Observations of a Transequatorial Migrant, Tyrannus savana Vieillot 1808 (Fork-tailed Flycatcher), in an Urban Setting on a Tropical Island
Wayne J. Arendt1*, Marvin A. Tórrez2, María M. Paulino3, Luis R. Paulino3
1International Institute of Tropical Forestry, USDA Forest Service, Sabana Field Research Station, HC 02 Box 6205, Luquillo, PR 00773. 2Universidad Centroamericana (UCA), Instituto Interdisciplinario de Ciencias Naturales, Managua, Nicaragua. 3Grupo Acción Ecológico, Calle Gabriel García, No. 105, Zona Colonial, Santo Domingo, Dominican Republic. *Corresponding author.
Urban Naturalist Notes, No. 5 (2022)
Abstract
Fork-tailed Flycatcher (Tyrannus savana, Vieillot 1808) is an easily recognized, widespread inhabitant of grassland and open terrain throughout its extensive range from southern Mexico south to Argentina. In the Caribbean, although sightings are fluid and can increase daily on some islands, especially those close to source continents and during peak migration, its numbers have continued to increase over a 19-yr period in the Caribbean Basin (2002–2021). We report a 2018 sighting of a vagrant Forktailed Flycatcher in the Dominican Republic’s capital city of Santo Domingo. Our recent observations of this vagrant attest to the important natural and anthropogenic regulatory ecosystem services provided by migratory species in general. These include the potential control of arthropod plagues and disease vectors, as well as supportive services such as pollination and seed dispersal. Conversely, the city of Santo Domingo provides reciprocal ecological services to resident and migratory birds alike.
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Volume 9, 2022 Urban Naturalist Notes No. 5
Observations of a
Transequatorial Migrant,
Tyrannus savana Vieillot
1808 (Fork-tailed
Flycatcher), in an Urban
Setting on a Tropical Island
Wayne J. Arendt, Marvin A. Tórrez,
María M. Paulino, and Luis R. Paulino
Urban Naturalist
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Cover Photograph: A vagrant, immature Fork-tailed Flycatcher in Santo Domingo, Dominican Republic, shown here
with its congener competitor, the Gray Kingbird. Photograph © Luis Paulino.
1
1International Institute of Tropical Forestry, USDA Forest Service, Sabana Field Research Station, HC
02 Box 6205, Luquillo, PR 00773. 2Universidad Centroamericana (UCA), Instituto Interdisciplinario de
Ciencias Naturales, Managua, Nicaragua. 3Grupo Acción Ecológico, Calle Gabriel García, No. 105, Zona
Colonial, Santo Domingo, Dominican Republic. *Corresponding author: waynearendt@gmail.com.
Associate Editor: Sabina Caula, Universidad de Carabobo.
Observations of a Transequatorial Migrant, Tyrannus savana
Vieillot 1808 (Fork-tailed Flycatcher), in an Urban Setting on a
Tropical Island
Wayne J. Arendt1*, Marvin A. Tórrez2, María M. Paulino3, Luis R. Paulino3
Abstract - Fork-tailed Flycatcher (Tyrannus savana, Vieillot 1808) is an easily recognized, widespread
inhabitant of grassland and open terrain throughout its extensive range from southern Mexico south to
Argentina. In the Caribbean, although sightings are fluid and can increase daily on some islands, especially
those close to source continents and during peak migration, its numbers have continued to increase
over a 19-yr period in the Caribbean Basin (2002–2021). We report a 2018 sighting of a vagrant Forktailed
Flycatcher in the Dominican Republic’s capital city of Santo Domingo. Our recent observations of
this vagrant attest to the important natural and anthropogenic regulatory ecosystem services provided by
migratory species in general. These include the potential control of arthropod plagues and disease vectors,
as well as supportive services such as pollination and seed dispersal. Conversely, the city of Santo
Domingo provides reciprocal ecological services to resident and migratory birds alike.
