ORNITOLOGIA NEOTROPICAL 15 (Suppl.): 83–99, 2004
© The Neotropical Ornithological Society

RAPTOR MIGRATION IN THE NEOTROPICS: PATTERNS,PROCESSES, AND CONSEQUENCES
Keith L. Bildstein
Hawk Mountain Sanctuary Acopian Center, 410 Summer Valley Road, Orwigsburg,
Pennsylvania 17961, USA. E-mail: Bildstein@hawkmtn.org

Resumen. – Migración de rapaces en el Neotrópico: patrones, procesos y consecuencias. – El Neotrópico alberga poblaciones reproductivas y no reproductivas de 104 de las 109 especies de rapaces del Nuevo
Mundo (i.e., miembros del suborden Falconides y de la subfamilia Cathartinae), incluyendo 4 migrantes
obligatorios, 36 migrantes parciales, 28 migrantes irregulares o locales, y 36 especies que se presume queno migran. Conteos estandarizados de migración visible iniciados en la década de los 1990, junto con una
recopilación de literatura, nos proveen con una idea general de la migración de rapaces en la región. Aquídescribo los movimientos de las principales especies migratorias y detallo la geografía de la migración en el
Neotrópico. El Corredor Terrestre Mesoamericano es la ruta de migración mas utilizada en la región.
Tres especies que se reproducen en el Neártico, el Elanio Colinegro (Ictina mississippiensis), el Gavilán Aludo(Buteo platypterus) y el Gavilán de Swainson (B. swainsoni), de los cuales todos son migrantes obligatorios,
junto con las poblaciones norteamericanas del Zopilote Cabecirrojo (Cathartes aura), dominan numéricamente este vuelo norteño o “boreal”. Cantidades mucho menores de Aguilas Pescadoras (Pandion
haliaetus), Elanios Tijereta (Elanoides forficatus), Esmerejónes (Falco columbarius) y Halcones Peregrinos(Falco peregrinus), ingresan y abandonan el Neotrópico rutinariamente utilizando rutas que atraviesan el
Mar Caribe y el Golfo de México. Los movimientos sureños o “australes” e intra-tropicales, incluyendo
la dispersión y la colonización en respuesta a cambios en el hábitat, son conocidos pero permanecen
relativamente poco estudiados. Habiendo detallado los patrones y procesos de la migración de rapaces
en la región, discuto lo que se conoce sobre las consecuencias ecológicas de estos movimientos,
incluyendo las interacciones competitivas entre especies migratorias y residentes. Luego exploro como la
migración de larga distancia ha moldeado la distribución y abundancia de los rapaces en el Neotrópico.
Concluyo proponiendo que la configuración de las masas continentales en forma de reloj de arena y laorientación norte-sur de las cordilleras del Nuevo Mundo, no sólo facilitan la migración terrestre de larga
distancia de rapaces en esta región, sino que también aumentan la diversidad regional de rapaces por medio
de un proceso de especiación llamado “dosis de migración” (migration dosing). El proceso de “dosis demigración” involucra una serie de eventos en los cuales ciertas “dosis” de migrantes de larga distancia sedesvían y (1) llegan a áreas que son tangencialeso que están más allá de sus principales rutas de migración,(2) en vez de regresar a sus áreas reproductivas tradicionales permanecen ahí durante el siguiente período
reproductivo y (3) eventualmente divergen de la linea parental, aislados geográficamente. Ejemplos
probables de “dosis de migración” se presentan de dos géneros ricos en especies y casi cosmopolitas, Accipiter y Buteo.
Abstract. – The Neotropics host breeding or non-breeding populations of 104 of the 109 species of New
World raptors (i.e., members of the suborder Falconides and the subfamily Cathartinae), including 4 complete migrants, 36 partial migrants, 28 irregular or local migrants, and 36 presumed non-migrants. Standardized counts of visible migration initiated in the 1990s, together with a focused literature search
provide an emerging picture of raptor migration in the region. Here I describe the movements of the principal species of migrants, and detail migration geography in the Neotropics. The Mesoamerican Land Cor-83

BILDSTEIN

ridor is by far the most concentrated migration flyway in the region. Three species of Nearctic breeders,the Mississippi Kite (Ictinia mississippiensis), the Broad-winged Hawk (Buteo platypterus), and the Swainson’s
Hawk (B. swainsoni), all of which are complete migrants, together with western North American populations of Turkey Vultures (Cathartes aura), numerically dominate this northern or “boreal” flight. Muchsmaller numbers of Ospreys (Pandion haliaetus), Swallow-tailed Kites (Elanoides forficatus), Merlins (Falco
columbarius), and Peregrine Falcons (F. peregrinus), routinely enter and leave the Neotropics via trans-Caribbean and trans-Gulf of Mexico routes. Southern or “austral” and intratropical movements, including dispersal and colonization in response to habitat change, are known but remain relatively little studied.
Having detailed the patterns and processes of raptor migration in the region, I discuss what is known
about the ecological consequences of these movements, including competitive interactions between
migrant and resident species. I then go on to explore the role that long-distance migration has played in
shaping raptor species distributions and abundances in the Neotropics. I conclude by proposing that thehourglass configuration of continental landmasses and the north-south orientation of mountain ranges in
the New World, not only facilitate long-distance land-based raptor migration in the region, but alsoenhance regional raptor diversity via a speciation process called “migration dosing.” The process of migration dosing involves a series of events in which “doses” of diverted long-distance migrants (1) arrive inareas tangential to or beyond their major migration flyways, (2) remain there the following breeding season
rather than returning to traditional breeding areas, and (3) eventually diverge from parental stock in geographic isolation. Likely examples of migration dosing are presented from two species-rich near-cosmopolitan genera, Accipiter and Buteo. Accepted 13 December 2003.
Key words: Raptors, Falconiformes, migration, Neotropics, patterns, processes, consequences.

