Reproductive Success of the Puerto Rican Vireo in a Montane Habitat

The Wilson Journal of Ornithology 120(3):460–466, 2008

REPRODUCTIVE SUCCESS OF THE PUERTO RICAN VIREO IN A MONTANE HABITAT

ADRIANNE G. TOSSAS1,2

ABSTRACT.—I studied the reproductive success of the Puerto Rican Vireo (Vireo latimeri) from 1998 to 2000 in Maricao State Forest, a montane reserve in the southwestern part of Puerto Rico. No parasitism by the Shiny Cowbird (Molothrus bonariensis) was found but 63% of 38 active nests were depredated with an overall daily nest survival of 0.932 Ϯ 0.007 (Ϯ SE). Mean nest survival estimates following Hurricane Georges did not vary signiﬁcantly before and after the hurricane. However, a return rate of only 39% was estimated for color- marked adults the year including the hurricane compared to 72% for the year without a hurricane. A 26% decline in density of territorial males was observed in the post-hurricane year. Received 26 May 2007. Accepted 11 November 2007.

The nesting success of a bird species can cess of the PRVI and the effects of cowbird vary geographically as a result of differences parasitism in habitats other than Guańica. Par- in abundance of brood parasites, particularly asitism rates may differ within the island, par- if habitat characteristics favor the parasitic ticularly since the Shiny Cowbird is rare or species (Ward and Smith 2000, Purcell 2006). absent in some habitats. Cowbird parasitism These effects can be exacerbated if the host of vireo nests has been found to be less inten- has not evolved nest defense strategies follow- sive in highly forested montane habitats than ing recent colonization by the parasitic species in the more fragmented coastal habitats (Cruz (Rothstein 1990). This may be the case of the et al. 1985, Pe´rez-Rivera 1986). Pe´rez-Rivera Puerto Rican Vireo (PRVI; Vireo latimeri), (1986) reported that 36% of PRVI nests and which has experienced sharp decreases in 35% of Black-whiskered Vireo (Vireo altilo- population size in Guańica State Forest quus) nests in the central mountain region (henceforth Guańica) on the dry coastal plain were parasitized. In contrast, cowbirds para- in southern Puerto Rico (Faaborg et al. 1997). sitized 87% of the Black-whiskered Vireo However, the species is still commonly found nests in coastal forests (Cruz et al. 1985). throughout the island (Raffaele 1989). The The objectives of my study were to exam- population decline has been mainly associated ine if the Puerto Rican Vireo was affected by with high rates (73–83%; Woodworth 1997) Shiny Cowbird parasitism in Maricao as has of brood parasitism by the Shiny Cowbird been reported in Guańica (Woodworth 1997, (Molothrus bonariensis), a species first re- 1999; Woodworth et al. 1998, 1999), and to ported in Guańica by 1969 (Kepler and Kepler compare the breeding biology between the 1970), while expanding its range into the Ca- two populations. The incidental passage of ribbean region from its native distribution in Hurricane Georges through the Maricao study South America (Cruz et al. 1985). The PRVI area in September 1998 provided an oppor- population in Guańica from 1973 to 1996 tunity to evaluate how reproductive parame- showed an average annual decrease of 5% ters were affected by the natural event. I hy- (Faaborg et al. 1997). Mean annual survival pothesized that higher nesting success should estimates for the PRVI in Guańica decreased occur at Maricao due to the scarcity of cow- from 0.68 in 1973–1990 (Faaborg and Arendt birds at this site, compared to Guańica, and 1995) to 0.61 for a longer time period includ- that parameters related to the breeding biology ing data through 1996 (Faaborg et al. 1997). of the Puerto Rican Vireo should differ in the Little is known about the reproductive suc- years before and after the passage of the hurricane.