Introduction. Fork-tailed Flycatcher (Tyrannus savana, Vieillot 1808) is comprised of
four subspecies; three sedentary species: T. s. sanctaemartae, circumdatus and monachus, and
one long-distance migrant: the nominate subspecies T. s. savana (Gómez-Bahamón et al. 2020,
Jahn and Tuero 2020, Zimmer 1937). Owing to the restricted ranges of two of the three sedentary
subspecies, T. s. sanctaemartae in northwestern Colombia and T. s. circumdatus in the
State of Amazonas and northern Brazil (Gómez-Bahamón et al. 2020, Jahn and Tuero 2020),
only the non-migratory but wide-ranging T. s. monachus and the long-distance transequatorial
migrant T. s. savana will be discussed here.
The sedentary T. s. monachus ranges and breeds from North America (southern Mexico)
throughout Central America (except El Salvador) to South America, primarily Venezuela, Colombia,
Surinam, and north-central Brazil, including several land-bridge and offshore islands
(BirdLife 2016, Gómez-Bahamón et al. 2020, Mobley 2004, Jahn and Tuero 2020, Teul et al.
2007). Central American populations generally breed from March to August (Jahn and Tuero
2020) but from April to August in Belize (Teul et al. 2007) and March to June in Costa Rica
(A.E. Jahn, Indiana University, 2022, in litt.). Northern South American populations of T. s.
monachus generally breed from March to May in Venezuela (Restall et al. 2006), January to
May in Colombia (Mobley 2004). Annual molt takes place primarily from July to September
(Jahn and Tuero 2020).
T. s. savana has an expansive distribution and migratory range but breeds mainly from
September to December in central Brazil (Marini et al. 2009), southern and southeastern
Brazil, northern and eastern Bolivia, Paraguay and Uruguay; October-January in Argentina
Vol. 9, 2022 URBAN NATURALIST NOTES N5:1–10
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(Mezquida 2002). Southern populations migrate north to Colombia, Venezuela, the Guianas
and the State of Amazonas (Jahn and Tuero 2020, Marini et al. 2009, Mobley 2004, Ridgely
and Tudor 1994). T. s. savana overwinters in the State of Amazonas, a large portion of northern
South America, i.e., within the Orinoco River Basin (Jahn et al. 2013) and Trinidad and Tobago,
occasionally appearing in the West Indies (Ridgely and Tudor 1994). T. s. savana molts primarily
from April to July (Jahn et al. 2016, Pyle 1997).
The nominate subspecies, and to a lesser extent monachus, occur regularly in North America
(Jahn and Tuero 2020, McCaskie and Patten 1994, Shepherd and Smith 1996), with most
sightings occurring in fall (September–November) and a few in spring and early summer, e.g.,
May–June (Jahn and Tuero 2020, McCaskie and Patten 1994). During migration, roosts of
10,000 Fork-tailed Flycatchers have been documented (Jahn and Tuero 2020).
Historically, T. s. savana was reported occasionally from the West Indies (Jahn and Tuero
2020, Ridgely and Tudor 1994). However, there has been a sharp increase of Fork-tailed Flycatcher
sightings of both T. s. savana and T. s. monachus throughout the region in recent years
(eBird 2019). There is at least one previous record (subspecies not mentioned) from the Dominican
Republic in 2005 from Punta Cana (a verbal description without photograph; in Latta
et al. 2006).
In the past (1976–2017), all of our observations of vagrant birds (rare or accidental species)
in the Dominican Republic, especially within and around Santo Domingo, consisted of
Nearctic-Neotropical migratory species, mostly warblers and shorebirds. However, during our
current research (2016–present), we observed this austral long-range migratory Fork-tailed
Flycatcher that, although reported from North America as early as 1834 (J. J. Audubon, in Mc-
Caskie and Patten 1994), prior to 1975 had not been documented in the Caribbean (see ebird.
org checklist: Jack Kelly, Grenada, 26 June 1975, with photo). Herein, we report a much more
recent sighting.
