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Nature Of Migration In The Species
Nearly the entire population migrates annually between breeding areas in North America and wintering grounds in pampas of South America, a round-trip that can exceed 20,000 km. Individuals are absent from breeding grounds for 5–6 mo in central California, 205–217 d in ne. California (Woodbridge 1991), and 7–8 mo in Saskatchewan (CSH). Generally migrate in flocks that can be as large as 5,000–10,000 individuals, always during daylight, typically soaring in thermals, and rarely over water (Fig. 2). Although overall migration period at any given location in spring or fall may last 2 mo, most individuals pass through Central America during brief, concentrated period (roughly 2 wk) during both seasons (Smith 1980, Ruelas Inzunza et al. 1993). In Mexico and Central America, associates with flocks of Broad-winged Hawks, Turkey Vultures (Cathartes aura), and Mississippi Kites (Ictinia mississippiensis), creating a river of hawks called “one of the most spectacular and easily observed movements of birds in the New World, and possibly anywhere” (Smith 1980: 52). No known sexual differences in migration pattern or timing.
Timing And Routes Of Migration
Flocks of several hundred immatures and postbreeding adults gather by late Aug and early Sep, fattening on grasshoppers (Orthoptera; McGrath 1988, Houston 1990). With aid of northerly winds, they then travel south in flocks. In 1995, 2 adult hawks (1 female and 1 male) with satellite transmitters were tracked from their territories near Hanna, Alberta, to Argentina. On 14 Sep 1995 these birds were on their territories; reached sw. Saskatchewan by 23 Sep. The male (position transmitted weekly) passed through Nebraska on 1 Oct; Tamaulipas, Mexico, on 7 Oct; Honduras on 14 Oct; and reached Marcos Juárez, Argentina, on 7 Nov. The female (position reported twice weekly) reached Kansas on 5 Oct; Tamaulipas, Mexico, on 9 Oct; Oaxaca, Mexico, on 12 Oct; Nicaragua on 19 Oct; Panama on 22 Oct; passed through Colombia on 25–28 Oct; south of Santa Cruz, Bolivia, on 4 Nov; and reached Armstrong, Argentina, on 7 Nov. Male took up to 54 d, averaging 194 km/d, while female took up to 53 d and averaged 198 km/d. During two 3-d periods in early Nov, female averaged 565 km/d and 473 km/d, respectively. In the month after arrival, female continued another 650 km south within Argentina (Schmutz et al. 1996).
In 1996 and 1997, 34 Swainson’s hawks were captured throughout the breeding range in North America and instrumented with satellite radios Fuller et al. 1998, (Bechard et al. 2006). Only 27 of these completed the southward migration with a mean cumulative distance of 13,504 km and a mean direct distance of 10,139 km, averaging 188 km/day. They converged in eastern Mexico on the Gulf of Mexico coast, then followed a narrow, well-defined path through Central America, across the Andes Mountains in Columbia, and east of the Andes to central Argentina where they all spent the austral summer (Fuller et al. 1998).
Band recovery data and ground observations give comparable migration dates. Most individuals have left British Columbia by late Aug or early Sep (recorded to 30 Oct; Campbell et al. 1990). Transients observed mid-Aug–early Oct in s. California (extremes late Jul–late Oct; Garrett and Dunn 1981); late Aug–early Oct, peaking in late Sep in Minnesota (recorded to 7 Nov; Janssen 1987); late Aug through third week of Oct, peaking in late Sep or early Oct, in Missouri (Robbins and Easterla 1992); Oct–Nov in Mexico (Howell and Webb 1995); Oct–early Nov in Panama (occasionally late Sep; Smith 1985a, Ridgely and Gwynne 1989); and Sep–early Nov in Colombia (occasionally early Aug; Hilty and Brown 1986). Earliest band recovery in Guatemala was 7 Oct, in Colombia 22 Oct, and in Argentina 10 Nov (Houston and Schmutz 1995b).