1 Department of Biology, University of Puerto Rico, METHODS Rıó Piedras, Puerto Rico 00931, USA. Њ Ј Њ Ј 2 Current address: Villas del Rıó, 1100 Bambu´, Ma- Study Area.—Maricao (18 09 N, 66 59 yaguëz, Puerto Rico 00681, USA; W) is in the western part of the central moun- e-mail: [email protected] tain range of Puerto Rico, 16 km northwest of 460 Tossas • REPRODUCTIVE SUCCESS OF THE PUERTO RICAN VIREO 461

Guańica. It comprises 4,150 ha with eleva- as signs of brood parasitism. Predation was tions ranging from 150 to 875 m (Silander et assumed if eggs disappeared before their al. 1986). Annual rainfall and temperature hatching date. Chicks were considered pre- from 1961 to 1990 averaged 232.6 cm and dated if they disappeared from the area around 21.7Њ C, respectively. The area contains sub- the nest before the expected fledging date. In- tropical moist forest, subtropical wet forest, cubation and nestling stages are 15 and 12 and lower montane wet forest (Ewel and days, respectively (Woodworth 1997). The Whitmore 1973). Dominant tree species in- dates marking the initiation or end of each clude Micropholis chrysophylloides, Tere- stage were assigned by direct observation or braria resinosa, Linociera dominguensis, by back-dating or forward-dating from other Homalium racemosum, Tabebuia schumanni- observed events. Nest abandonment was re- ana, and Eugenia stahlii (Silander et al. corded if adults or chicks were not seen in the 1986). nest or in the vicinity during three consecutive Maricao was struck by Hurricane Georges visits of 30 min each. I noted if abandoned on 21–22 September 1998. This hurricane nests included eggs or were empty. (category 3 on Saffir-Simpson scale of 5) Nest searching in 1999 was limited because damaged the forest structure with sustained of changes in forest structure related to Hur- winds of 184 kph and gusts of 240 kph (Ben- ricane Georges and management practices that nett and Mojica 1998). The strong winds restricted access to trails. However, I moni- opened the canopy by severe defoliation, tree tored pairs throughout the breeding season falls, and stem and branch breakage, all of and was able to ascertain their productivity which affected the resident avifauna (Tossas even when I did not find the nest structures. 2006). Breeding season length was calculated for Nest Searching and Monitoring.—Thirty- each study year from the day when the first six adult PRVIs were color-banded from 1998 pair started building a new nest until no active to 2000 to facilitate behavioral observations. nests were found. Individuals were lured to mist nets by playing Nest Success.—Daily nest survival rates the male’s song on a tape recorder near or and standard errors were generated following within their territories. Gender, age, and Mayfield (1975) and Johnson (1979). Esti- breeding condition were assigned for captured mates were calculated for two nesting inter- vireos when possible. Twenty adult vireos vals separately, from egg laying to incubation, were confidently separated by gender, 15 were and for the nestling period. An overall esti- males and five were females. All captured in- mate was calculated for both intervals com- dividuals were marked with a USGS alumi- bined. num band and a unique combination of two Index of Annual Reproductive Success.—An plastic color bands. At least one member of estimate of annual reproductive success was all pairs studied was color-banded. calculated for all nests built by a pair moni- Vireo pairs were monitored from March to tored throughout the entire breeding season. July each year from 1998 to 2000 along three However, only single nests were found for trails with similar floristic composition and most pairs in 1998 (17/22) and 2000 (11/22), vegetation structure. I followed singing males and only three nests were monitored in 1999. looking for behavioral cues that could lead to Thus, identifying whether a pair had success- their nests, or pairs carrying nesting material fully raised chicks was based on observations up to ϳ50 m into the forest on each side of of juveniles attended by color-marked adults the trails surveyed. Nest contents were ex- at the end of the breeding season. This method amined every 2–5 days for signs of predation, was possible because juvenile PRVIs remain parasitism or nest abandonment. Nests were within their natal territories and are fed by checked directly or with a mirror fixed to a adults for as long as 2 months after fledging 6-m pole. Nests were checked when possible (pers. obs.). from a distance of ϳ10 m to avoid disturbing Territories of color-marked individuals were breeding pairs, altering the nesting habitat, or visited every 2–5 days throughout the breed- increasing the risk of predation. I looked for ing season in search of nests, but searches for Shiny Cowbird eggs or chicks in PRVI nests juveniles were more intensive from June to 462 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 120, No. 3, September 2008