INTRODUCTION
Diurnal birds of prey or raptors (suborder
Falconides and the subfamily Cathartinae) area diverse group of approximately 300 speciesof hawks, eagles, falcons, vultures, and theirclose relatives, one third of which occur in the
Neotropics (del Hoyo et al. 1994). Found on
many oceanic islands and on all continents
except for Antarctica, the group includes several of the most broadly distributed and mostmobile of all species of birds. Each year wellover five million raptors migrate into andwithin the Neotropics. Unlike many of theregion’s migrants that travel principally atnight, birds of prey migrate almost entirely by
day, often along well-established flyways.
Most are relatively large-bodied, lightly wingloaded species, and many depend upon soaring to complete their long-distance journeys(Bildstein & Zalles 1998, 2001; Zalles & Bildstein 2000, Ferguson-Lees & Christie 2001).
Zalles & Bildstein (2000) summarize
known migration choreography in the region.
84

Overall, raptor movements are far betterstudied in Central America than in SouthAmerica, and far more is known about thelong-distance “northern” movements ofNorth American breeders than about
migrants that both breed and “overwinter” inthe region. The limited information available
indicates that raptor migration in the Neotropics is both complex and confusing (Bildstein& Zalles 1998).
At least 40 of 104 raptors in the region arecomplete or partial migrants, 28 others are
irregular or local migrants. Long-distancemovements, the overwhelming majority ofwhich are made by latitudinal migrants, arenumerically dominated by the influx of North
American breeders in boreal autumn. The
Mesoamerican Land Corridor, which is partof the larger Transamerican Flyway, is by far
the most heavily used migration pathway in
the Neotropics (Fig. 1). Lesser numbers ofNorth American breeders travel into and outof Central and South America via the Gulf of
Mexico and the Caribbean Sea. At least 22

RAPTOR MIGRATION IN THE NEOTROPIICS

migration has affected species distributions
and raptor diversity in the region. I conclude
by detailing priority areas for future research.

METHODS

FIG. 1. The Transamerican Flyway is used by 5-10
million outbound raptor migrants each autumn.
Cardel, Mexico, Kekoldi, Costa Rica, and Ancon
Hill, Panama, are three important migration
watchsites along the Mesoamerican Land Corridor
portion of this flyway. Both the geography of the
flyway and the timing of the flight, are far better
understood in North America and Central America than in South America.

species of Neotropical raptors are intratropical migrants, 23 are southern or “austral”
migrants, and at least 16 are altitudinalmigrants (Table 1) (Zalles & Bildstein 2000).
Northern or boreal migrations are far better
understood and appreciated than are intratropical, austral, and altitudinal movements.
Here I (1) summarize and update themigratory characteristics of Neotropical raptors, (2) describe the geography, timing, andmagnitude of long-distance raptor migrationin the region, (3) assess the forces responsible,(4) detail ecological consequences of the
movements, and (5) describe how raptor

Definitions. In this analysis, raptors include the
307 species that belong to the suborder Falconides and the subfamily Cathartinae (sensu
Sibley & Ahlquist 1990). Species taxonomy
and nomenclature follow del Hoyo et al. 1994,
as updated by more recent findings. Neotropical raptors are the 104 species known to breed
within or to regularly visit the Neotropical
Realm (sensu Darlington 1957). Migration isdefined as directed and generally long-distance, recurring movements that alternate indirection and that are temporally and spatiallypredictable. Geographically, migration “flyways” are hemispheric networks of regional“corridors” and local “routes” along which
raptors concentrate while migrating. Complete migrants are species in which > 90% of
all individuals leave their breeding range during the non-breeding season. Partial migrants
are species in which < 90% of all individuals
migrate. Irregular migrants are species inwhich the extent of migratory movements
varies annually, typically in response toamong-year shifts in prey abundance. Local
migrants include species in which most populations, except for those at the latitudinal
perimeters of the range, do not migrate, or
nomadic species in which individuals tend to
wander during the non-breeding season.

RESULTSThe raptors
Faunal comparisons. With 34% (104) of theworld’s 307 species of raptors and 46% (37) of
its 81 genera, the Neotropical Realm is arguably the most raptor-diverse biogeographicalrealm on earth. The region’s 18 endemic genera make it one of the most distinctive raptor
85

BILDSTEIN

TABLE 1. Migratory tendencies and the tropical extent of breeding and wintering range of 104 Neotropical raptors.
SpeciesComplete migrants
Pandion haliaetus
Ictinia mississippiensis
Buteo platypterus
Buteo swainsoni
Partial migrants
Cathartes aura
Coragyps atratus
Elanoides forficatus
Elanus leucurus
Rostrhamus sociabilis
Ictinia plumbea
Haliaeetus leucocephalus
Circus buffoni
Circus cyaneus
Circus cinereus
Accipiter poliogaster
Accipiter striatus
Accipiter cooperii
Accipiter bicolor
Accipiter chilensis
Accipiter gentilis
Buteogallus anthracinus
Buteogallus meridionalis
Parabuteo unicinctus
Geranoaetus melanoleucus
Buteo nitidus
Buteo lineatus
Buteo brachyurus
Buteo albigula
Buteo albicaudatus
Buteo polyosoma
Buteo albonotatus
Buteo jamaicensis
Buteo regalis
Aquila chrysaetos
Milvago chimango
Falco sparverius
Falco femoralis
Falco columbarius
Falco mexicanus
Falco peregrinus