Migration proceeds on broad front through s. Canada and U.S., but as birds converge in Middle America, huge flocks form as many birds pass through much narrower areas. On 6 Oct 1953, 16 km west of San Antonio, TX, 25,000 birds passed over 0.4-km observation strip; flight was 6 km wide and extrapolated to have contained up to 375,000 hawks (Fox 1956). On 2 Oct 1991, 5,014 Swainson’s Hawks were counted flying over Medina River, slightly farther west of San Antonio, in the hour before noon; that same morning, with a cold front, 15,000 flew over Mitchell Lake south of San Antonio; and a week later 10,000 passed northwest of Medina River (Fox 1956, Economidy 1992b). In e. Mexico, even larger concentrations passed just west of Gulf of Mexico and over Veracruz (Ruelas Inzunza et al. 1996; see Demography and populations: population status). Between Mexico and Panama, most transients occur on Pacific slope (Howell and Webb 1995). They usually fly over Nicaragua, for which there are no specimen records, without stopping (T. Howell pers. comm.). Large numbers are counted at Talamanca, Costa Rica at a newly recognized migration site, second only to Cardel in Vera Cruz, Mexico (Porras-Penaranda et al. 2004). They cross the upper Andes in Colombia and travel along eastern foothills of Andes, south through w. Brazil and e. Bolivia to Argentina. As birds enter n. South America and disperse over larger area, size of migrating flocks decreases (Hilty and Brown 1986).
Observations in e. North America have increased since 1970s, particularly in Ontario (12 fall and 5 spring records; Duncan 1986), Pennsylvania (3 in 1995; Hohenleitner 1995), Massachusetts (2 in 1988; Veit and Petersen 1993), and New Jersey (≤7 sightings/yr at Cape May alone; Sibley 1993). Still considered very rare along East Coast in fall. First mention in Canadian maritime provinces occurred in Nova Scotia in 1964 (Tufts 1986). The first Nova Scotia specimen, carrying a band applied at Cape May, New Jersey the previous September, was found dead in spring 1989 at Advocate Harbour, Nova Scotia (Houston and Schmutz 1995). The first fall sighting in Newfoundland was on 5 Oct 1998 (Mactavish 1999) and the first spring sighting was on 8 May 1999 (Maybank 1999). Other first sightings on the East Coast included New Brunswick 10-11 Oct 2002 (Mactavish 2003), North Carolina 3 Oct 1993 (Dean and Dinsmore 1997), and Long Island 18 Sept-4 Oct 1998 (DiCostanzo and Hays 1999). Status of these birds is unknown; they could winter in Florida where a small number arrive in Oct (Stevenson and Anderson 1994). The increase in eastern sightings may reflect an increase in birdwatchers and raptor enthusiasts, but increasing observations at some eastern hawk watches that have been carefully monitored for years suggests a possible long-term trend.
Sightings in the Caribbean were first reported in the mid-1990s on islands as far south as Trinidad (Hayes 2001).
Band recoveries and satellite telemetry data indicate central Argentina to be the main austral ground but there are also sightings in Brazil and museum specimens collected in Chile in 1875 and 1923 (Marin 2000). Banded birds, with adults over-represented from ne. California, Saskatchewan, and Colorado co-occurred in e. La Pampa Province, Argentina, in Jan 1994 (Woodbridge et al. 1995b); in 1994–1995, birds with satellite transmitters from ne. California, Idaho, Alberta, Saskatchewan, and Colorado co-occurred in same region (MJB). These results suggest that flocks in South America consist of individuals from different regions of breeding range (Woodbridge et al. 1995b). Geographic origins of overwintering populations in California and Florida unknown.
Large concentrations in Argentina begin to disperse in mid-Feb and begin moving north in late Feb–mid-Mar (MJB, B. Woodbridge pers. comm.). Transients recorded Feb–Mar in Colombia (Hilty and Brown 1986); Mar–early Apr (occasionally late Feb–late Apr) in Panama (Ridgely and Gwynne 1989); Mar–Apr in Mexico (Howell and Webb 1995), typically peaking in Veracruz, Mexico, 10–13 Apr (Ruelas Inzunza et al. 1996); mid-Mar–May (occasionally early Mar), peaking first half of Apr, in s. California (Garrett and Dunn 1981); peak late Apr–early May (earliest date 7 Mar) in British Columbia (Campbell et al. 1990); early Apr (earliest 14 Mar), peaking mid–late Apr, in Missouri (Robbins and Easterla 1992); and peak late Apr (recorded late Mar–mid-May) in Minnesota (Janssen 1987). Most generally return to Alberta and Saskatchewan between late Apr and 10 May (CSH).
