Migration

Figure 1. Distribution of the American Goldfinch.

Figure 6. Annual cycle of the American Goldfinch.

Nature Of Migration In The Species

Northern populations (in Canada and the upper midwestern US) breeding only, southernmost populations (from North Carolina south and west, through Texas, Mexico, California and w. Oregon) wintering only, with year-round populations in between (Wiseman 1975, Middleton 1978; Fig. 1). Females winter further south than males, young males winter further north than adults (Prescott and Middleton 1990). Some cover long distances; e.g., first winter male banded in March at Guelph, ON, recovered dead 8 mo later, 1,626 km south southwest, at Olla, LA (ALAM, Speirs 1985); most journeys shorter (Prescott and Middleton 1990). Presumed daytime migrant. A commonly banded species, with over 3 million records. Pre-1974 data of limited value because most records lack reliable sex and age information (Prescott and Middleton 1990).

Timing And Routes Of Movement

Migratory activity peaks mid-Apr to early Jun and late Oct to mid-Dec (see Fig. 6; Middleton 1978). Extent, direction, and timing may vary with locality (Tyler 1968, Wiseman 1975, Dawson et al. 1983, ALAM). Spring migration occurs simultaneously with Prealternate molt, but fall migration follows Prebasic molt (Middleton 1978). Regional movement common in wintering populations; e.g. female banded Jan, Guelph, ON, retrapped Apr same year, Cambridge, ON, 24 km southwest; male banded Dec, Guelph, retrapped Toronto, ON, late next Feb, 50 km east (ALAM). Much daytime movement between feeding stations in resident populations; direct distance movements of 7 km between feeders not uncommon (ALAM).

Analysis of banding data (ALAM, D. Bystrak pers. comm.) show birds from maritime Canada and New England states follow coast line; those from central Canada and states east of the Rockies move towards Mississippi drainage and thence southward, west of Appalachian Mountains. In far West, north-south movement follows major valley systems and the coastline.

Migratory Behavior

Limited recovery data suggest birds do not migrate until their second winter (Prescott et al. 1989, Prescott and Middleton 1990, ALAM). Extent and direction of migration by individuals may vary annually (Dawson et al. 1983, ALAM). Birds move in flocks.

Control And Physiology Of Migration

Not studied directly. Goldfinches breeding at high latitudes either migrate or overwinter (Middleton 1978, Dawson et al. 1983, Prescott and Middleton 1990). Energetic consequences of both strategies similar and depend on substantial aerobic endurance and extensive power outputs by flight muscles or on shivering, fueled by catabolism of fat (Dawson et al. 1983). Winter and summer acclimatized goldfinches differ in physiological capabilities, which suggests close links with migratory preparedness. Although basal metabolism in summer and winter is similar, when exposed to severe cold (-70°C), summer goldfinches remain homeothermic for about 1 h, winter birds for 6–8 h (Dawson and Carey 1976).

Seasonal weight changes mainly result from deposition or depletion of lipids (Dawson and Marsh 1986). Glycogen stores in pectoral muscles are significantly higher in winter (Carey et al. 1978) as are basal metabolic rate and cold tolerance (Liknes et al. 2002). Basic (winter) plumage heavier than Alternate (summer) plumage (Dawson and Carey 1976, Middleton 1986), and feathers from same tracts have different structure in 2 plumages (Middleton 1986). Physiological linkages and timing of above processes may be related primarily to winter adaptation (Dawson and Carey 1976, Middleton 1986, see Feeding), but are also important for migratory preparedness. Temperature hypothesized as an ultimate factor stimulating seasonal fattening (Dawson and Marsh 1986) but proximate factor remains unknown; photoperiod implicated (Middleton 1978, Gluck 1982b).

Northern limit of winter range approximates the minimum Jan temperature isocline for 0°F (Root 1988). North of this line, climate is characterized by long spells of sub-freezing temperatures and snow accumulation. The latter limits access to food and could impose a natural limit to goldfinch overwintering.