July, the final weeks of the breeding season. I (n ϭ 13). Males and females were pooled for played the song of the PRVI in a tape recorder analysis due to small sample size and inability 1–2 times from the center of each territory for to differentiate gender of all individuals. Re- 5-min intervals and looked for both members turn rates were calculated for two time peri- of the pair during visits that lasted from 30 ods, a year including the hurricane event min to 1 hr. Adults usually approached the (1998–1999) and a year without a hurricane source of the intruder’s song and responded (1999–2000). Confidence intervals for return with alarm calls, providing an opportunity to rates were calculated assuming binomial sam- observe accompanying juveniles. Each terri- pling. tory was visited 10–15 times, and 20–22 pairs Spot maps were prepared for PRVIs along were monitored each year with at least one three transects to measure size and density of member of the pair being observed in every male territories. I assumed territorial bound- visit. aries occurred where males or pairs engaged I calculated the proportion of juveniles at- in aggressive encounters with neighbors and tended by adults as an index of annual repro- intruders (Bibby et al. 2000). The location of ductive success by the end of the breeding singing males, nests, and movements of indi- season. The index was the mean number of viduals also helped delineate territories. The young fledged/pair/year. This method may un- mating status of males was assigned from ob- derestimate reproductive success if chicks servations during repeated visits to the terri- were depredated after leaving the nest, if their tories throughout the breeding season. Un- presence was not detected by being concealed paired males were excluded from the total and quiet, or if the brood was split between density of territorial males (Wenny et al. the pair. However, even with these biases, the 1993). All results are presented as mean (Ϯ index allows comparisons among study years. SD), except for nest survival rates (x¯ Ϯ SE). Shiny Cowbird Surveys.—I performed 60 point counts with playbacks of the Shiny RESULTS Cowbird chatter call along the two trails Nest Success.—I found 38 active PRVI where I did most nest searching. Counts were nests in Maricao during the breeding seasons conducted in June 1998 (n ϭ 30) and 1999 (n of 1998 to 2000. Most were found during nest ϭ 30) when PRVIs and most resident bird building or early in the incubation period. All species were breeding. Fifteen point counts of cases of nesting failure (24 of 38 nests) were unlimited radius, 100 m apart, were conducted attributed to predation, resulting in loss of the in each trail. I played the call in the center of entire clutch or brood. Average clutch size each plot for 5 min and waited 5 min for a was 2.03 Ϯ 0.16 eggs (range ϭ 2–3, n ϭ 38). cowbird aural or visual response. I also care- Mean nest height was 4.9 Ϯ 1.29 m (range ϭ fully looked for indications of cowbird pres- 2.4–7.0 m, n ϭ 19). Breeding seasons lasted ence during field visits throughout the project. 90, 107, and 88 days in 1998 (26 Apr to 24 Return Rates and Territory Mapping.— Jul), 1999 (23 Mar to 15 Jul), and 2000 (14 Marked PRVIs were followed over time to ob- Apr to 10 Jul), respectively. Nesting activity tain estimates of their return rates (finite sur- was not observed after 15 July of any year. vival rate described by Krebs 1989). The an- None of the PRVI nests was parasitized by nual return rate was estimated as the number the Shiny Cowbird. Cowbirds were not de- of marked adults present in their territories in tected on any of the point counts in the forest year t that were recaptured or reobserved in interior with or without playbacks. I only had their territories in year t ϩ 1. The probability incidental observations of individual cowbirds of relocating banded PRVIs using this method outside of the study sites, along edges or in is high (Woodworth et al. 1999), although it disturbed areas such as around the forest head- does not account for individuals that may be quarters and near the communications anten- alive but not in the sampling area. Thus, nae and vacation center. I observed single adults were assumed to have died or dispersed cowbirds on two occasions following Greater outside the search area if not observed the fol- Antillean Orioles (Icterus dominicensis) at the lowing year. The study included 26 adult vir- forest edge and twice observed single cow- eos color-banded in 1998 (n ϭ 13) and 1999 birds flying high over the forest canopy. Tossas • REPRODUCTIVE SUCCESS OF THE PUERTO RICAN VIREO 463

TABLE 1. Nest survival rates (x¯ Ϯ SE) of the Puerto Rican Vireo in Maricao State Forest, Puerto Rico, 1998–2000.