Urban Bird Project. Our observations of a vagrant Fork-tailed Flycatcher took place in
the Dominican Republic’s capital city of Santo Domingo (Fig. 1). This sighting is a result of
our international Urban Long-term Research Area (ULTRA) and green areas (Baghdadi and
Figure 1. Left: Map showing geospatial coordinates of the island of Hispaniola and the general location of
the Fork-tailed Flycatcher sighting. The inset depicts the Caribbean Basin and Hispaniola’s geographical
position within the Greater Antilles. Right: Satellite image of Santo Domingo’s commercial San Carlos
sector within i-Tree plot SC-16 known as “Or Solar” (18°28′37.71′″ N, 69°53′48.89′″ W) where the
immature Fork-tailed Flycatcher was observed. It is noteworthy that this intercontinental vagrant used
an urban gray infrastructure area (Baghdadi and Zribi 2016) over several days, rather than relocating
to one of the more anticipated urban green areas to rest and forage. Satellite imagery was taken from
Landsat 7 and Landsat 8, extracted from Google Earth Engine, 2018.
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W.J. Arendt et. al.
Vol. 9, 2022 N5:1–10
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Zribi 2016) avian research project initiated in September 2016 and currently underway within
four sectors of the city. The Fork-tailed Flycatcher was observed within the highly commercial
sector of San Carlos (Fig. 1) at i-Tree (Nowak et al. 2008) plot SC-16 named “Or Solar”
(18°28′37.71″ N, 69°53′48.89″ W). The “Or Solar” plot is a diverse matrix comprised of 60%
commercial/industrial and a 40% transportation ground usage with an average of only 7.5%
tree and shrub cover (range: 5–10%), an average of 30% built ground cover, e.g., buildings,
cement and pavement (range: 10–50%), and an average of 10% grassy ground cover overall.
Weather Conditions and Disclaimer. All wind and temperature readings on Tuesday, 23
October, 2018 were taken with a Kestrel 4500 NV pocket Weather Tracker® (The use of trade
or firm names in this publication is for reader information and does not imply endorsement
by the U.S. Department of Agriculture of any product or service). See the following link for
more detailed wind and temperature readings as well as meteorological parameters in addition
to those we recorded on our Kestrel: https://weatherspark.com/h/d/27168/2018/10/23/Historical-
Weather-on-Tuesday-October-23-2018-in-Santo-Domingo-Dominican-Republic#Figures-
Temperature
Statistical Analysis of eBird Data. Fork-tailed Flycatcher sightings were compiled over
the 121-year period covered by the eBird.org (2019) online database from 1900 to mid-August
2021. However, for a more balanced design, we excluded the partial year 2021, which constituted
only 7.5 mo. (January to mid-Aug.). To ensure a standardized and more robust analysis of
the data, only years covered from the inception of eBird in 2002 onward to the year 2020 are
Figure 2. An immature Fork-tailed
Flycatcher photographed in the
commercial San Carlos sector (i-Tree plot
SC-16). It was observed either perching
alone on a utility cable or, in between
bouts of sallying for aerial insects, it
rested together with its congener in
the kingbird genus Tyrannus, the yearround
resident Gray Kingbird. It also
hover-hunted for arthropods and foraged
on palm fruit in a raceme of a Royal
Palm, and is seen here displaying in
its bill a Royal Palm seed with a fleshy
and nutritious mesocarp. This species,
accidental in the Dominican Republic, is
providing the capital city (and potentially
beyond) regulatory and supportive
ecosystem services by negatively
impacting numbers of potentially
harmful arthropods and dispersing
seeds, such as those of the Royal Palm
eaten by resident and migratory birds
alike. In reciprocity, the city of Santo
Domingo provides many ecosystem
services to resident and migratory birds,
including diverse food sources, natural
and anthropogenic perches such as public
utility cables shown here. Photographs ©
Luis R. Paulino.
Urban Naturalist Notes
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Vol. 9, 2022 N5:1–10
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included in the statistical analyses. We categorized as dependent variables all sightings of Forktailed
Flycatcher and the corresponding checklists submitted to eBird.org each month from
2002 to 2020. The total numbers of checklists and party hours per year were taken as random
independent variables. General linear models were run in R (R Core Team 2017) for regression
and comparisons using data previously tested for their probability distribution. We chose the
model that best-fit the distribution of the observations using the AIC method. All statistically
generated graphs and comparisons were completed in R, except Figures 3 and 8, which were
created in XLSTAT (Addinsoft 2021).
