Migration

Figure 1. Distribution of the White-throated Sparrow.

Nature Of Migration In The Species

Short distance nocturnal migrant strongly influenced by weather. Although breeding and winter ranges overlap (Figure 1), most if not all populations are migratory. Spring migrants arrive when snow melt exposes portions of forest floor; latest departures in fall are prior to first permanent snowfall. In winter individuals seek areas with minimal snow cover (e.g. se. U.S.), facilitating ground foraging.

Timing And Routes Of Migration

Migration routes not documented in detail. Recoveries of birds banded in summer and winter show convergence from broad area in summer (Newfoundland to Alberta) to narrower wintering area in se. U.S. (Bird Banding Laboratory). Birds from western populations winter west of central and eastern birds (Mazerolle et al. 2005). There is also a wintering population along the U.S. west coast (Figure 1), possibly derived from breeders west of the Rocky Mountains.

Females winter further south than males, as shown by sex ratios at different latitudes (Odum 1958, Jenkins and Cristol 2002) and stable hydrogen isotopes in head feathers grown on the wintering ground (Mazerolle and Hobson 2005, 2007). First year females may winter further south than adults (Odum 1958). Males leave earlier than females in spring (Odum 1949) and arrive at staging and breeding areas 1–2 wk ahead of females (Mills 2005, Caldwell and Mills 2006, Mazerolle and Hobson 2007). At Delta, Manitoba, wintering latitude rather than previous breeding latitude, as well as body size and minimum April temperatures, influence spring arrival dates of males (Mazerolle and Hobson 2007). White-stripe (WS) birds, especially females, arrive a few days earlier than Tan-stripe (TS) birds (Knapton et al. 1984, Caldwell and Mills 2006, Mazerolle and Hobson 2007). The precision of these differences is limited by the difficulty of sexing migrants (see Measurements). Experienced breeders of both sexes arrive earlier than novices (Tuttle 1993).

Spring migration occurs from mid Mar to late May. Early arrivals begin in late Mar (s. Illinois), early to mid-Apr (central Ohio, s. Michigan), mid- to late Apr (Nova Scotia, New Hampshire, New York, Quebec, Pennsylvania, n. Illinois, Indiana, n. Ohio, n. Michigan, Ontario, Minnesota, N. Dakota, Manitoba, Saskatchewan, Alberta), or early to mid-May (n. British Columbia, Newfoundland, Yukon). In s. Ontario, peak of migration is last week of Apr to mid-May. In Manitoba, most birds arrive in mid-May, late May to early Jun in far north (Knapton 2003). Most migrants have left wintering areas by late Apr (Texas), mid-May (N. and S. Carolina, Tennessee, Missouri, Indiana) or late May (Maryland, Ohio, s. Michigan, N. Dakota).

At Long Point, Ontario, fall adults of both sexes and immatures migrate together (J. D. McCracken unpubl. data), but young arrive later than adults on wintering areas in N. Carolina (Piper 1995). Fall migration proceeds more slowly than spring migration and stopovers are longer (Borror 1948, Piper 1995). Fall migration begins in late Aug (Yukon, Sinclair et al. 2003) or Sep with peak in late Sep in central Alberta (Semenchuck 1992), mid-Sep through Oct in Nova Scotia (Tufts 1986), early Oct in s. Ontario. Last migrants leave in late Sep in n. British Columbia (Campbell et al. 2001), mid-late Oct (Manitoba, N.Dakota, Nebraska, Minnesota, Iowa, n. Michigan, New Hampshire), or early to mid-Nov (Ontario, Ohio, n. Indiana, n. Illinois, Pennsylvania). Fall arrivals begin in mid-Sep in n. US (N. Dakota, Minnesota, s. Michigan), in late Sep in the next tier of states (Iowa, Ohio, Indiana, Massachusetts, Maryland), in early to mid-Oct in the main wintering areas (N. and S. Carolina) and in November further south (Mississippi, Texas). Above migration dates are based mainly on Lowther and Falls (1968) where many more may be found.

Migratory Behavior

Few details. Usually migrates at night in loose flocks; utters seep-like vocalizations. Nocturnal activity of captives in spring migratory condition correlates with weather conditions. Activity increases with high or increasing temperature, falling barometric pressure, and south winds. Birds in fall migratory condition show random activity patterns with respect to weather (Muller 1976).

In daylight, spring and fall migrants associate as flocks in dense vegetation where they forage on or just above ground. Individuals maintain contact using seep vocalizations; noisy during early morning hours. In spring and fall, some migrants sing. Songs are degraded in fall flocks, less so in spring.

White-throated Sparrows take off at dusk and migrate at night; several cues are available for orientation. They can use sun compass (Able and Dillon 1977, Bingham and Able 1979), polarized light (Able and Cherry 1986) and, at least under artificial sky, patterns of stars (Gauthreaux 1969). Birds captured during migration, released aloft under clear skies, and tracked by radar orient in the predicted migratory direction. If fitted with frosted lenses prior to release, birds head downwind (Able et al. 1982).