86

Longdistancemigrantsa

Transequatorial
migrantsb

Yes
Yes
Yes
Yes

Yes
Yes
Yes
Yes

Yes

Yes

Yes

Yes

Altitudinalmigrantsc

Water
Maximum Tropicalcrossing flock sizee speciesf
behaviord
Long

> 50
> 100*> 1000*> 1000*MainlyModerate> 1000*> 10*> 100*

Moderate> 10*> 100*< 10

MainlyMainlyMainlyMainlyMainlyMainly

Short
Short
Short
Yes
Yes

MainlyMainly
Yes

Long
Short

< 10

Yes
Yes
Yes

Short
Short

< 10
< 10

Yes

Short

Yes

MainlyYes

MainlyMainlyMainlyMainlyMainlyMainly
Short

< 10
< 10

Yes
< 10

Yes

Yes
Yes
Yes

Yes
Yes

Yes

Short

< 10
> 10
< 10

Moderate
Short
Moderate
Short
Long

< 10

Long

MainlyMainlyMainlyMainlyMainly< 10

RAPTOR MIGRATION IN THE NEOTROPIICSTABLE 1. Continuation.
Species

Irregular or local migrants
Cathartes burrovianus
Cathartes melambrotus
Sarcoramphus papa
Vultur gryphus
Chondrohierax uncinatus
Harpagus bidentatus
Harpagus diodon
Geranospiza caerulescens
Leucopternis lacernulata
Leucopternis polionota
Buteogallus urubitinga
Busarellus nigricollis
Harpyhaliaetus solitarius
Harpyhaliaetus coronatus
Buteo magnirostris
Buteo leucorrhous
Buteo poecilochrous
Buteo ventralis
Harpia harpyja
Spizaetus tyrannus
Spizaetus ornatus
Phalcoboenus carunculatus
Phalcoboenus megalopterus
Phalcoboenus australis
Polyborus plancus
Milvago chimachima
Falco rufigularis
Falco deiroleucus
Nonmigrants
Leptodon cayanensis
Leptodon forbesi
Gampsonyx swainsonii
Rostrhamus hamatus
Accipiter superciliosus
Accipiter collaris
Accipiter chionogaster
Accipiter ventralis
Accipiter erythronemius
Accipiter gundlachi
Leucopternis plumbea
Leucopternis schistacea
Leucopternis princeps

Longdistancemigrantsa

Transequatorial
migrantsb

Altitudinalmigrantsc

Water
Maximum Tropicalcrossing flock sizee speciesf
behaviord
MainlyWhollyMainly

Yes

Yes
Yes

Yes

MainlyMainlyMainlyMainlyWhollyMainlyMainlyMainlyMainlyMainlyMainlyMainlyMainlyMainlyMainlyMainlyMainlyWhollyMainlyMainlyMainlyMainlyYes

Yes

MainlyWhollyMainlyMainlyMainlyWhollyWhollyWhollyMainlyWhollyWhollyWhollyWholly87

BILDSTEIN

TABLE 1. Continuation.
Species

Longdistancemigrantsa

Transequatorial
migrantsb

Leucopternis melanops
Leucopternis kuhli
Leucopternis semiplumbea
Leucopternis albicollis
Leucopternis occidentalis
Buteogallus aequinoctialis
Buteogallus subtilis
Buteo ridgwayi
Buteo galapagoensis
Morphnus guianensis
Spizastur melanoleucus
Oroaetus isidori
Daptrius ater
Daptrius americanus
Phalcoboenus albogularis
Herpetotheres cachinnans
Micrastur ruficollis
Micrastur plumbeus
Micrastur gilvicollis
Micrastur mirandollei
Micrastur semitorquatus
Micrastur buckleyi
Spiziapteryx circumcinctus

Altitudinalmigrantsc

Water
Maximum Tropicalcrossing flock sizee speciesf
behaviord
WhollyWhollyWhollyWhollyWhollyMainlyWhollyWhollyWhollyMainlyMainlyMainlyWhollyMainlyMainlyMainlyWhollyWhollyWhollyMainlyWhollyComplete or partial migrants, ≥ 20% of whose populations (sometimes entire subspecies) regularly
migrate > 1500 kms.
b
Long-distance migrants, ≥ 20% of whose populations (sometimes entire subspecies) regularly migrate
across the Equator.
cSpecies in which at least some populations are known or suspected to migrate altitudinally.
d
Short, species that regularly cross water barriers of < 25 km; moderate, 25–100 km; long, > 100 km.
eSpecies whose maximum flock sizes are marked with an asterisk (*) regularly migrate in flocks, all otherswith maximum flock sizes listed flock occasionally.
fSpecies in which at least 50% (mainly) or all (wholly) of the range falls within the Tropics of Cancer andCapricorn.
afaunas as well. In addition, the Neotropics isthe center of generic diversity in the family
Falconidae (del Hoyo et al. 1994), as well as
the ancestral home of the genus Buteo (Amadon 1982, Voous & deVries 1978, Riesing etal. 2003).
Seventy-nine percent of the Neotropics
88

lies between 30º S and N, and most raptors in
the region are wholly (23%) or mainly (53%)
tropical species, many of which are nonmigratory (Table 1) (Bildstein et al. 1998).
Indeed, there are only 4 species of complete
migrants and 7 species of long-distance,trans-equatorial migrants in the region, com-RAPTOR MIGRATION IN THE NEOTROPIICS

pared with 11 and 14, respectively, in the
Afrotropical Realm. And overall, the Neotropical raptor community is less migratory thanthat of any other continental region excepting
Indomalaysia and Australasia (Bildstein et al.
1998, Zalles & Bildstein 2000).

56ºS) for mainland South America in austral
winter (Humphrey et al. 1970). Concentrated
movements of hundreds of White-throatedHawks (Buteo albigula) are reported for themountains of central Chile (33º–34º S) (Pavez
2000, Zalles & Bildstein 2000).

Complete migrants. Four species of complete
migrants occur in the Neotropics (Table 1).
Adult Osprey (Pandion haliaetus) breed in small
numbers and over-winter in large numbers in
the region, and North American juveniles
over-summer there as well (Poole et al. 2002).
North American (i.e., world) breeding populations of Mississippi Kite (Ictinia mississippiensis),
Broad-winged Hawk (Buteo platypterus), andSwainson’s Hawk (B. swainsoni) overwinter inthe region, and all three species typically travelto, within, and out of the region in large,
multi-thousand bird migratory flocks (Zalles& Bildstein 2000).

