Wilson Bull., 114(1), 2002, pp. 99±105

SITE FIDELITY AND EPHEMERAL HABITAT OCCUPANCY: NORTHERN USE OF PUERTO RICAN BLACK MANGROVES DURING THE NONBREEDING SEASON

LEONARD REITSMA,1,2,6 PAMELA HUNT,1,3 SHERMAN L. BURSON, III,1,4 AND BENJAMIN B. STEELE1,5

ABSTRACT.ÐWe studied the dispersion of Northern (Seiurus novaboracensis) in southwestern during four nonbreeding seasons, 1996±1999. Densities were high (up to 13 /ha) on a 3-ha mature black mangrove (Avicennia germinans) study plot, but were signi®cantly lower during periods of high water levels. Individuals exhibited site ®delity within and between seasons. Feeding areas were small (mean ϭ 0.074 ha Ϯ 0.041 SD) and there was considerable overlap tolerated among conspeci®cs. Waterthrush density decreased when water submerged their primary foraging substrate: woody debris and pneumatophores. Inter- annual returns were similar to other Neotropical migrants (mean ϭ 50%) but site persistence was low due to periodic ¯ooding. In September 1998, hurricane Georges ¯ooded the plot and blew down Ͼ90% of the black mangrove trees. This drastic habitat alteration was followed by a drastic decline in waterthrushes using the study area. Individuals left feeding areas for overnight roost sites in red mangroves (Rhizophora mangle). This latter ®nding, coupled with site ®delity and high return rates concurrent with low site persistence, suggests that waterthrushes exhibit high plasticity in their use of habitat during the nonbreeding season, but may rely upon mangroves for overwinter survival. Received 15 May 2001, accepted 13 March 2002.

Nearctic-Neotropical migratory Waide 1994). Already, forest cover is esti- spend over half of each year in the tropics, mated to have declined to about 21% of island commonly returning to the same sites (Faa- land area in the Caribbean (Wunderle and borg and Arendt 1984, Lynch et al. 1985, Waide 1994). Kricher and Davis 1986, Holmes et al. 1989, Mangrove forests have some of the highest Winker et al. 1990). This realization has likely abundances of overwintering migrants in the in¯uenced the increase in studies that focus Caribbean (Arendt 1992, Lefebvre et al. 1992, on winter habitat loss and its effect on popu- Robbins et al. 1992, Sliwa and Sherry 1992, lations of migrants. While almost all migrants Wunderle and Waide 1993). However, few de- are territorial during the breeding season, win- tailed population studies have been conducted ter habitat use varies widely between species in mangroves (but see Holmes and Sherry and many species' use of habitat is still poorly 1992; Lefebvre et al. 1992, 1994; Lefebvre understood. Population studies recently have and Poulin 1996; Marra and Holmes 2001). demonstrated that optimal nonbreeding habitat Several studies have used marked individuals may be limiting, at least for some species on the wintering grounds to quantify overwin- (Rappole et al. 1989, Marra et al. 1998), thus ter site persistence, annual return rates, and the high rate of deforestation in Middle Amer- dispersion (Winker et al. 1990, Holmes and ica and the Caribbean potentially will have a Sherry 1992, Mabey and Morton 1992, Stai- great impact on migrant populations cer 1992, Wunderle 1992, Marra et al. 1998). (Rappole and Powell 1986, Wunderle and Some migrants in mangroves defend territo- ries (Holmes et al. 1989, Rappole et al. 1989, 1 New England Inst. for Landscape Ecology, Box Mabey and Morton 1992). 38-H, RR2, Canaan, NH 03741, USA. 2 Current address: Dept. of Natural Sciences, Plym- The Northern Waterthrush (Seiurus nova- outh State College, Plymouth, NH 03264, USA. boracensis) has been documented as the most 3 Current address: Audubon Society of New Hamp- abundant migrant in some man- shire, 3 Silk Farm Rd., Concord, NH 03301, USA. groves (Lefebvre et al. 1992, Wunderle and 4 Current address: P.O. Box 47, Denali Park, AK Waide 1993). Lefebrve et al. (1994) and Le- 99755, USA. febvre and Poulin (1996) concluded this spe- 5 Current address: Dept. of Natural Sciences, Colby- Sawyer College, New London, NH 03257, USA. cies was not territorial in either Venezuelan or 6 Corresponding author; E-mail: Panamanian mangroves, respectively, in con- [email protected] trast to Schwartz's (1964) reports of territori- 99 100 THE WILSON BULLETIN • Vol. 114, No. 1, March 2002 ality in this species within an irrigated botan- We collected data in January during the 1995±1996 ical garden in . nonbreeding season (September through early April), We monitored the use of a mature black and in October, January, and March for the following 3 years. We captured Northern Waterthrushes in mist mangrove (Avicennia germinans) forest by a nets, and color banded each bird. We measured weight, marked population of Northern Waterthrushes fat (0±4 visual index of interfurcular fat), and un¯at- over four nonbreeding seasons to determine tened wing chord. During the last 2 years, we collected whether this species (1) exhibits site ®delity 5 ␮l of blood from the brachial vein of birds to sex in this habitat, and (2) responds to changes in individuals of this monochromatic species. We used a habitat suitability within and between seasons. combination of PCR and electophoretic detection of diagnostic protein bands to determine sex (Grif®ths et Natural changes that occurred during our al. 1996) for some individuals, and nonoverlapping study allowed us to document the degree