Migration

Complete migrant, leaving breeding grounds in eastern U.S. for wintering areas from s. Florida to n. South America. Long-distance migration known from seasonal changes in regional occurrence and from recoveries of banded birds. Observation of numbers and sequence of arrival dates from Galveston to n. Florida indicates most birds cross the Gulf of Mexico (Stevenson 1957); flocks in Veracruz, Mexico, suggest some movement around the Gulf, avoiding an overwater crossing (Rappole and Warner 1980).

Fall

In autumn, leave northern breeding areas in Sep; migrants common in se. U.S. in late Sep and Oct (Taber and Johnston 1968). Nearly all birds have departed breeding grounds by mid-Oct; first arrivals in Neotropics in mid-Sep. Migration moves in a broad front and not along mountain ridges (Alleghenies, Hall 1983). Individuals appear on Campeche Bank (islands 120—160km northwest of Yucatán Peninsula and 750 km south of Mississippi Delta; 22°07'N 91°25'W) during Oct, but not in winter (Howell 1989). Likewise on cays of Belize, appear only during migration (Russell 1964), but present throughout mainland Belize Oct–15 Apr, peak numbers Oct 1 –15Apr (Jones 2004).

Spring

Leave wintering areas in late Apr and May; arrive on breeding grounds late Apr to early Jun (Trautman 1940, Taber and Johnston 1968, Payne 1991). Considerable variation in timing of migration: some individuals still on wintering grounds in Guatemala through April; others appear in Louisiana late Mar through early May (Lowery 1974, Rogers et al. 1982). Moving with the buntings to band them in spring migration and recover them on their breeding grounds (he had no recoveries), Nickell (1966) noted some buntings arrived in their breeding areas in Michigan before the last buntings arrived in migration in Texas and Louisiana. In spring migration buntings are seen over water approaching the northern Gulf coast. Migrants arrive on the n. Gulf coast from late Apr through May (Burleigh 1944, Nickell 1967, 1968). 11 males recaptured during spring migration in inland North America were also found on breeding grounds late May to Jul. These birds tended to arrive earlier at mid-latitudes when the distance to their northern breeding site was greater. Five recoveries in Fig. 4 suggest a flight over Gulf of Mexico followed by a stopover to feed near the southern coast of the U.S., and a later northward flight to the breeding area.

On northern breeding grounds, arrive over a period of several weeks. In Missouri and s. Indiana, males first appear at breeding sites in late Apr; in s. Michigan, in early May (Carey 1982, Quay 1987, Payne 1989, 1991). Overlap in arrival dates of age classes is considerable: although adult males arrive earlier than yearling males on average, less than 10% of the total variance ina rrival dates in May in s. Michigan is variance between the two age groups; > 90% of the variance is within age groups (Payne 1991, Payne and Payne 1996).

Females migrate later on average than males, and although arrival dates of females are more difficult to determine by direct observation than the conspicuous males, there is a two-week difference in the arrival dates of the first males and the first females. Time of arrival is thought to result from differences in time of leaving the wintering area: premigratory fattening schedules in Jamaica indicate a two-week lag between the onset of spring migration of males and females (Johnston and Downer 1968).

In Jamaica, groups of 40 or so birds remain together all winter, and banded birds observed near a feeding station from Dec to (more often) Feb stay through mid-Apr; a few sing before departing in spring migration (Johnston and Downer 1968, Downer and Sutton 1990).

Band Recoveries

Migration routes determined from banding recoveries reported through March 2003 (USGS Bird Banding Laboratory BBL). Nearly all individuals banded as adults in spring or autumn, or as birds of the year in autumn; few banded as nestlings; most banded in May, Sep and Oct. Buntings were banded in the breeding or migration season in 45 states in the United States and six provinces in Canada, and in winter in Florida, Texas, Mexico, Belize, Guatamala, Honduras, El Salvador, Costa Rica, Panamá, Columbia, the Greater Antilles (Cuba, Puerto Rico, Jamaica), the Lesser Antilles, and the Bahama Islands. Of 149,738 buntings banded from 1927 through 2002, an average of 2,000 birds a year, only 340 birds were encountered again; and of these, only 116, a total of 0.078% of buntings banded, were recovered away from their banding site (bird banding and encounter data). Encounters within 10 km of the banding site were excluded from the comparison.

Migration recoveries between the seasons indicate that individuals that breed in eastern and midwestern regions of North America remain geographically separate into winter, with the net linear flight tracks of migration between sites east and west being parallel between seasons. In comparing the east-to-west sites in the winter and breeding regions, the longitudinal order of sites in the breeding and the wintering range are closely correlated (Spearman rank-roder correlation coefficient r_s= 0.93, P < 0.01). These recoveries provide evidence for a migratory link between spatially distinct breeding and wintering populations. Bunting populations thus maintain their spatial structure ("connectivity", Greenberg and Marra 2005), from east to west, across the seasons.

Within North America, spring migration recoveries trace a net linear movement towards the northeast (Fig. 4). Ten males encountered south of 35°N were controlled in passage from 2–13 May and during the breeding season from 18 May–5 Aug. Their flight tracts were extended northward to 40°N on a map plot to allow comparison of their east-west geography at this common northern latitude (Fig. 4). Birds in the se. United States migrated to the eastern part of the northern breeding region, and birds further west along the Gulf coast migrated to the northern midwestern region. The spatial orders of southern sites in spring migration and of sites in the spring and the summer breeding region were closely correlated (r_s= 0.89, P < 0.01).

