|Peters Family Name
|Sibley Monroe Family Name
|Gill Family Name
||ACCIPITRIDAE:Hawks,Eagles and Kites
|New 2013 Family Name
||ACCIPITRIDAE,Buteoninae:Buteos and Allies
$Nisus hiemalis Cuvier,1829,Le règne animal distribué d'après son organisation,2nded.,1,p.334.
Nature Of Migration In The Species
One of 5 North American diurnal birds of prey that are complete migrants. Continental populations vacate their breeding range during winter; endemic Caribbean subspecies are resident.
Generally migrates in large flocks, or "kettles," ranging from several individuals to thousands. Migrants frequently soar in thermals. Kettles of tens of thousands of birds are regularly sighted in s. Texas, Mexico, and Central America, occasionally along shores of Great Lakes, more rarely elsewhere. Lone migrants are rare; only 12% of fall migrants sighted in central New York flew alone.
Although the overall migration period at any given location in spring and fall may last 2 months, most individuals pass during a brief and concentrated 2 wk period during both seasons.
In Mexico and Central America, associates with flocks of Swainson's Hawks, Turkey Vultures (Cathartes aura), and Mississippi Kites (Ictinia mississippiensis) as well as with solitary raptor migrants such as Osprey (Pandion haliaetus) or Cooper's Hawk (Accipiter cooperii).
Routes Of Migration
It has been suggested that this species uses an elliptical migration path to compensate for prevailing winds in spring and fall; hypothesized route from and to e. Canada is more easterly in fall and more westerly in spring.
Broad-front migrant throughout its breeding range in e. United Sstates and Canada, as well in eastern plains states and e. New Mexico; rare but regular through w. U.S., along California coast, and north to British Columbia. Numbers concentrate, however, where topography funnels migrants, or where human-made features, such as cities, increase thermal strength. Migration routes skirt large bodies of water, although short water crossings occur regularly, especially where peninsulas or islands shorten the distance (e.g., over Bay of Fundy between Nova Scotia and Maine or New Brunswick); occasional wider crossings reported (e.g., Florida Keys to Cuba).
Autunl. Individuals from e. and central Canada head south skirting Great Lakes and Atlantic shoreline; thousands pass northern and western shores of Great Lakes (e.g., 228,176 on western shore of Lake Erie on 19 Sep 1994), e. Appalachian Mtns., and Connecticut and New York coasts. Eastern migration route moves inland through n. New Jersey and se. Pennsylvania; flight lines disperse along southern Appalachian ridges, with smaller flocks observed from Maryland to Tennessee. Most migrants presumably head southwesterly, through Louisiana and e. Texas, to fly around the Gulf of Mexico. Central Flyway birds appear to disperse south through Mississippi Valley to converge with Eastern Flyway birds in Texas.
Migration routes west of Mississippi River poorly known. Small, but increasing, numbers follow Rocky Mtns. south, with annual counts averaging 25-30 birds in Goshutes Mtns., Nevada. Small numbers, 50-150/yr, occur along California coast and in Baja California.
Migrants flying south along ridges in Pennsylvania are predominantly adult (63-85% from 1987 to 1994; LJG); those sighted along Atlantic Coast are mostly juveniles.
Spring. Except for Great Lakes region, where birds concentrate along southern shorelines, and s. Texas, where thousands may pass each day, spring migration is more dispersed than fall, with few notable concentrations reported.
South Of U
Migration south of U.S. not as well documented. Most Broad-winged Hawks leave and enter U.S. through se. Texas. In Mexico, migrants follow Gulf of Mexico slope and adjacent foothills, and also Pacific slope south of the Isthmus of Tehuantepec, during spring and fall. Migrants dispersed more broadly along Atlantic slope in spring than in fall.
In Guatemala, large flights follow the foothills on the Atlantic slope. In Honduras, huge flocks reported during October on the Pacific slope. In Costa Rica, migrants appear to use both slopes in spring but primarily the Atlantic slope and highlands in fall. In Panama, primary route is same as that used by Swainson's Hawk (near Pacific Coast in central Panama and in central mountains in east and wes). Migrants of both species occur in such large numbers in Panama that they "blacken the sky".