of wing chord length distinctions in others (Ͼ78 mm for plasticity that this species exhibits in response males, Ͻ71 mm for females; Pyle et al. 1987). to both short term and long term habitat We systematically searched the plot for Northern changes. Waterthrushes during each time period, and locations and movements of banded and unbanded birds were STUDY AREA AND METHODS recorded. A bird was considered a resident if it was We mapped the spacing pattern, and measured den- relocated Ն4 days in the same area in one time period sity and site persistence of Northern Waterthrushes on of a single season. All observations were compiled a 3-ha plot within a mature black mangrove forest. during each trip to produce minimum activity ranges, This site (18Њ 01Ј N, 67Њ 10Ј W) was located on the which were the minimum convex polygons encom- Boqueron Bird Refuge, a 182-ha reserve located 8 km passing all observations of individual birds (Holmes et south of Cabo Rojo in southwestern Puerto Rico and al. 1989). We refer to these minimum activity ranges managed by the Puerto Rican Dept. of Natural Re- as feeding areas rather than territories because individ- sources. This forest was separated from a red man- uals did not aggressively exclude conspeci®cs, but grove (Rizophora mangle) stand and open lagoons by they did exhibit within and between season site ®del- a manmade dike. The plot had a mean canopy height ity, and they actively fed in these restricted areas. In- of 12 m and a mean of 256 trees/ha, nearly all of which terannual return rates and site persistence were based were black mangroves. The forest ¯oor was a combi- upon resightings and recaptures. We investigated nation of shallow water (Ͻ25% cover when not ¯ood- movement of birds toward and away from the plot us- ed), leaf litter, mangrove pneumatophores, and woody ing mist netting during all three times of the nonbreed- debris. Waterthrushes foraged predominantly on the ing season. An 80±120 m mist net lane was operated forest ¯oor, but also searched for food on woody de- for 4 h after dawn and 4 h preceding dusk on the dike bris, upturned root masses, and in black mangrove along the west edge of the plot. Travel direction was trees. recorded for each bird captured. Periodic ¯ooding of the plot occurred throughout We used simple regression to correlate water depth our study as a result of variations in rainfall between on the plot with bird density, and to correlate density September and December. We measured water depth with the amount of overlap in feeding areas. Overlap beginning in January 1997 using a gauge placed in a was quanti®ed as the percent cover of the plot in which channel adjacent to the plot and calibrated the channel two or more feeding areas overlapped. We used chi depth to the plot water depth. For time periods prior square analysis to test for differences in travel direc- to the placement of the gauge in the ®eld, we estimated tion of birds into the mist net lane after dawn com- the water levels by comparing the relative exposure of pared to before dusk. Seasonal changes in body mass/ pneumatophores and percent open water cover on the wing chord ratios were analyzed using ANOVA to plot. March 1997 and 1998 and January 1998 were measure weight loss during the nonbreeding season. similar in water level to the conditions prior to instal- To obtain an adequate sample size for analysis of all ling the gauge, and measurements during these three three time periods of each season, the sample included intervals allowed reasonably accurate estimates of the all Northern Waterthrushes caught on the plot and on two periods previous to installation. Before Hurricane the dike adjacent to the plot, most of which did not Georges, the mangrove forest had a closed canopy and have feeding areas on the plot. We also used a t-test little foliage below 2.5 m. The hurricane dramatically to analyze differences in body mass/wing chord ratios altered this habitat in September 1998, leaving Ͻ10% for resident birds on the plot during October and Jan- of the trees standing. The plot was still ¯ooded 6 uary, for which we had adequate sample sizes. months after the hurricane, and little vegetative re- growth had occurred. We estimated the water levels RESULTS for the periods after the hurricane by comparing the The number of Northern Waterthrushes on height of water on prominent standing trees and other unaltered features on and around the plot to ¯ooded the 3-ha study plot ranged from 39 in October periods when the gauge was in place, such as January 1996 to 0 in March 1999 (Fig. 1). Feeding and October 1997. areas were small (mean ϭ 0.074 ha Ϯ 0.041 Reitsma et al. • NORTHERN WATERTHRUSHES IN PUERTO RICO 101

FIG. 1. Feeding areas of Northern Waterthrushes on a 3-ha mature black mangrove study plot at the Bo- queron Bird Refuge in southwestern Puerto Rico during four nonbreeding seasons. Each polygon encloses all the observations of individually color-marked birds that were resighted Ն4 times on the plot during each des- ignated time period. The number of mapped birds and the mean water depth of the plot (WD, in cm) is shown. The entire plot is covered with water when WD Ͼ 25 cm. The west side of the plot is bounded by a dike. Hurricane Georges drastically altered the structure of this forest plot in September 1998 just prior to Year Four. By March 1999, no resident birds remained.