Irregular and local migrants. Twenty-eight irregular and local migrants occur in the region(Table 1). They include 22 mainly and 3
wholly tropical species, and 3 species whose
distributions fall mainly within the non-tropical “southern cone” of South America. Mostof these species are best characterized as local
migrants with relatively small “latitudinallyperipheral” migratory or partially migratory
populations, either north or south of the tropics or both. Examples include Hook-billed
Kite (Chondrohierax uncinatus) in Mexico (E.
Ruelas pers. com.) which are at the northern
limits of their range, and the Rufous-thighed
Kite (Harpagus diodon) which is migratory intemperate South America at the southern
limits of its range (Hayes et al. 1994, Ridgely &Greenfield 2001). Nomadic species include
the regionally migratory Savanna Vulture(C. burrovianus) in Brazil and Panama (Sick1993), and Snail Kite in many parts of its
extensive range (Angehr 1999, Ferguson-Lees& Christie 2001).
Recent unconfirmed sightings of southbound flocks of as many as several hundredWhite-tailed Hawks (B. albicaudatus) near Santa
Cruz, Bolivia, in mid-November raise the possibility of significant regional movements inthis species as well (Olivo 2003). However, as
previous reports of migration in this species[e.g., Hudson (1920) in Argentina, and Wetmore (1943) in Mexico] are thought to have
involved either individuals that had briefly
joined flocks of true migrants, and were notthemselves migrating (Farquhar 1992), or misidentified Swainson’s Hawks (Bildstein &Zalles 2001), and because Bolivia falls along amajor flightline for Swainson’s Hawks, these

Partial migrants. Thirty-six partial migrants
occur in the region (Table 1). They include 13
species that breed largely in North America
and that over-winter mostly or entirely inCentral America, 12 species that breed mainlyin the Neotropics and that over-winter there,
and 11 species that breed and over-winterwholly within the Neotropics. North American breeding populations of Turkey Vulture(Cathartes aura meridionalis) are by far the mostnumerous partial migrant. Other partial
migrants whose movements involve > 10,000
birds include Swallow-tailed Kite (Elanoides
forficatus), Plumbeous Kite (Ictinea plumbea),
and Snail Kite (Rostrhamus sociabilis). Populations of nine species, Turkey Vulture, Cinereous Harrier (Circus cinereus), Bicolored Hawk
(Accipiter bicolor), Red-backed Hawk (Buteo
polyosoma), Rufous-tailed Hawk (B. ventralis),
Striated Caracara (Phalcoebanus australis), American Kestrel (Falco sparverius), Aplomado
Falcon (F. femoralis), and Peregrine Falcon(F. peregrinus) evacuate Tierra del Fuego (53º–89BILDSTEIN

recent sightings require careful confirmation.
Truly irruptive migrants include theWhite-tailed Kite (Elanus leucurus), a species
that in Buenos Aires Province, Argentina,
tracks rodent cycles (Ferguson-Lees &Christie 2001). Nomadic migrants include the
Snail Kite, a species that feeds almost exclusively upon apple snails, and which established a breeding population at Lake Gatun,
Panama, from populations at least 350 km
away, fewer than 10 years after apple snails
were introduced to the lake (Angehr 1999).
Altitudinal migrants. At least 16 species are altitudinal migrants in the region (Table 1). They
include Andean Condors (Vultur gryphus),
Hook-billed Kites, White-throated Hawks,
Puna Hawks (Buteo poecilochrous), and Aplomado Falcons, populations of which breed in
the high Andes and migrate to lower elevations at other times of the year (Fjeldså &Krabbe 1990, Ferguson-Lees & Christie
2001). Condors, for example, regularly travel
both to near and distant lowland feeding sites
on a seasonal basis, including as far as Mato
Grosso, southwestern Brazil, which requires aroundtrip journey of several thousand kilometers (Sick 1993). A recent analysis that indicates low genetic variability in this species(Hendrickson 2003), despite its partially disjunct distribution, indicates the likelihood of
long-distance intra-Andean movements aswell. The Rufous-tailed Hawk also may be an
altitudinal migrant (Ferguson-Lees & Christie
2001).
Migration geography in the Neotropics
Transamerican Flyway. The Transamerican Flyway, the southern two thirds of which
stretches from northwestern Mexico to thetemperate southern cone of South America(Fig. 1), far and away is the most numericallyimportant raptor migration flyway in theregion. Thirty-two species, including three
accipiters, eight buteos, and five falcons travel
90

along the flyway’s central 4000-km
Mesoamerican Land Corridor that extends
from southern Texas through eastern
Panama. Numerically significant migrants
using the corridor include Turkey Vultures[2,000,000 (minimal estimate of numberof birds based on autumn-watchsite
counts), Osprey (5000), Swallow-tailed
Kite (1000), Plumbeous Kite (1000), Mississippi Kite (200,000), Northern Harrier(Circus cyaneus; 1000), Sharp-shinned Hawk
(Accipiter striatus; 5000, northern part only),
Cooper’s Hawk (A. cooperii; 2000, northernpart only), Broad-winged Hawk (1.5 million),
Swainson’s Hawk (800,000), American Kestrel(3000, northern part only), Merlin (Falco
columbarius; 1000), and Peregrine Falcon(5000).
During outbound migration in boreal
autumn, flight lines of the four numericallydominant migrants using the corridor converge in coastal Veracruz, Mexico, before
splitting west of the Isthmus of Tehuantepec.
Although the bulk of the flight crosses to the
Pacific slope at this point, a substantial portion and perhaps most of the broadwing
flight, continues east along the Caribbean
slope into eastern Guatemala and northwestern Honduras.
Visual counts of migrants have been sporadic in El Salvador, Honduras, and Nicaragua, and the geography of the migration
corridor is less well understood there thanelsewhere in Mesoamerica. Satellite trackingsof outbound Broad-winged Hawks (Haines etal. in press), and Swainson’s Hawks (Fuller etal. 1998), suggest that large numbers ofbroadwings travel along the Caribbean slopeof the continental divide, as well as along thenear-coastal Pacific slope, as far south as theGulf of Fonseca, where the flight again coalesces on the Pacific side, whereas the vast
majority of Swainson’s Hawks migrate along
the coastal Pacific slope throughout this portion of the route. Observations in the western