Field Observations. On Tuesday, 23 October 2018, at 0728 h during a Santo Domingo
ULTRA (Grimm et al. 2000) urban bird survey, within one of ca. 70 i-Tree plots (Nowak
et al. 2008), our research field team observed an immature Fork-tailed Flycatcher (Tyrannus
savanna; Fig. 2). We conclude that this visitor was an immature because T. savana employs
a complex basic strategy in the Wolfe-Ryder-Pyle molt system (Wolfe et al. 2010, A.E. Jahn,
Indiana University, 2022, in litt.). Molt limits are visible in various photos of the flycatcher in
Figure 3. Results of Fork-tailed Flycatcher sightings by Caribbean Island. Data compiled from eBird.
org (2019) revealed that between 26 June 1975 and mid-August 2021, 816 Fork-tailed Flycatchers
(the white and black numbers inside and outside of the bars, respectively) were reported on 16 Greater
Caribbean islands. The purpose of this Pareto diagram is to distinguish the importance of the “vital
few from the trivial many” islands. As anticipated, the closest islands to South American source
populations, i.e., the first five islands on the x-axis, e.g., Curaçao, Aruba, Grenada, the Grenadines
group (Mayreau and Union) and Bonaire, constitute 96% of the 816 Fork-tailed Flycatcher sightings in
the Greater Caribbean region; and the first two islands (Curaçao and Aruba) account for almost 80%
of sightings. Not all islands are shown in the graph because they are included in larger political groupings.
For example, whereas there were no Fork-tailed Flycatcher sightings from St. Vincent during this period,
there were 48 sightings in total on two Grenadine islands (Mayreau and Union Island). The “G. & L.
Cayman” entry on the x-axis represents the Grand and Little Cayman Islands group. The right vertical
axis has percent demarcations. A red cumulation frequency curve representing the cumulative frequency
distribution of the grouped islands was used to sum the percentages generated from each island.
Urban Naturalist Notes
W.J. Arendt et. al.
Vol. 9, 2022 N5:1–10
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Figure 2. Thus, this vagrant is in its first cycle formative (FCF) plumage (Howell et al. 2003,
Pyle 1997, Wolfe et al. 2010).
On the day of observation, the sky was clear, the temperature averaged 27 °C, and wind
speed averaged 2 km/h. The Fork-tailed Flycatcher was first sighted in Roystonea regia (Kunth)
O.F. Cook (royal palm tree) as it hover-hunted among several racemes foraging on palm fruits
and insects attracted to them (Fig. 2). The flycatcher foraged among 24 Gray Kingbirds (Tyrannus
dominicensis, Gmelin) and a single Palmchat (Dulus dominicus, Linnaeus). The Gray Kingbirds
frequently attacked their congener, a similar behavior reported by Colón López (ebird.
org 2010) on nearby Puerto Rico. The avian foraging aggregation was ca. 8 m off the ground.
Individual Gray Kingbirds and the Fork-tailed Flycatcher occasionally flew up and perched on
electrical wires ca. 15 m above ground level (Fig. 2).
eBird Data Results. From 26 June 1975 (ebird.org checklist: Jack Kelly, Grenada; photo) to
mid-August 2021, 816 sightings of the Fork-tailed Flycatcher on 16 Caribbean islands were submitted
to the eBird online database (Sullivan et al. 2009). Most sightings come from the southern
Netherlands Antilles (Curaçao, Aruba, Bonaire), Grenada and the Grenadines (Mayreau
and Union), all of which are islands closest to the South American continent. Sightings diminish
northward and westward throughout the Lesser and Greater Antilles. Not surprisingly,
islands closest to T. s. monachus’ breeding grounds have received the bulk of the birds that
digress from traditional migration routes (Fig. 3). Although Cornell University’s online database
“ebird.org” (Sullivan et al. 2009) was in operation by 2002, the number of eBird checklists
reporting Fork-tailed Flycatcher remained stable, not increasing significantly for more than
Figure 4. Number of checklists reporting
Fork-tailed Flycatcher submitted to eBird each
year. Although Cornell University′s online
database, ebird.org (Sullivan et al. 2009) was
in operation by 2002, the number of eBird
checklists reporting Fork-tailed Flycatcher did
not increase significantly until more than a
decade later, in 2016, and continued to increase
prominently to 2020.