Daily survival ratesa

Egg laying and Overall (egg laying Year Nests Observation days incubation Nestling through nestling) 1998 20 411 0.981 Ϯ 0.008 0.953 Ϯ 0.017 0.935 Ϯ 0.029 1999 2 27 b 2000 16 246 0.951 Ϯ 0.017 0.976 Ϯ 0.017 0.928 Ϯ 0.013 Totals 38 684 0.968 Ϯ 0.008 0.963 Ϯ 0.012 0.932 Ϯ 0.007

a Incubation and nestling stages consist of 15 and 12 days, respectively (Woodworth 1997). b Analysis not possible due to small sample size, data added to total.

Fourteen nests were lost during 440 nest- turn rate (x¯ Ϯ SD) of territorial adults from days of incubation and nine nests were lost 1998 to 1999 was 0.39 Ϯ 0.14 (n ϭ 5/13; 95% during 244 days of the nestling stage (Table CI ϭ 0.14–0.68). Thirteen individuals marked 1). The probabilities (x¯ Ϯ SE) that a nest in 1999 were added to the sample group of would survive 15 days of incubation and 12 five survivors from 1998. The return rate from days of the nestling stage were 0.62 Ϯ 0.008 1999 to 2000 was 0.72 Ϯ 0.11 (n ϭ 13/18; and 0.64 Ϯ 0.012, respectively. The probabil- 95% CI ϭ 0.47–0.90). None of the individuals ity of survival from incubation to fledging was marked in 1998 and missing in the 1999 sam- 40%. Daily survival rates did not differ be- pling period was reobserved in 2000. tween the incubation and nestling periods Mean (Ϯ SD) territory size was 0.86 Ϯ 0.20 ␹2 ϭ ϭ ϭ ( 1 4.08, P 0.25) or between study years ha and ranged from 0.56 to 1.08 ha (n 9). ␹2 ϭ ϭ ( 3 0.05, P 0.83). I did not observe territory switching in Annual Reproductive Success.—Puerto Ri- marked individuals. Ninety-one percent of all can Vireo pairs in Maricao renested as many territorial males (1998–2000) were paired (n as three times in a single season after losses ϭ 69). This number ranged from 96% in 1998 to predation, but most pairs (17/22, based on (n ϭ 23) to 87% in 1999 (n ϭ 23) and 91% 1998 data) had a single nest. The percent of in 2000 (n ϭ 23). Density of male territories successful pairs ranged from 36 in 1998 to 65 was 0.57, 0.44, and 0.41/ha, in 1998, 1999, in 1999, and was 55% in 2000. None of the and 2000, respectively. pairs had two successful clutches in one season. A total of 40 fledglings was produced by DISCUSSION 33 successful pairs (n ϭ 64 total pairs) during The main difference in nesting success be- the 3 years. Successful pairs produced an av- tween Maricao and Guańica PRVI populations erage (Ϯ SD) of 1.21 Ϯ 0.55 fledglings/year, was related to parasitism rates. Brood parasit- ranging from 1.15 Ϯ 0.55 fledglings/pair in ism was absent in the montane population, but 1999 to 1.25 Ϯ 0.62 fledglings/pair in 2000. 73–83% of the nests in the lowland were par- I was able to follow 16 PRVI pairs for more asitized by the Shiny Cowbird (Woodworth than one breeding season. Most pairs (81%) 1997). The dissimilarity is related to high were successful in at least 1 of the 3 years. cowbird abundance in the southern coastal Four of the nine pairs I observed during 1998- plain where the species is favored by land 2000 were successful once, two were success- uses such as agriculture and cattle ranching. ful twice, and three pairs failed to produce Large pastures in the vicinity of the forest re- fledglings in any year. None of the pairs was serve in Guańica (S. Molina-Coloń, pers. successful in all 3 years. Seven pairs were fol- comm.) contrast to only 4% pastures in 19,382 lowed in two breeding seasons. Three suc- ha surrounding Maricao (Tossas and Thomlin- cessfully reared young in both years, while the son 2007). The lower and more open structure rest were successful in 1 year. Fifty-one per- of the dry forest in Guańica may also facilitate cent of all pairs (n ϭ 64 pair years) produced nest searching by cowbirds. offspring in at least 1 year. Differences in reproductive success be- Return Rates and Territory Size.