Control And Physiology

Migration of White-throated Sparrows is part of a complex annual cycle involving underlying neuroendocrine rhythms and sensitivity to photoperiod (Meier 1976, Meier and Russo 1985). Lengthening days lead to Prealternate molt, premigratory fattening, gonadal development, and northward migration. In summer, birds become photorefractory (unresponsive to changes in day length), body fat decreases, gonads regress, Prebasic molt occurs followed by fattening and fall migration (Kuenzel and Helms 1974). Migrants that have depleted their body fat require stopovers in which to feed, the more severe the fat loss the longer the stopover (Wolfson 1954a, b). See also Food Habits: Metabolism and Temperature Regulation.

Nocturnal restlessness (Zugunruhe) in White-throated Sparrows caged outdoors occurs at times of migration (Weise 1956, 1963, Helms 1963); it is more precise, intense and continuous in spring than fall (Kuenzel and Helms 1974), and exhibits circadian (daily) rhythm in birds maintained in constant dim light (McMillan et al. 1970). Riker (1978) presents a detailed description of day and night activity of caged birds in relation to seasons and various light regimes, and also explores related brain chemistry. Exposing birds maintained on short days to long days induces Zugunruhe, with movements oriented northward (as in spring migration) in spite of actual season (Miller and Weise 1978). No relationship has been found between migratory restlessness and stored energy reserves in birds nearing the end of spring migration (Smith and Norment 2005).

In fall, White-throated Sparrows remain photorefractory until mid-Dec. Maintenance on long days prolongs this condition, exposure to short days in summer shortens it (Wolfson 1958, Shank 1959). Winter (photosensitive) birds exposed to increased day length in captivity show gonadal development (Harris and Turek 1982) and fat deposition (Wolfson 1953). Manipulation of day length can also induce Prealternate and Prebasic molts out of season (Lesher and Kendeigh 1941), as well as migratory restlessness (above).

Duration of dark period of light:dark (l:d) cycle affects response. Birds maintained on short days with interrupted nights show gonadal development (Jenner and Engels 1952). Timing of interruption appears critical (Meier 1976). Birds maintained on l:d 5:1 (5 h light:1 h dark) exhibit gonadal development and fat deposition after 3–6 wk and sing frequently (Wolfson 1953). Regular daily disturbance by handling is also stimulatory (Meier 1976). Disturbance 6 h after onset of light of l:d 14:10 cycle stimulates gonadal development and fat deposition, disturbance at onset of light causes fat loss, and disturbance 12 h after onset of light has no effect.

Seasonal variation of circadian hormonal rhythms affects migration and metabolic changes leading up to migration; temporal synergism of corticosterone and prolactin are apparently responsible for most effects. A peak of plasma corticosterone just before sunrise followed by a peak of pituitary prolactin 12 h later (naturally occurring in May) results in premigratory fattening and northward orientation; a corticosterone peak at sunset is followed by prolactin peak 4 h later (naturally occurring in Oct) results in premigratory fattening and southward orientation. A 6- to 8-h interval between corticosterone and prolactin peaks (naturally occurring in Aug) produces no migratory effects but stimulates Prebasic molt. Experimental manipulations of hormone levels produce unseasonal behavior and explain different responses of birds in May and Aug when photoperiods are similar (Meier 1976); for further details see Meier and Russo (1985).

Gonadal hormones play a role in migratory physiology. Castration eliminates premigratory fattening and delays and reduces nocturnal restlessness. Late night activity is intensified in castrated birds, an effect seen during gonadal regression in normal birds (Weise 1967). Testosterone treatment of birds in photorefractory condition inhibits gonadal regression and delays onset of photosensitive condition (Turek et al. 1980).

Migratory flights can only be sustained by high rates of fatty acid uptake by flight muscles. Enhanced protein-mediated transport of fatty acids from blood circulation into muscle makes this possible. Recent studies on White-throated Sparrows in winter, spring and fall show greatly increased concentrations of cystolic heart-type fatty acid binding protein (H-FABP), fatty acid translocase (FAT/CD36) and plasma membrane fatty acid binding protein (FABPpm) in pectoralis muscle during migratory seasons. Shown for the first time in birds, the latter two are known mammalian proteins that facilitate transport of fatty acid through the muscle membrane. Increased activity of other metabolic enzymes was also demonstrated (McFarlan et al. 2009).

Feeding experiments show a clear effect of dietary fatty acid profile on performance in White-throated Sparrows. Evidence suggest that diets rich in omega 6 rather than omega 3 fatty acid could lead to better migratory performance (Price and Guglielmo 2009). Klaiman et al. (2009) found seasonal changes in adipose and muscle fatty acid composition in free-living White-throated Sparrows, which were attributed to changes in diet.