SD, n ϭ 158). During times of high density, January 1997 with a corresponding Ͼ50% de- there was greater overlap in feeding areas (F1,8 cline in density (Fig. 1). Flooding also oc- ϭ 26.49, P ϭ 0.0009, r2 ϭ 0.768). Density curred prior to October 1997 and October ¯uctuated widely within and between seasons 1998 and was associated with much lower and was negatively correlated with water densities compared to October 1996. The low- 2 depth (F1,7 ϭ 5.403, P ϭ 0.049, r ϭ 0.403). est densities occurred after severe hurricane Flooding occurred between October 1996 and damage (October, January, and March of the 102 THE WILSON BULLETIN • Vol. 114, No. 1, March 2002 fourth season). Overall site persistence was DNA and 13 were males (28.9%), a propor- low (42%) and varied from one interval to tion consistent with that of the residents. another. Of 23 Northern Waterthrushes with well- DISCUSSION documented feeding areas on the plot during When not ¯ooded, this mature black man- January of 1996, 12 (52%) returned the fol- grove forest had among the highest density lowing season and ®ve (22%) returned again (13 birds/ha) of marked resident birds of any the next season. Return rates were lower be- previously recorded Neotropical migrant pop- tween the second and third seasons of this ulation (but see Marra and Holmes 2001, who study (8 of 26 birds, 31%) and lower still be- reported 15 American Redstarts, Setophaga tween the third and fourth seasons (2 of 14 ruticilla, per ha during fall and 12/ha during birds, 14%). Of 63 well-mapped birds over the spring). However, the conditions of this and 4-year study, 21 (33% of the original cohort) other habitats (PH unpubl. data) used by had feeding areas in Ն2 consecutive years and Northern Waterthrushes were not stable. On ®ve (7.9% of the original cohort) had feeding the black mangrove plot, densities were in- areas in three consecutive years. One of the versely associated with water level. A similar 3-year occupants was recaptured the fourth pattern was documented in other habitats in season but did not have a known feeding area southwestern Puerto Rico (LR unpubl. data). on the plot. Moist ground appears to be the most suitable Waterthrushes lost mass throughout the sea- feeding substrate for waterthrushes in black son. The body mass/wing chord ratio of all mangroves, but standing water that submerges waterthrushes, including transients, decreased pneumatophore tips precludes their ability to throughout the nonbreeding season from Oc- forage on the ground. Woody debris and dead tober (mean ϭ 0.23 g/mm Ϯ 0.02 SD, n ϭ trunks remain useable when pneumatophores 158) to January (0.22 Ϯ 0.01 SD, n ϭ 102) are submerged, but the majority of water- and then further in March (0.21 Ϯ 0.02 SD, n thrushes we observed foraging were walking

ϭ 9; F2,226 ϭ 4.71, P ϭ 0.01). For resident on the ground among the pneumatophores. birds, we had adequate sample sizes to com- The linear dispersion of individuals along the pare only October to January, and the pattern drier dike during periods of ¯ooding (Fig. 1: was consistent (October mean ϭ 0.23 Ϯ 0.02 October 1997 and March 1998) provides fur- SD, n ϭ 23, and January mean ϭ 0.22 Ϯ 0.02 ther evidence for the importance of water lev- SD, n ϭ 47; t ϭ 1.998, df ϭ 68, P ϭ 0.05). el. These individuals had moist ground sub- Birds consistently moved across the dike strate for foraging, allowing them to persist from the red mangrove side in the morning along the edge of the plot. and from the black mangrove side at dusk (n The response of Northern Waterthrushes to ϭ 100, ␹2 ϭ 37.37, df ϭ 1, P Ͻ 0.001). Of Hurricane Georges was marked. The hurricane the birds captured in the morning, 64% (n ϭ covered the plot with an estimated mean of 45) were going from red to black mangrove. 