RAPTOR MIGRATION IN THE NEOTROPIICS

coastal plain of Chiapas, southern Mexico,
suggest that the bulk of the Turkey Vulture
flight also follows the Pacific slope in thisregion (Tilly et al. 1990).
Farther south the principal flightline
enters the lowlands of northwestern Costa
Rica where it largely tracks the foothills of theAtlantic slope southeast into Panama. In Panama, most of the flight crosses to the Pacificslope east of Volcan Baru, before entering
northwestern Colombia via the Darien. Falcons migrating coastally through Mesoamerica apparently join the flight whenever itpasses within several kilometers of the coast.
Once the birds reach South America, theflightline turns south, where many and perhaps most of the migrants follow the
Magdalena Valley south into Tolima in central
Colombia, before entering the Amazon lowlands en route to east-central Bolivia and,from there, the Pampas of northeastern
Argentina. At least some of the flight, including large numbers of Swallow-tailed Kites,
pass through Colombia along the Pacificslope of the Andes, before crossing themountains to the east near the border with
Ecuador (K. Meyer pers. com.).
Broadwings begin to drop out of the flightin large numbers and into wintering areas atleast as far north as El Salvador, and mosthave left the main flightline before southern
Colombia, as have most Turkey Vultures.
Large numbers of Mississippi Kites continue
at least as far south as central Bolivia, andmost Swainson’s Hawks remain on the flightline into Argentina (Zalles & Bildstein 2000,
M. Bechard, C. Marquez, & K. Meyer pers.
com.).
So far as is known, in spring, the less studied return flightline largely retraces theoutbound flightline, excepting that proportionately more birds travel along the Atlantic
slope while crossing the Panama Canal in central Panama (Smith 1985), and the Pacificslope in southeastern Costa Rica (Skutch1945) than occurs in autumn, presumably
because of seasonal differences in theweather.
Austral flyways. Austral or South American
migration system migrants (sensu Joseph 1997)
are species that breed in the temperate, southern cone area of South America and that
migrate north toward the Equator in austral
winter (Chesser 1994, Joseph 1997). Austral
movements are particularly pronounced along
the southern Andes and in wetlands associated with the Paraguay River, where notablemigrants include White-throated Hawk andSnail and Plumbeous kites, respectively (Pavez
2000, Hayes et al. 1994). Diversion lines andmigration bottlenecks are lacking in theregion, however, and details regarding themagnitude of such movements, as well as theorigins and destinations of most of the
migrants involved remain uncertain.
Hayes et al. (1994) recognized two types ofaustral migrants, northern and southern, coinciding with Joseph’s (1997) temperate-tropicaland cool, temperate South American
migrants, respectively. Regional populationsof at least 23 species of raptors (18 partial
migrants and 5 irruptive or local migrants)
have been characterized as northern or southern austral migrants, or both. They include
the Turkey Vulture, four species of kites, twoharriers, two accipiters, two buteos, and threefalcons, all but four of which are wholly orlargely Neotropical species (Chesser 1994,
Hayes et al. 1994, Zalles & Bildstein 2000).
Much remains to be learned about austral raptor migration, and careful observations ofthese movements offer the promise of substantial new information.
Intratropical flyways. Because many are local,
regional, irruptive, or are mixtures of all three
types of movements, and because the numbers of individual migrants involved apparently are relatively few, intratropical
91

BILDSTEIN

movements are decidedly less well understood than is either northern or austral migration. The situation in the eastern lowlands of
Bolivia, a region that falls along an important
north-bound route for populations of North
American Swallow-tailed Kite, Mississippi
Kite, and Swainson’s Hawk, as well as Neotropical populations of Snail Kite, Plumbeous
Kite, and, possibly, Swallow-tailed Kite
(Zalles & Bildstein 2000) typifies the challenges involved. Although the North American origins of Mississippi Kites andSwainson’s Hawks in Bolivia is certain basedon their breeding distributions, and whereas
satellite telemetry substantiates the North
American origins of at least some of the Swallow-tailed Kites traveling through the country(K. Meyer pers. com.), the origins and destinations of thousands of Plumbeous Kites andhundreds of Snail Kites migrating in the area
are largely unknown.
Consequences of raptor migration
Competition between long-distance migratory and sedentary raptors. Competition between migratory
and resident Neotropical raptors is little studied. However, the numbers of migrants
involved (i.e., 5-6 million birds), similarities inecology between many of the migratory andsedentary species, and a detailed study ofinteractions between migratory and resident
Turkey Vultures in Venezuela, all suggest thelikelihood of significant, widespread competition. Consider, for example, the situation in
Costa Rica involving the closely related
Broad-winged Hawk and Roadside Hawk (B.
magnirostris). Both species are abundant territorial inhabitants in lowland and mid-elevational disturbed areas and forest edges in the
country, and both spend much of their timeperch hunting for the same prey (i.e., large
insects, reptiles, and small mammals) (Stiles &Skutch 1989, Ridgely & Gwynne 1989).
Although competitive interactions between
these two raptors have yet to be documented,
92