South of Panama there are few data on the routes (or flight timing). Thousands appear to roost along eastern slope of Colombian Cordillera in sites such as Combeima Canyon between Bogota and Cali.
Although most birds reach winter destinations via Texas and Middle America, possible limited over-water route; individuals observed flying south from Florida Keys in fall, although some fly northwest, after having turned back from an extended over-water flight across the Florida Strait. Recher and Recher (1966) reported spring flights of B. p. platypterus over Puerto Rico, and others report flocks (e.g., 40-200 birds) on Tobago, and Little Tobago. Occasional sightings on Trinidad. Migrants reported consistently but rarely from Dominican Republic.
Timing Of Migration
One of the earliest fall migrants of any North American raptor, and one of the latest in spring. Timing probably influenced by reliance on cold-blooded prey.
In e. North America, fall migration mid-August to early October with peak flights (mid-Atlantic) 10-20 September. Breeders leave territories in New York and Pennsylvania in late August to mid-September. Timing of migration is compressed at any given latitude; e.g., at Hawk Mtn., PA, 95% of all fall migrants pass within a 2-wk period in mid-September. In s. Texas, major flights occur in late Sep and first week of October, with smaller flights continuing through late October (e.g., 1-day records of >250,000 birds in s. Gulf Coast); late migrants recorded into November.
Fall migration in Mexico occurs September-October with peak in late September or early October; peak 1-day flights of >400,000 recorded in central Veracruz. Fall migrants recorded in Costa Rica and Panama from late September to mid-November, but most pass in October.
Birds begin to arrive on wintering sites by late October; appear widespread through Amazonia, Brazil, after 31 October.
Spring migration spans the period March through May and, rarely, into June. Migrants recorded in Panama from March to early April, with most migration occurring mid- to late March. Spring migrants recorded in Costa Rica from early March to late May, with most passing in mid-March.
In Mexico, migrants recorded from March to mid-May, with peak flights last week of Mar to early April. Birds then fly north into Texas by early April. Movements in Texas peak in the last week of March and the first 1-2 week of April, with thousands of birds per day passing sites such as Santa Ana National Wildlife Refuge.
Spring migration peaks in mid- to late April from mid-Atlantic states north. Peak movements (>10,000 birds/day) along the south shore of the Great Lakes occur in late April and early May. Late migrants, predominantly subadults, often return to breeding areas in late May or June.
No known differences in migration pattern or timing by sexes. Few data on timing of fall migration by age, although observations at Hawk Mtn., PA, suggest that immatures may migrate earlier than adults. Immatures migrate north several weeks later than adults in spring, with peak flights of adults in April and of immatures in early May.
Geographic Origins Of Overwinter Populations
No association noted between origins of individuals and their winter recovery locations. U.S. Fish and Wildlife Service band recoveries for birds banded during the breeding season or fall migration and recovered between Dec and Feb (n = 22 [1926-1985]), showed 6 (27.3%) recoveries in South America, 10 (45.5%) in Central America, 2 (9.1%) in Mexico, and 4 (18.2%) in U.S. Recovery data may be biased toward U.S. recoveries.
Segregation On The Wintering Grounds Among Age And/Or Sex Classes
A sample of 191 specimens from 14 scientific museums suggests little if any segregation among age and sex classes on wintering grounds in s. Central America and nw. South South America. When birds wintering in Florida are included in this sample, juveniles appear to winter farther north than adults. South Florida wintering population may represent birds that have separated from the main flight. Three females banded as juveniles in s. Florida were recovered in following years as adults in Mexico and Guatemala, suggesting that Florida juveniles migrate to Central America in subsequent years.
Males may winter farther south than females, as a slightly greater proportion of male specimens was recovered in South America, although the difference is not significant if Florida birds are excluded).