The directions of migration tracks between winter and breeding site (Fig. 3) were compared with the directions between passage site and breeding site (Fig. 4). Directional angle of tracks from south or southwest to northeast was measured with a protractor, where each linear track was between a southern site south of 40°N and a northern site, and where the linear tracks were longer than 300km. The direction of linear tracks differed between the winter and breeding site, and the spring passage and breeding site (Mann-Whitney U test, z = 17.7, P < 0.001). During the season of spring migration the winds over the Gulf of Mexico blow from the southeast, and further north the prevailing winds over the continent of North America are from the southwest (Gauthreaux et al. 2005). these banding recoveries thus suggest that buntings fly downwind both in crossing the Gulf and in overland migration in spring between landfall north of the Gulf and their journey to breeding grounds, and these winds differ in direction along their journey in spring, first from the southeast and then from the southwest.

The one male was banded in se. Pennsylvania on 15 May 1956 and recovered seven days later in Massachusetts. The recovery file also lists four females with seasonal movement in spring. The most distant recovery was a female banded in Michigan on 27 May 1966, and found 1,020 km to the east in Massachusetts on 5 July in the same year; perhaps it was still in migration when banded in late May. In four recoveries north of 35°N in spring, the later date in the breeding season was within 2° latitude south of their earlier site. These recoveries indicate flight reversals: when a bird flew too far northward it then tracked back to the south.

Migratory Behavior

Nocturnal migrant. Rate of feeding increases before migration, as birds deposit fat that is used to fuel migration (Johnston 1965, 1973, Emlen 1969). In migration season birds become active at night (Emlen 1967a, b, 1969); gather together in flocks during weeks before a trans-Gulf or other long migratory flight. Depart in flight at night, and sometimes continue to fly into daylight. Unknown how individuals detect direction and speed of winds before they depart on a migratory flight. Apparantly not social in flight, and do not call extensively; only one record of song in migration flight (Taber and Johnston 1968). One estimate of flight speed of migrating buntings: about 32 km per hour (Cooke 1937).

Control And Physiology Of Migration

In autumn, accumulate subdermal and abdominal deposits of fat before they migrate; become as heavy as 26g, a 50% increase in mass (Johnston 1965, 1973). Some "migrants," including birds killed in passage flight, are only 15g (Odum et al. 1961, Blake 1969), no heavier than birds in breeding season. These birds lack sufficient fuel to complete a long-distance flight; may feed again before a long trans-Gulf flight. Based on calculations of energy metabolism, a bunting needs about 30% of body mass in available fat to complete the nearly 1,000 km flight from n. Florida across the Gulf of Mexico (Odum 1960). Buntings ≥ 18 g may have suffiicient flight-fuel energy to complete a trans-Gulf migration unaided by winds (Johnston 1965, Johnston and Downer 1968).

On wintering grounds, in Panama, Guatemala and Belize, individuals in Mar and Apr were on average only 15g and did not increase mass before they disappeared; because their fat deposits are insufficient to sustain an active flight from Central America across the Gulf, they probably stop and feed in Mexico before flight over the Gulf (Rogers and Odum 1966, Rogers et al. 1982, Wetmore et al. 1984). In Jamaica, birds add fat before migrating north in spring: mean mass of males increases from 14.5 g in Jan to 18–19 g in Apr, just before migrating (Johnston and Down 1968). Buntings arriving after a Gulf crossings in spring stop (mean, 4 d) and feed near the coast in Lousiana, where these fat-depleted birds gain mass at faster rate in woodland habitats with low density of migrant birds than in habitats with more migrant birds (Moore and Yong 1991, Moore 1999). Further north on the mainland, buntings feed before they arrive at breeding sites; may restore depleted energy reserves and fuel their next flight in migration. In s. Michigan, most males arrive on breeding grounds lighter and leaner than they are later in breeding season. Seven of 8 heavy birds (16.1—17.9 g) captured in a woodlot in May left the area (apparently still in migration) whereas 9 of the 15 lightest birds (13.8—15.8 g) stayed at least 20 d and most bred there, a significant difference (RPB). For body mass of migrants in sw. Pennsylvania, see Measurements.

In migration season, captive buntings become restless at night, a behavior appropriate to the season and the direction of migration. They orient their activity southwards in autumn, and northwards in spring. Main cue to orientation is the star pattern in night sky, shown by tests of behavior toward star patterns in a planetarium (Emlen 1967a, b, 1969, 1970, Wiltschko et al 1980). Uncertain whether magnetic fields influence migratory orientation (Emlen et al 1976, Sandberg et al. 2000). Experienced adults return to previous breeding sites in the field even when they are held captive thorugh winter and released far from their normal wintering area (Sniegowski et al. 1988).

Molt and migration generally occur in different seasons. In early May on breeding grounds, a few males arrive with blue head and body feathers still growing (RBP). After breeding, molt is generally completed on breeding grounds before migration (RBP), and occasional nocturnal migrants have growing primaries and body plumage in early Oct (Tordoff and Mengel 1956). Some birds may interrupt their molt in autumn migration, and resume molt after they arrive on their wintering grounds (Yuri 2002). See also Appearance.