within Talamanca, Costa Rica, the homeranges of the two often interdigitate, and it
seems reasonable to assume that the twosometimes interact competitively (but see
Smith 1980). Intraspecifically, in Cuba, where
wintering migratory American Kestrels (F. s.
sparverius) co-occur with the island’s smaller
resident form (F. s. sparveroides), migrants typically displace residents from preferred feeding areas (F. Rodriguez pers. com.). In thellanos of Venezuela, migratory Turkey
Vultures (C. a. meridionalis) socially dominate
smaller residents (C. a. ruficollis) when thetwo commingle at carcasses, and resident
vultures withdraw from savanna feedingareas and forage almost entirely within gallery
forests when the larger migrants arrive inthe region. Significantly, the body conditionof resident vultures declines at this time, particularly when the migrants overwinter inlarge numbers, whereas that of migrant vultures improves during their stay in the area
(Kirk & Gosler 1994, Kirk & Houston 1995).
Suggestively, in western Panama, where
migrants are believed to make up 45% of theTurkey Vulture population in winter, residents
begin egg-laying in February after many ofthe migrants have left (Smith 1980). Other
potentially competitive migrant-resident pairs
include Osprey and Black-collared Hawk
(Busarellus nigricollis), Mississippi Kite andPlumbeous Kite, and Swainson’s Hawk andWhite-tailed Hawk. Obviously, there is needfor additional careful study of this phenomenon in the region.
Migration-dosing speciation. One unintended
consequence of large-scale raptor movementsin the Neotropics is that each year somemigrants are displaced from their principal
migration flyways, either by weather or by
failed navigational systems, and once lost
eventually wind up in areas that are geographically isolated from the species’ traditional
wintering grounds. The so-called “vagrants”

RAPTOR MIGRATION IN THE NEOTROPIICS

face three potential outcomes. They can die in
the new-found area without breeding, they
can reorient themselves and successfully relocate their intended destinations, or they andother simultaneous vagrants can survive thewinter, forego return movements the following spring, and subsequently settle and breed
in isolation from their parental stock. This last
scenario can result in the phenomenon of
migration dosing speciation (sensu Bildstein &Zalles in press).
Simply put, migration dosing is orchestrated vagrancy in which viable groups ofpotential propagules in the form of winddrifted or otherwise navigationally challenged
flocks of migrants (1) simultaneously arrive atsites tangential to or beyond their traditional
wintering areas, (2) fail to return to theirintended destinations, and (3) via random
genetic drift, natural selection in response to
the new circumstances, or both, eventually
diverge from their parental stock and speciate
in isolation (Bildstein & Zalles in press). Most
likely, inexperienced juveniles constitute themajority of “dosed’ propagules, as they often
migrate together, are particularly vulnerable towind drift (Kerlinger 1989, Thorup et al.
2003), and have a propensity to develop new
migratory habits (Viverette et al. 1996).
Note that migration dosing differs from
adaptive radiation, in that adaptive radiation
involves “simultaneous divergence of numerous lines from much the same adaptive type
into different [and] also diverging adaptive
zones” (Simpson 1953), whereas migration
dosing refers to changes from relatively specialized highly migratory forms into otherspecialized, or in some instances – especiallyon islands – into generalized sedentary forms.
Although migration dosing has notreceived widespread attention as a speciation
mechanism in birds, conditions associatedwith it, including long-distance migration,
migration in large flocks, vulnerability to winddrift, and high rates of vagrancy among

migratory birds (Alerstam 1990, Newton2003), as well as numerous examples of speciation that are most simply explained by thisprocess, are well described in the avian literature (e.g., Snow 1978, Bildstein & Zalles inpress).
Neotropical migratory raptors appear tobe particularly prone to migration dosing forseveral reasons. Most prey on nutritionallysubstitutable prey rather than specific taxa,
which makes it easy for them to adapt to new
prey types in new areas. Many soar on migration and are particularly vulnerable to winddrift. And many, and in particular most longdistance migrants, migrate in large flocks,
thereby increasing the likelihood of simultaneous vagrancy (Kerlinger 1989, Schulter2000). The tropics are an ideal location for
migration dosing speciation because of theincreased likelihood of sedentary behavior
there, and the subsequent greater impact ofgeographic barriers (Mayr 1969).
Migration dosing is most common when
raptors are blown off course and onto relatively isolated islands from which return is difficult. The most dramatic raptor example ofthis phenomenon involves the genus Accipiterin the South Pacific. Nineteen species, or fully
38% of the world’s 50 species of accipiters,
occur on islands in Melanesia (18 species) andin Australia (3 species) east of Wallace’s Line(del Hoyo et al. 1994). The likely origins ofmost of these forms appear to be tied to theseasonal long-distance over-water migrationsof close to half a million Chinese Goshawks(Accipiter soloensis). In most years thesemigrants complete their outbound migrations
aided by monsoonal northwesterly winds andtheir return movements aided by easterly
trade winds. Each autumn, however, their
movements carry the birds across an active
tropical cyclone region in which typhoons canscatter the migrants, and in the springs of El
Niño-Southern Oscillation events, strong
westerly winds replace the easterly trade
93

BILDSTEIN

winds, making return migration difficult for
many of the birds. Both of these potential disruptions provide the backdrop for episodic
misdirected movements that place some of
the migrants on islands east of Wallace’s Line
far beyond their normal flight line fromwhich they are unable to return to their continental breeding areas. The “dosed’ vagrants,
in turn, appear to have provided the seedstock for much of the region’s high level ofendemism (Bildstein & Zalles in press).
Neotropical examples of accipiters thatmost likely speciated via migration dosing
include the critically endangered Cuban Gundlach’s Hawk (Accipiter gundlachi), the Central
American White-breasted Hawk (A. chionogaster), and the South American Plain-breasted(A. ventralis) and Rufous-thighed (A. erthronemius) hawks. The first appears to be derived
from the migratory Cooper’s Hawk, and thelast three from the far more migratory Sharpshinned Hawk. Intriguingly, several of thenon-migratory Neotropical forms tend to be
decidedly more intensely rufous ventrally, adifference that parallels that between the
Palaearctic migratory Eurasian Sparrowhawk(A. nisus) and the Rufous-breasted Sparrowhawk (A. rufiventris), its non-migratory
African derivative (Snow 1978). High-frequency episodic or continual gene flow
between the North American migratory subspecies of the Sharp-shinned Hawk (velox) andthe five largely Neotropical non-migratory
subspecies [suttoni (extreme s. New Mexico
south to s. Veracruz), madrensis (Guerro andpossibly w. Oaxaca, Mexico), striatus (Hispaniola), fringilloides (Cuba), and venator (Puerto
Rico)], all of which occur north of the threefull-species forms associated with this northAmerican migrant, apparently precludes speciation via migration dosing in these forms.
Two South American partial migrants, Bicolored Hawk and Chilean Hawk (A. chilensis),both of which are close relatives of the Cooper’s Hawk, also most likely resulted directly
94