—The re- tween PRVIs in Maricao and Guańica may be 464 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 120, No. 3, September 2008 affecting the amount of effort invested by (Margarops fuscatus), Puerto Rican Lizard pairs in nest building and egg-laying. Pairs in Cuckoo (Coccyzus vieilloti), and Red-legged Maricao built new nests after a nesting failure Thrush (Turdus plumbeus). I cannot dismiss but not after a successful nest. However, pairs the possibility that nest losses were caused by in Guańica renested as many as six times after rodents due to the difficulty of predator iden- previous failures and also had second broods tification based on nest damage characteristics (Woodworth 1997). This behavior may com- (Marini and Melo 1998). pensate for high rates of nest loss and low Woodworth (1999) indicated that Guańica reproductive success. Weather condition may is a population sink for the PRVI due to high be the ultimate factor affecting the number of levels of nest parasitism and depredation. A nesting attempts and population dynamics computer simulation model of the PRVI pop- overall, as extremely dry conditions have been ulation with the levels of reproductive success shown to have a direct relationship with bird found at Maricao implies this population has population declines in Guańica (Faaborg et al. a positive growth rate even when the produc- 1984). Woodworth (1997) found the arrival of tivity index used in the model is probably an rains early in 1991 facilitated rapid initiation underestimate of the real number of fledglings of breeding behavior by the PRVI in Guańica produced (Tossas 2002). The differences in with increased opportunities for laying second nest success between the two populations con- clutches. That year, six pairs that had fledged tribute to the idea of a source-sink metapop- young by the beginning of June were able to ulation structure, but the short dispersal dis- initiate second broods 1–2 weeks later. In ad- tances reported for juvenile PRVIs (Wood- dition to higher chances of reproductive suc- worth et al. 1998) suggest that Maricao indi- cess, nests initiated early in the season were viduals may not migrate as far as Guańica. less likely to be parasitized (Woodworth Alternatively, the surplus of individuals pro- 1997). duced at Maricao may disperse to forest frag- Maricao and Guańica can be considered the ments surrounding this forest reserve (Tossas extremes of a gradient differing in elevation and Thomlinson 2007). above sea level and habitat characteristics, but Hurricanes directly increase avian mortality aspects of the biology of PRVI were similar rates due to collisions during the storm, or in- in both populations. For example, breeding directly by destroying the resources on which season lengths at Maricao were similar to they depend (Wiley and Wunderle 1993). A those observed in Guańica (106 days in 1991, study of the impacts of Hurricane Georges in 69 days in 1993; Woodworth 1997). High lev- 1998 to the Maricao avian community showed els of nest predation were responsible for nest that 16 of 21 species, including the PRVI, de- losses at both sites with 63% of nests at Mar- clined after the hurricane (Tossas 2006). The icao (this study) and 70% of nests at Guańica reasons for the decline of each species may (Woodworth 1997) lost to predation. These vary since they have particular ecological re- numbers are typical for open-cup understory quirements and evidence on how resources are nests in the Neotropics, as 67% of nests in wet affected is scant. Nesting success parameters forests in Costa Rica (Skutch 1985), and 62 of PRVI did not vary significantly before and and 68% of nests in lowland Panama, in 1996 after the hurricane, but a low return rate of and 1997, respectively (Robinson et al. 2000) color-banded adults was observed in 1999 were lost to predation. suggesting that disappearance of individuals The identity of predators could not be as- was related to the effects of the hurricane. certained in most cases in the present study, This finding