birdlife. Getting tensive forest. Acta Oecol. Gen. 8:285-292. to know birds and where they live. Univ. Virgin ROBERTSON.W. B., JR. 1962. Observations on the Islands Cooperative Extension Service, Extension birds of St. John, Virgin Islands. Auk 7944-76. Handbook 3. SCHWARTZ.P. 1964. The Northern Waterthrush in WILLIS.E. 0. 1973. Local distribution of mixed flocks Venezuela. Living Bird 3: 169-184. in’Puerto Rico. Wilson Bull. 85~75-77. SEAMAN,G. A. 1952. The mongooseand Caribbean WOODBURY,R. O., AND P. L. WEAVER. 1987. The wildlife. Trans. N. Amer. Wildl. Conf. 17:188- vegetation of St. John and Hassel Island, U.S. 197. Virgin Islands. U.S. National Park Research/Re- SEAMAN,G. A., ANDJ. E. RANDALL. 1962. The mon- sources Manage. Repr. SER-83, Southeast Re- gooseas a predator in the Virgin Islands. J. Mam- gional Office, Atlanta. mal. 43544-546.