or indirectly from migration dosing. The origins of the region’s three remaining accipiters,
Grey-bellied Goshawk (A. poliogaster), Tiny
Hawk (A. superciliosus) and Semi-collared
Hawk (A. collaris) are less certain (see Wattel
1973)
There are putative Buteo examples of
migration dosing as well. Seventeen species of
buteos, or 60% of the world’s 28 species,
including two complete and nine partial
migrants, occur in the region. Two Neotropical species, Broad-winged Hawk and Swainson’s Hawk, are long-distance, transequatorial
migrants that breed in the Nearctic and overwinter in the Neotropics. Both of the latter
regularly “dose” the Neotropics with at least
hundreds and possibly thousands of misguided individuals, and both appear to havespawned several new species via migration
dosing. Two of four range-restricted endemics in the genus, Galapagos Hawk (B. galapagoensis) and Hawaiian Hawk (B. solitarius)
represent examples of migration dosing via
swainsoni (Mayr 1943, Riesing et al. 2003). Bothof these island forms almost certainly arederived from wind-drifted flocks of Swainson’s Hawks that were driven off-course, possibly by tropical cyclones, and thereafter
carried westwards across the Pacific Ocean
via easterly trade winds, which are known tofacilitate round-the-clock soaring via largescale sea thermals (Augstein 1980). Tworecent molecular phylogenies (Fleischer &McIntosh 2001, Riesing et al. 2003) place solitarius and galapagoensis together with swainsoni
and the Central and South American Shorttailed Hawk (B. brachyurus) and the SouthAmerican White-throated Hawk. This raises
the possibility that the latter two continental
forms also arose via migration dosing.
Another range-restricted island endemic,
the Ridgway’s Hawk (B. ridgwayi), most likely
represents a migration dosing example via thefar less migratory Red-shouldered Hawk (B.
lineatus) although its origins via platypterus can-

RAPTOR MIGRATION IN THE NEOTROPIICS

not be ruled out (Riesing et al. 2003). The
Rufous-tailed Hawk appears to be an end-ofthe-migration-route continental form of thenorthern Red-tailed Hawk (B. jamaicensis).
On the other hand, five insular, Caribbean
basin races of the Broad-winged Hawk [e.g.,
cubanenis (Cuba), brunnescens (Puerto Rico),insulicola (Antigua), rivierei (Lesser Antilles),and antillarum (St. Vincent, Grenada, Trinidad& Tobago)], apparently are “dosed” toofrequently by continental forms, or havebeen isolated too recently, for speciation to
occur.
The origins of the remaining truly Neotropical buteos are a bit more difficult toassess. Red-backed, White-tailed, Gurney’s
hawks and, somewhat less so, the Gray Hawk
(B. nitidus), appear basal to the genus. The
largely non-migratory Roadside Hawk andWhite-rumped Hawk (B. leucorrhous) may beeven more basal, possibly to the point ofbeing in different genera (Riesing 2003).
Other likely migration dosed species in
the region include the Orange-breasted Falcon (F. deiroleucus) that is probably derived
from northern migratory populations of the
Peregrine Falcon (F. peregrinus anatum or tundrius), or, possibly, from the Bat Falcon (F.
rufigularis), which appears to be closely related
to the highly migratory Northern Hobby (F.
subbuteo) (Ferguson-Lees & Christie 2001).

DISCUSSION
The closing of the Central American landbridge approximately three million years ago
opened the way to substantial and significant
long-distance, land-based raptor migration in
the Neotropics. The Mesoamerican Land
Corridor, together with relatively extensive
land masses north of 30º in North America
and south of 30º in South America, is largely
responsible for most long-distance northernand austral migration in the region. The latitudinal location (20º–25º S), north-south orien-

tation, and seasonal nature of Paraguay Riverwetlands facilitate additional austral movements, and the High Andes produce the backdrop for considerable altitudinal migration in
the region.
As is true of many other groups of North
America migrants (see Keast & Morton 1980
and references therein), most, if not all, northern raptors overwintering in the Neotropics
do so principally in (1) coastal or inland
aquatic and wetland habitats (e.g., Osprey,
Peregrine Falcon, Turkey Vulture), (2) grasslands and open savannas (e.g., Mississippi
Kite, Swallow-tailed Kite, Swainson’s Hawk),or (3) forest edges and second growth forests(Broad-winged Hawk), rather than in large,
undisturbed forest tracts. Presumably most of
this reflects the fact that the northernmigrants inhabit similar habitats on theirbreeding grounds. As there is no evidence forcompetitive exclusion or increased predation
risk for migrants inhabiting undisturbed forests, it seems likely that the habitat shifts
occur because of the presence of “superabundant and/or sporadically available” prey inmany open areas (Karr 1976), coupled with alack of nest-site constraints during the nonbreeding season.
Somewhat surprisingly, the region’s twolargest seasonal tropical freshwater wetlandcomplexes, the 300,000-km2 VenezuelanColombian llanos and 200,000-km2 BrazilianBolivian-Paraguayan pantanal (Scott & Carbonell 1986), do not appear to host large
numbers of long-distance migrants, althoughintratropical migrants are well represented
there. A recent two-year survey of raptors
in the central and western Venezuelan llanos,
for example, indicates that North American
migrants, Osprey, American Kestrel, andPeregrine Falcon, comprise only 4.2% of all
raptors seen, and that when kestrels – which
are both resident and migratory in the area –are eliminated from the analysis, the tworemaining migrants make up only 1.2% of the95