Figure 5. Total number of Fork-tailed
Flycatchers per checklist-hour divided into
three comparative yearly ranges from 2002
to 2020. Fork-tailed Flycatcher numbers per
checklist-hour did not increase significantly
in the Caribbean until about a decade after the
advent of eBird (p < 0.0001). Distinct letters
refer to significant differences between years
(p < 0.05). The three yearly range periods are
the results of the observed data distribution
among histogram bins (buckets); horizontal
bars within boxes are medians; lower and
upper limits of each box are the first and third
quartiles, respectively.
Urban Naturalist Notes
W.J. Arendt et. al.
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another decade, i.e., until 2016 (t = 3.821, p = 0.0001; Fig. 4) A similar pattern was found in the
number of Fork-tailed Flycatchers per checklist-hour (t = 4.535, p = 0.0003; Fig. 5). Conjointly,
we observed an increase in the number of Fork-tailed Flycatchers detected each year (t = 5.911,
p < 0.0001) (Fig. 6).
To test whether the number of Fork-tailed Flycatcher individuals reported per checklist
increased over the 19-year period (2002–2020), we compared the number of Fork-tailed Flycatchers
reported per checklist per year (Fig. 7). There was a significant correlation (t = 3.481,
p = 0.003). As further confirmation of a significant increase of Fork-tailed Flycatcher sightings
during the last 10-yr period of our analysis, we compared the median number of Fork-tailed Flycatchers
per checklist during the first decade (2002–2010) to the median number submitted during
the second decade of the study (2011–2020). There was a significant increase in the median
number of Fork-tailed Flycatchers sighted within the last years of our survey period (Fig. 8).
Origin of the Fork-tailed Flycatcher Vagrant. It remains uncertain whether or not the
Santo Domingo Fork-tailed Flycatcher is a wayward individual of the nominate and migratory
subspecies T. s. savana, or a vagrant of the non-migratory but vagile subspecies T. s monachus
(Gómez-Bahamón et al. 2020; Zimmer 1937). However, because our observations took place in
October, when austral spring migration of T. savana savana occurs in South America as they
migrate south to their breeding grounds (Jahn and Tuero 2020, Tuero et al. 2019), the Santo
Domingo bird most likely is of the nominate subspecies T. s. savana. It could have undertaken
a reverse migration, which occasionally occurs, especially in juvenile and immature migratory
birds (Nilsson and Sjöberg 2016).
Figure 6. Number of Fork-tailed Flycatcher
Individuals reported in eBird checklists
between 2002 and 2020. Similar to the
number of eBird checklists reporting Forktailed
Flycatcher sightings each year (Figure
4), numbers of individual flycatchers reported
did not increase significantly (p = < 0.0001)
until more than a decade later in 2017 and 2018
when numbers doubled from 50 to a mean of
100 (range: 87–112), and then almost doubled
again by 2020.
Figure 7. Number of Fork-tailed Flycatchers per
checklist over a 19-yr period. Numbers varied
between 0 and 3 per checklist for almost the
first decade until 2010, averaging 1.44, whereas
during the second half of the study (2011–2020)
the average was 8.5 Fork-tailed Flycatchers per
checklist, which was significant at α = 0.05 (p =
0.005; Addinsoft 2021: Wilcoxon Signed Rank
Test, Hollander et al. 1999).
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Figure 8. Visual results of the Wilcoxon Signed Rank Test (Addinsoft 2021) comparing the median
number of Fork-tailed Flycatchers per checklist during the first decade (2002–2010) and the second
decade of the study (2011–2020). Red crosses within boxes correspond to the means; central horizontal
bars are the medians; lower and upper limits of each box are the first and third quartiles, respectively.
South American populations of T. s. monachus breed earlier (Jan–May in Colombia,
March–May in Venezuela) than Central American populations (March–August). Yet, even
though T. s. monachus begins to breed earlier in northern South America than Central America,
the breeding seasons in both regions end at about the same time (A.E. Jahn, Indiana University,
2019, in litt.). Likewise, although the Greater Antilles are much closer to the Yucatán peninsula
than Colombia or Venezuela and annual feather molt might take place earlier in either its South
American breeding or non-breeding grounds, one cannot be certain of the origin of the Santo
Domingo vagrant because vagrants of all avian species are capable of “island hopping” (A.E.