0.75 m of water and destroyed most of the Of the birds captured in the evening, 89% (n canopy trees. Even though many birds were ϭ 55) were going from black to red man- caught in nets on the dike and on the plot grove. within 3 weeks after the hurricane, these birds We determined the sex of 25 resident birds, likely were spending most of their time off the 10 using blood DNA and 15 using wing chord plot because only 2 of 48 caught on or near lengths. Genetic analysis of blood showed no the plot were resighted. Bird activity on the sex bias (6 of 10 sexed were males). The ma- plot declined during the 2 weeks following the jority (14 of 15, 93%) of all birds sexed by hurricane, October 6±22, 1998, and was mark- wing chord length were females. Therefore, 7 edly lower still in January and March 1999. of 25 birds of known gender with well- This probably was due to a lack of preferred mapped feeding areas on the plot were male foraging substrate because of the ¯ooding and (28%). The sample sizes were too small to the disappearance of the canopy, which pro- compare sex ratios within or between seasons. vided shade (see Schwartz 1964 for proposed An additional 45 transients caught on or ad- minimum habitat requirements). jacent to the plot were sexed using blood The fact that abundance on the plot varied Reitsma et al. • NORTHERN WATERTHRUSHES IN PUERTO RICO 103 so greatly during each winter (19 to 39, 10 to the nonbreeding season among a local popu- 21, and 0 to 8) demonstrates the ability, and lation of Northern Waterthrushes within an ur- perhaps the necessity of waterthrushes to shift ban park in Venezuela, but this territorial de- their daytime feeding areas. Although most of fense may have been in response to arti®cial these birds were found repeatedly on one ground moisture supplied by irrigation. Al- small part of the plot while they were resi- though most wintering migrants are territorial dents, they went elsewhere when water cov- or occur in mixed species ¯ocks, species with ered the foraging substrate and they presum- overlapping home ranges have been reported ably found suitable habitat elsewhere. Individ- elsewhere in Puerto Rico (Staicer 1992). uals banded during October sometimes were The consistent predictable movement of in- not present during January, but were seen on dividuals from black to red mangrove for the plot in March, indicating that the disap- overnight roosting was further supported by a pearance of birds from the plot during high minimal amount of radio telemetry (LR un- water did not represent mortality but move- publ. data). Individuals that fed in the black ment to other areas. This is consistent with the mangroves traveled to the red mangroves lack of season-long residence by waterthrush- across the dike in the evening and then back es in Panamanian mangroves (Lefebrve and to the black mangroves the next morning. This Poulin 1996). In Panama, waterthrushes ap- use of red mangroves was further corroborat- peared to be tracking food abundance, and the ed by more detailed studies at a site 6 km to authors suggested that the movement they the south and a site on the east side of Puerto documented may represent a broader pattern Rico (LR unpubl. data). of midwinter migration for at least some spe- Body mass to wing chord ratios declined cies of Neotropical migrants. The large num- throughout the nonbreeding season from Oc- ber of transient birds captured on or near the tober to March. This analysis was consistent plot and never resighted suggests that many for resident birds and for residents and tran- individuals simply moved through the plot. sients combined. The pattern is similar to that Recent evidence from telemetry suggests that which occurred in American Redstarts in Ja- many ``transients'' move considerable distanc- maica (Marra et al. 1998) and Belize (S. Baird es through many habitat types en route from unpubl. data). Body condition might be ex- regular feeding areas to roost sites (LR un- pected to decline given the general drying publ. data). trend in most of Puerto Rico, and especially Although many species of Neotropical mi- in the southwest, from January to the end of grants are territorial during the nonbreeding March, which often reduces arthropod abun- season (Lynch et al. 1985, Holmes et al. 1989, dance. Rappole et al. 1989, Marra et al. 1998), North- Individuals of both sexes had feeding areas ern Waterthrushes are not when they occupy on the plot. Birds that moved through the plot mangroves (see also Lefebvre et al. 1994 and and those that remained on it to feed both