contrasts with the vireo’s high re- but the lack of damage to the nests suggests turn rate and territorial fidelity in a year with- birds or reptiles were responsible. Snakes (Al- out a hurricane. Thus, the 1999–2000 annual sophis portoricensis, Epicrates inornatus) that return rates of 72% was likely typical for the could predate bird eggs or young are uncom- species, while the 39% return estimated from mon in Maricao, but potential avian predators 1998 to 1999 was probably caused by either were frequently observed. The latter include mortality or dispersal during or immediately the Puerto Rican Sharp-shinned Hawk (Accip- after the hurricane. The estimated return in iter striatus venator), Pearly-eyed Thrasher 1999–2000 is similar to the annual survival Tossas • REPRODUCTIVE SUCCESS OF THE PUERTO RICAN VIREO 465 probabilities (68–74%) of PRVIs at Guańica FAABORG,J.AND W. J. ARENDT. 1995. Survival rates reported by Woodworth et al. (1999), Faaborg of Puerto Rican birds: are islands really that dif- and Arendt (1995), and Faaborg et al. (1997). ferent? Auk 112:503–507. FAABORG, J., W. J. ARENDT, AND M. S. KAISER. 1984. The decline in adult return rate resulted in Rainfall correlates of bird population fluctuations 26% lower density of territorial males the year in a Puerto Rican dry forest: a nine year study. following the hurricane. Wilson Bulletin 96:575–593. Nest parasitism was not found to threaten FAABORG, J., K. M. DUGGER,W.J.ARENDT,B.L. the PRVI in this study, but the Shiny Cowbird WOODWORTH, AND M. E. BALTZ. 1997. Population population may increase at Maricao if hurri- declines of the Puerto Rican Vireo in Guańica Forest. Wilson Bulletin 109:195–202. canes facilitate their colonization by opening JOHNSON, D. H. 1979. Estimating nest success: the the canopy and altering microhabitat charac- Mayfield method and an alternative. Auk 96:651– teristics. Brood parasites and predators may 661. become more abundant if the amount of dense KEPLER,C.B.AND A. K. KEPLER. 1970. Preliminary forest in the area surrounding Maricao is af- comparison of bird species diversity and density fected by fragmentation and deforestation. in Luquillo and Guańica forests, Pages E183–191 Changes in land-use practices affecting habitat in A tropical rain forest (H. T. Odum, Editor). U.S. Atomic Energy Commission, Oak Ridge, Tennes- continuity include rural development by an in- see, USA. creased human population and substitution of KREBS, C. J. 1989. Ecological methodology. Harper traditional shade-coffee plantations for sun- and Row, Publishers, New York, USA. grown varieties or other crops with higher MARINI,M.Aˆ . AND C. MELO. 1998. Predators of quail yields. eggs, and the evidence of the remains: implica- tions for nest predation studies. Condor 100:395– ACKNOWLEDGMENTS 399. MAYFIELD, H. F. 1975. Suggestions for calculating nest This work was supported by the Department of Bi- success. Wilson Bulletin 87:456–466. ology at the University of Puerto Rico, Rıó Piedras PE´ REZ-RIVERA, R. A. 1986. Parasitism by the Shiny and grants from the National Science Foundation Cowbird in the interior parts of Puerto Rico. Jour- (Center for Research Excellence in Science and Tech- nal of Field Ornithology 57:99–104. nology, HRD-9353549; Alliance for Graduate Educa- PURCELL, K. L. 2006. Abundance and productivity of tion and the Professoriate, HRD-9817642). The Puerto Warbling Vireos across an elevational gradient in Rico Department of Natural and Environmental Re- the Sierra Nevada. Condor 108:315–325. sources provided permits to work in Maricao State RAFFAELE, H. A. 1989. A guide to the birds of Puerto Forest. Beatriz Hernańdez and L. A. Mun˜iz-Campos Rico and the Virgin Islands. Princeton University were helpful in the field. I am grateful to J. M. Wun- Press, Princeton, New Jersey, USA. derle Jr. and R. B. Waide for guidance during the de- ROBINSON, W. D., T. R. 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