Jahn, Indiana University, 2019, in litt.; Arendt 2006: Ch. 6, p. 71).
History of Fork-tailed Flycatcher eBird Sightings in the Caribbean. Based on contemporary
knowledge and escalating vagrant sightings in both North America and the Caribbean,
it is possible that the Fork-tailed Flycatcher, historically accidental in the Caribbean but apparently
rapidly increasing in number (Fig. 6), may be increasing its migratory range northward
as well. Such sightings of austral species appearing in diverse geographical locations
and varied ecological environments in North America and the Neotropics continue to increase
and may continue to mount as the effects of global climate change acerbate. Increasingly
acyclic precipitation regimes and warming temperatures are causing worldwide plant and
Urban Naturalist Notes
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8
animal phenology mismatches, resulting in resource constraints and food shortages for many
bird species (Franklin et al. 2022, Youngflesh et al. 2021). Noteworthy and during similar time
periods, is mention of a handful of Elaenia parvirostris von Pelzeln, 1868 (South American
Small-billed Elaenia) showing up for the first time in the USA in April, 2012 (Chicago, Illinois)
and again in November, 2021 (Waukegan, Illinois), as well as Quebec, Canada in October, 2021
(Davis 2021). Fork-tailed Flycatcher numbers may continue to increase exponentially as evidenced
in Figure 6, as the repercussions of global warming and climate change, often referred
to as Anthropogenic climate change (ACC), continually escalate (Cotton 2003, Parmesan and
Hanley 2015). Arguably, however, many additional factors undoubtedly govern this species’
migration patterns, population dynamics and behavior. Thus, research into alternative governing
mechanisms must be conducted before a definitive conclusion can be made.
Study Limitations. Our results are based on different variables and sampling efforts, e.g.,
number of checklists submitted and number of Fork-tailed Flycatcher reported each year, per
checklist and sampling hours, all of which varied among years. This led us to limit our conclusions
to the number of checklists and components of sampling hours solely involving reports of
Fork-tailed Flycatcher. Also noteworthy, is that the lack of reports of Fork-tailed Flycatcher in
decades prior to our study is not considered as an absence of the species. But rather, it reflects
the period preceding the advent of eBird, the popular geospatial tool used worldwide by thousands
of contemporaries to permanently record and disseminate avian abundance, distribution
and population trend data, together with other spatiotemporal information.
Conclusions. Our results support a predominant increase in Fork-tailed Flycatcher numbers
in the Caribbean Basin, especially during the latter years of our review period. Pairing
that with the fact that the number of individual Fork-tailed Flycatchers per checklist continues
to increase (t=3.481, p=0.0033), suggests there is an effect resulting from the combination of
the number of checklists, plus the number of individuals per checklist, both of which reflect
the overall increase of Fork-tailed Flycatcher sightings in recent years. As a result, increasing
numbers of Fork-tailed Flycatcher contributions reported by eBirders every year are expanding
our knowledge of the species’ population dynamics and giving us a better understanding of the
Fork-tailed Flycatcher’s presence, abundance and distribution in the Caribbean Basin.
Acknowledgments
We thank Leonardo Cortés for preparing the satellite image of Santo Domingo’s commercial San
Carlos sector depicted in Figure 1. We thank Jerry Bauer for his guidance as Project Leader and, especially,
his enviable drive and continuous technical support. We thank I. Davies for assisting us in retrieving
eBird data, as well as answering our undoubtedly superfluous queries. A.E. Jahn, Indiana University,
gave generously of his time and imparted his insightful knowledge of the species, while enhancing early
drafts of the manuscript. Our observations and analyses of eBird records were made possible through the
financial and logistical support of the USDA Forest Service, International Institute of Tropical Forestry,
and our Santo Domingo ULTRA project entitled “Conservation and Sustainable Management of Urban
Watersheds as Mechanisms for Adaptation to Climate Change”. The project is a joint USDA Forest Service
“Participation Agency Program Agreement” (PAPA-517-AID-AEG-T-00-07-00003) entered into
with the US Agency for International Development (USAID/Dominican Republic).
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