Swainson’s hawks’ northward migration largely retraced their southward route. Averaging only 150 km/day (n = 17 complete northward migration tracks), it took them 60 days to complete the shorter 11,952 km migration back to North America (Fuller et al. 1998, Bechard et al. 2006).
Migration is diurnal and timed to take advantage of rising thermals of hot air. In s. Texas, “the spectacular flights of fall migrants are, on sunny days, usually high in the air, where birds soar along, seldom flapping. They pass in long straggling lines or in clusters, which sometimes pause to wheel even higher on thermal updrafts” (Oberholser 1974: 234). May move in large flocks because thermal energy occurs in patches, zones, or waves (Smith 1985a). Hawks that were followed in a glider at 700 m entered early stages of developing thermal “streets” (i.e., lenticular zones of constant thermal uplift), then glided, largely out of sight from the ground, in lower 3 m of long, flat clouds (Smith 1985a). A hawk entering such a “street” may sail 60 km or more in straight line without losing altitude (Smith 1985b). Can also exploit lift along face of storm fronts, and slope- and wave-soar up and into the dry-season inversion zone to altitudes exceeding 6,000 m (Smith 1985a). Rarely fly over ocean, but in 1982, after unusually persistent northeast trade winds blew them onto the Azeuro Peninsula of Panama, they flew 30 km over Bay of Panama (Smith 1985a). At night, will roost in flocks of >100 and begin migration the following morning when thermals form (Smith 1980). During northward migration, late Feb–Apr, individuals tack in and out of mountains against and with northeast trade winds, often achieving very high speeds (Smith 1985a).
Control And Physiology
No information available on hormonal control of migration or physiological changes. Species is almost unique in switching from diet of primarily small mammals when raising young to insects when nonbreeding birds collect in midsummer flocks (Johnson et al. 1987), or when adults and immatures prepare to migrate.
Body fat increases prior to migration (Smith et al. 1986), but controversy persists about how far and how long Swainson’s Hawks can travel using these fat stores without food (Goldstein and Smith 1991, Kirkley 1991). However, the fasting migration model has had little empirical support based on body masses of hawks during migration (Bechard et al. 2006). There are occasional places en route through Central America and n. South America where they can forage; e.g., one account documents Swainson’s Hawks eating caterpillars on the ground in Costa Rica (P. Slud in Smith 1980). However, Skutch failed to see any sign of excretion below roosts of migrating Swainson’s Hawks; this was in keeping with his failure to see any sign of eating or hunting as the birds passed over El General, Costa Rica. (Skutch 1998). Likewise, where thousands of migrants roosted overnight in Panama there were no excreta, suggesting fasting birds at that location (Smith 1980).
Migration from Mexico to Argentina can be covered in approximately 3 wk (see Migration: timing and routes of migration); given that some suitable foraging habitat does occur in Central and n. South America, extended periods of fasting may not be common. Report that some individuals arrive in Argentina so weak they can be picked up by hand (C. C. Olrog in Smith 1980) could be due to fasting, or alternatively could be result of birds that encounter bad weather which extends the migration period and prevents foraging, or possibly birds that encounter pesticides en route to Argentina (B. Woodbridge pers. comm.).
An assessment of nutritional condition of free ranging hawks in Cordoba province, Argentina, revealed no statistical differences among sex and age groups for urea, uric acid and triglyceride serum concentration. However, cholesterol concentration was higher in male (2999 mg/liter) than female hawks (2167 mg/liter). (Sarasola et al. 2004).
Bechard, Marc J., C. Stuart Houston, Jose H. Sarasola and A. Sidney England. 2010. Swainson's Hawk (Buteo swainsoni), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/265