Lefebvre and Poulin 1996). Two to ®ve indi- showed similar sex ratios of three females to vidual feeding areas often overlapped, and one male. Despite the fact that the majority overlap increased signi®cantly with increasing (70%) of captured birds were not sexed and density on the plot. Two to three individuals therefore the complete demographic makeup occasionally were observed Ͻ2 m from each of this population was not known, our data other, and when ¯ushed they often would ¯y suggest that waterthrushes do not appear to in the same direction. Some individuals did, sexually segregate by habitat, at least to the however, exhibit site ®delity to feeding areas, same degree as some other migrant warblers returning to the same speci®c areas on the plot such as the American Redstart (Marra and on successive days and in successive years. Holmes 2001, Parrish and Sherry 1994). We Return rates were highest from ®rst to second conclude that both sexes were residents on the year (52%), and the lower rates in subsequent plot for the duration of our monitoring, but years may have been in¯uenced by the higher we do not know the precise sex ratio at any incidence of ¯ooding from 1997 through single interval of our study. 1998. Schwartz (1964) documented classic The high density of Northern Waterthrushes territorial behavior during the second half of documented in black mangroves in this study 104 THE WILSON BULLETIN • Vol. 114, No. 1, March 2002 and others (e.g., Lefebvre et al. 1994) indi- sizes and philopatry of winter resident warblers in cates the importance of this habitat to sustain- Puerto Rico. J. Field Ornithol. 55:376±378. ing waterthrush populations during the non- GRIFFITHS, R., S. DAAN, AND C. DIJKSTRA. 1996. Sex identi®cation in birds using two CHD genes. Proc. breeding season. This habitat is subject to R. Soc. Lond. Ser. B 263:1251±1256. considerable short term change, mostly due to HOLMES,R.T.AND T. W. S HERRY. 1992. Site ®delity changes in water levels, but also due to chang- of migratory warblers in temperate breeding and es in food abundance (Lefebvre and Poulin Neotropical wintering areas: implications for pop- 1996). This might particularly in¯uence its ulation dynamics, habitat selection, and conser- suitability for a ground forager such as the vation. Pp. 563±578 in Ecology and conservation of Neotropical migrant landbirds (J. M. Hagan, Northern Waterthrush. As a result, Northern III, and D. W. Johnston, Eds.). Smithsonian Inst. Waterthrushes must be capable of changing Press, Washington, D.C. locations over the course of any single non- HOLMES, R. T., T. W. SHERRY, AND L. REITSMA. 1989. breeding season, and other locations and hab- Population structure, territoriality, and overwinter itats may be essential to their survival. Lefeb- survival of two migrant warbler species in Jamai- vre and Poulin (1996) suggest that migrant ca. Condor 91:545±561. dwellers of mangroves may regularly migrate KRICHER,J.C.AND W. E. DAVIS,JR. 1986. Returns and winter-site ®delity of North American migrants into and out of habitat patches, tracking the banded in Belize, Central America. J. Field Or- phenology of arthropod food in each. In our nithol. 57:48±52. study, the changes in habitat quality resulting LEFEBVRE,G.AND B. POULIN. 1996. Seasonal abun- from ¯ooding also may eliminate the bene®ts dance of migrant birds and food resources in Pan- of territorial behavior in favor of more oppor- amanian mangrove forests. Wilson Bull. 108:748± tunistic occupation of suitable feeding sites. 759. LEFEBVRE, G., B. POULIN, AND R. MCNEIL. 1992. Abun- Despite the documented within season mo- dance, feeding behavior, and body condition of bility of this species, the high densities in Nearctic warblers wintering in Venezuelan man- mangroves, the return to mangroves when this groves. Wilson Bull. 104:400±412. habitat becomes suitable, and the daily move- LEFEBVRE, G., B. POULIN, AND R. MCNEIL. 1994. Spa- ment back into red mangroves to roost, all un- tial and social behaviour of Nearctic warblers win- derscore the importance of mangrove habitats tering in Venezuelan mangroves. Can. J. Zool. 72: to Northern Waterthrushes. The rapid human 757±764. LYNCH, J. F., E. S. MORTON, AND M. E. VAN DER destruction of mangroves has the potential to VOORT. 