BILDSTEIN

community. On the other hand, numbers ofseveral local and regional migrants either
increased [Yellow-headed Caracara (Milvago
chimachima), Savanna Hawk (Buteogallus meridionalis), and Crane Hawk (Geranospiza caerulescens)] or decreased (Snail Kite and Whitetailed Hawk) during the November–April dry
season (Jensen 2003). Unfortunately, thestudy did not survey the wetland’s New World
vultures, and the relative densities of migratory versus resident Turkey Vultures was
not determined. A detailed study of raptor
populations in the Pantanal is lacking, butthere are no reports of large numbers ofnorthern migrants over-wintering in thatarea either (see for example Sick 1993). Large
numbers of northern migrants, including
Swallow-tailed Kite, Mississippi Kite and,particularly, Swainson’s Hawk, do overwinterin South America’s semitropical and temperate grasslands, steppes, and savannas,
however. As is so in southern Africa, northern migrants often outnumber resident species in these habitat types, presumably
because of the seasonal nature of insect andsmall mammal abundances in these areas (seeSmeenk 1974).
As a result, the open habitats of SouthAmerica play host to some of the most temporally and spatially diverse raptor communities anywhere. The extent to which migrants
and residents compete for resources in thesecircumstances is unclear. Most, if not all, of
the long-distance migrants are principally
nomadic during the dry season, as are mostregional and local migrants. Many of the latterat least are attracted to large-scale seasonal
burns where they feed upon animals along
active fire lines (Sick 1993). Others, including
Swainson’s Hawk, track migrating swarms ofdragonflies (Aeshna bonariensis) (Jaramillo1993), and still others, including Swallowtailed Kite, track populations of ephemerally
swarming termite alates (Termitidae) (Bildstein pers. observ.). Many species feed at live96

stock carcasses that accumulate during
particularly severe wet or dry seasons.
Arguably, highly diverse and apparently
superabundant prey, together with a lifestylethat allows raptors to track food resources
regionally, may enable migrants and residents
to coexist in Neotropical open habitats withrelatively little in the way of inter-species
competition, particularly if intra-species competition (Kirk & Gosler 1994, Kirk & Houston 1995) and territoriality (Smith 1980) limit
densities within species. Nevertheless, muchremains to be learned regarding the ways inwhich northern migrants and Neotropicalresidents parse the resources available tothem, and careful observations in this area arelikely to provide rich sources of information
regarding competition between these twogroups of raptors.
So far, conservation biologists focusing
on the factors responsible for the creationand maintenance of biological diversity haveconcentrated their efforts on the roles thatgeological processes, past climates, dispersal,
and the current availability of local resourcesplay in shaping current biodiversity, with little,
if any, attention being paid to long-distancemigration (see for example Newton 2003).
My assessment of raptor movements in the
Neotropics suggests that migration plays asubstantial role in shaping patterns of species
richness of several widespread genera of raptors in the region. Based on examples presented above, migration dosing appears toprovide a plausible addition to the allopatric“refugium theory” (Haffer 1969) and parapatric speciation across vast distances (Knapp &Mallet 2003) in explaining high species diversity in the Neotropics, at least for the region’s
raptors.
As raptor migration is representative ofthat of birds in general, it seems reasonable toconclude that migration dosing as a speciation mechanism occurs in other groups ofbirds as well (see for example Snow 1978).

RAPTOR MIGRATION IN THE NEOTROPIICSTABLE 2. Priority areas of inquiry for raptor-migration science in the Neotropics.
Why don’t Broad-winged Hawks (Buteo platypterus) shift their flight line from the Atlantic slope to the
Pacific slope at the Isthmus of Tehuantepec as do Turkey Vultures (Cathartes aura) and Swainson’s
Hawks (B. swainsoni)?
Where do most Turkey Vultures, Mississippi Kites (I. mississippiensis), Broad-winged and Swainson’s hawks
overwinter in the region, both geographically and in terms of habitat?
What are the migratory patterns of intratropical and austral migrants in the region?
To what extent to long-distance migrants from North America compete with wholly Neotropical migrant
and resident species?
To what extent is migration dosing responsible for raptor species diversity and distribution in the region?

Thus, I strongly recommend that researchers
studying avian diversity in tropical regions,
including those working on molecular phylogenies, consider the possibility of migration
dosing when discussing their results.
Finally, much remains to be learned about
raptor migration in the Neotropics. The priority areas of inquiry listed in Table 2 offer astarting point for additional study.

ACKNOWLEDGMENTS
In addition to published sources, much of theinformation herein was gathered and submitted to Hawk Mountain Sanctuary by cooperators working on the Hawks Aloft Worldwide
migration atlas project. I thank all of them,and in particular Jorge Aguilar, Chelina
Batista, Ernesto Ruelas Inzunza, Cristian
Olivo, Pablo Porras, Freddy Rodriquez Santana, Marco Saborio, Cesar Sanchez, and Julio
Sanchez, for helping clarify my impressions of
raptor migration in the region. My colleague
and friend Jorje Zalles played a major role inintroducing me to the Spanish-language literature, as well as in helping refine my thoughts
on migration dosing. Jaime Jimenez and theco-organizers of the VIIth Neotropical Ornithological Congress graciously provided an
appropriate forum for presenting my ideas on
Neotropical raptor migration. Kyle McCarty
and Pablo Porras commented on earlier draftsof the ms. This is conservation science contri-

bution number 103 from Hawk Mountain
Sanctuary.

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