1985. Habitat segregation between the cause declines in the species' population. sexes of wintering Hooded Warblers (Wilsonia ci- trina). Auk 102:714±721. ACKNOWLEDGMENTS MABEY,S.E.AND E. S. MORTON. 1992. Demography and territorial behavior of wintering Kentucky This research was supported by a cooperative agree- Warblers in Panama. Pp. 329±336 in Ecology and ment with the U.S. Forest Service and the International conservation of Neotropical migrant landbirds (J. Inst. for Tropical Forestry, and by the New England M. Hagan, III and D. W. Johnston, Eds.). Smith- Inst. for Landscape Ecology. We thank M. Baltz for sonian Inst. Press, Washington, D.C. assistance in the early stages of the project, L. Gates, MARRA, P. P., K. A. HOBSON, AND R. T. HOLMES. 1998. D. Brunetti, W. DeLuca and S. Clarke for assistance in the ®eld, and L. Sheldon, K. Langlais, and K.-L. Linking winter and summer events in a migratory Boyle for sex determination from blood and feathers. bird by using stable-carbon isotopes. Science 282: B. Poulin provided valuable comments on the manu- 1884±1886. script. We are indebted to the staff of the Caribbean MARRA,P.P.AND R. T. HOLMES. 2001. Consequences Island National Wildlife Refuge for logistic support, of dominance-mediated habitat segregation in a and we thank the staff of the Boqueron Bird Refuge migrant passerine bird during the non-breeding for permission to establish our study plot. season. Auk 118:92±104. PARRISH,J.D.AND T. W. S HERRY. 1994. Sexual habitat LITERATURE CITED segregation by American Redstarts wintering in Jamaica: importance of resource seasonality. Auk ARENDT, W. J. 1992. Status of North American migrant 111:38±49. landbirds in the Caribbean region: a summary. Pp. PYLE, P. S., N. G. HOWELL,R.P.YUNICK, AND D. F. 143±174 in Ecology and conservation of Neotrop- DESANTE. 1987. Identi®cation guide to North ical migrant landbirds (J. M. Hagan, III and D. W. American passerines. Slate Creek Press, Bolinas, Johnston, Eds.). Smithsonian Inst. Press, Washing- California. ton, D.C. RAPPOLE,J.H.AND G. POWELL. 1986. The Hooded FAABORG,J.R.AND W. J. ARENDT. 1984. Population Warbler. Pp. 827±853 in Audubon wildlife report Reitsma et al. • NORTHERN WATERTHRUSHES IN PUERTO RICO 105

(R. L. Silvestro, Ed.). National Audubon Society, Parula, Cape May Warbler, and Prairie Warbler New York. wintering in second-growth forest in southwestern RAPPOLE, J. H., W. A. RAMOS, AND K. WINKLER. 1989. Puerto Rico. Pp. 308±320 in Ecology and conser- Wintering Wood movements and mortality vation of Neotropical migrant landbirds (J. M. Ha- in southern Veracruz. Auk 106:402±410. gan, III and D. W. Johnston, Eds.). Smithsonian ROBBINS, C. S., B. A. DOWELL,D.K.DAWSON,J.A. Inst. Press, Washington, D.C. COLON,R.ESTRADA,A.SUTTON,R.SUTTON, AND WINKER, K., J. H. RAPPOLE, AND M. A. RAMOS. 1990. D. WEYER. 1992. Comparison of Neotropical mi- Population dynamics of the Wood Thrush in grant landbird populations wintering in tropical southern Veracruz, Mexico. Condor 92:444±460. forest, isolated fragments, and agricultural habi- WUNDERLE, J. M., JR. 1992. Sexual habitat segregation tats. Pp. 207±210 in Ecology and conservation of in wintering Black-throated Blue Warblers in Puerto Rico. Pp. 299±307 in Ecology and conser- Neotropical migrant landbirds (J. M. Hagan, III vation of Neotropical migrant landbirds (J. M. Ha- and D. W. Johnston, Eds.). Smithsonian Inst. gan, III and D. W. Johnston, Eds.). Smithsonian Press, Washington, D.C. Inst. Press, Washington, D.C. SCHWARTZ, P. 1964. The Northern Waterthrush in Ve- WUNDERLE, J. M., JR. AND R. B. WAIDE. 1993. Distri- nezuela. Living Bird 3:169±184. bution of overwintering Nearctic migrants in the SLIWA,A.AND T. W. S HERRY. 1992. Surveying winter- Bahamas and Greater Antilles. Condor 95:904± ing warbler populations in Jamaica: point counts 933. with and without broadcast vocalizations. Condor WUNDERLE, J. M., JR. AND R. B. WAIDE. 1994. Future 94:924±936. prospects for overwintering Nearctic migrants in STAICER, C. A. 1992. Social behavior of the Northern the Caribbean. Bird Conserv. Int. 4:71±87.