Food Habits

American Goldfinch, feeding, WA, October

Figure 4. American Goldfinches often feed on thistle seeds.

Feeding

Main Foods Taken

Seeds of many annual plants, Composites preferred, and small seeds of various trees, e.g., alder (Alnus spp.), birch (Betula spp.), cedar (Thuja occidentalis), and elm (Ulmus spp.). Prefer black thistle among feeder seeds, but will take sunflower (shelled or as hearts; KJM). Insects only as encountered.

Microhabitat For Foraging

Prefers taking seeds while perched on plant (or feeder) to foraging on the ground (Newton 1972). Approaches food plants with hovering flight before landing and sidling towards tip of stem or branch. Will hang upside down from branches or thistle head to obtain difficult-to-reach seeds (Coutlee 1964, Newton 1972: 110), though they prefer to eat and take more seeds when eating upright (Horn 1995).

Food Capture And Consumption.

Daytime feeder. Feet used in feeding to hold swaying food plants and in manipulation of seeds. Seeds held lengthwise in bill (long axis parallel to cutting edges of mandibles), manipulated by tongue, cracked open by pressure from two mandibles, and rapidly hulled (Coutlee 1964). Husk is dropped and seed swallowed. Feeding frequently followed by bill wiping (Coutlee 1964).

Gregarious at all times and usually feed in company of others. In winter, flock feeding normal with flock sizes > 200 individuals common (Tyler 1968, ALAM). Flocks often include other species, e.g., Black-capped Chickadee (Parus atricapillus), Common Redpoll (Carduelis flammea ), and American Tree Sparrow (Spizella arborea ; Tyler 1968). Feed efficiently on comparatively large seeds and can delay satiation owing to storage capacity of gut (Middleton 1982, Benkman and Pulliam 1988). Scan less and feed more when in larger groups (Popp 1986). Winter flocks range widely because well adapted to use patchily distributed seed resources (Benkman and Pulliam 1988). Flocks frequently feed in rolling, “leap-frog” fashion, typical of cardueline finches, a habit that exploits food efficiently and offers protection from predation (Middleton 1970a, Newton 1972).

Feeds more frequently in sheltered stands or at feeders in winter, when seeds of annual plants are scarce or unavailable. At feeders, comparatively docile and easily supplanted by other species, including two carduelines, the Pine Siskin (Carduelis pinus; ALAM) and House Finch (Carpodacus mexicanus; Hill 1993). House Finch thought to be adversely affecting goldfinch populations in e. North America (Hill 1993, W. Dawson pers. comm.), but data equivocal (Hill 1993, ALAM). Species’s agility enables feeding from small cones, catkins, and feeders of various design (Dunn and Hussell 1991).

Remains gregarious in spring and breeding season, feeding in pairs or small groups. Attracted at all times to stands of flowering composites, particularly thistles (Fig. 4), from earliest to latest flowering. Like other carduelines (Newton 1967, 1972), will tear open unripe heads to extract seed in “milk.” Often results in apparently misshapen bill due to accumulation of plant latex, e.g., dandelion (Taraxacum officinale; ALAM). Also eats buds of fruit trees, birch, and Siberian elm (Ulmus pumila), and strips tender bark from twigs (ALAM).

Captive, second-year males observed allofeeding during winter (Popp 1987b), as a function of dominance status (as occurs in other carduelines; Senar 1984, McGraw and Hill 1999).

Diet

Exhaustive list of foods unavailable and probably unattainable because of wide distribution and choice of seeds. Highly granivorous, with strong preference for seeds of Compositae. Consumes a wide variety of seeds from grasses (Gramineae) and trees such as alder, birch, cedar, and elm (Dorsey 1963, Coutlee 1964, Tyler 1968). Of over 50 separate items collected from stomach contents (Guelph, ON; samples accidentally destroyed before specific identifications made), most were “weed” seeds; 3 were insects, predominantly aphids (Aphididae; ALAM). In spring, consumes buds and strips bark from terminal shoots, presumably in search of specific nutrients (ALAM). Known to consume other items as varied as green algae (Spirogyra porticalis; Dorsey 1963, Kilham 1980) and maple sap (Muzny 1982). Studies of closely related, and ecologically similar, Green-backed (Carduelis psaltria), Lawrence’s, and European (C. carduelis) goldfinches provide clues as to nature and variety of foods consumed by American Goldfinch (Linsdale 1957, Newton 1967, 1972, Middleton 1970a).

Food Selection And Storage

Among cardueline finches, food selection limited by bill morphology (Newton 1967, 1972). Food selection by American Goldfinch thus basically restricted by bill size. Selection also varies with season (Dorsey 1963), metabolic demands (e.g., breeding, molt, migration), and geographic location. Precise factors that influence selection unknown but probably include recognition of specific foods (Coutlee 1964, Popp 1988).

Not known to cache food or defend sources. Distensible oesophagus (rudimentary crop) enables goldfinches to accumulate undigested food in gut (Miller 1978). Used to transport food to nest, or provide extra supply to meet metabolic demands under adverse conditions or roosting (Middleton 1982).

Nutrition And Energetics

Gross energy content of many seeds and insects known, but less information on metabolizable energy content (e.g., Robel et al. 1979). Metabolic efficiency of consumer varies with food type (Browning and Robel 1981). Therefore, in absence of complete food list, and because of variation in food sources, both seasonally and regionally, meaningful nutritional studies for American Goldfinch difficult to obtain.

Basal metabolic rates of 4.24 ± 0.61 (SD) ml O2 (g/h) and 4.65 ± 0.34 (SD) ml O2 (g/h) calculated for summer and winter conditioned birds, respectively (Dawson and Carey 1976). Metabolic rate increased by daylight activity (Bakken and Lee 1992). Body temperature reduced and thermoregulatory metabolism increased under adverse conditions (Lustick et al. 1982, Bakken and Lee 1992). These metabolic capabilities have significance, particularly for overwinter survival (Dawson and Carey 1976, Carey et al. 1978, Lustick et al. 1982, Buttemer 1985), and must be closely linked to energetic content of foods and prevailing weather conditions. Winter fattening pronounced in many populations (Wiseman 1975 , Carey et al. 1978, Middleton 1986). See also Migration: control and physiology.

Metabolism And Temperature Regulation

One of best-studied passerines for seasonal acclimatization of metabolism (Liknes et al. 2002). Shows improved thermogenic (shivering) endurance in the cold due to skeletal muscle catabolism of fatty acids, which perhaps is under the control of thyroid hormone (Dawson et al. 1992). Fattening and thickening of the Basic plumage also play a role (see Migration).

Drinking, Pellet-Casting, And Defecation

Drinks by obtaining mouthful of water, closing bill, raising head and swallowing (Coutlee 1964). Drinks from various sources, e.g. slow-flowing streams, shallow pools, bird baths, and dew. In winter, where freezing conditions prevail, observed to drink melt water on roofs and to eat snow (ALAM). Captive molting birds consumed ca. 2 ml of water daily when fed a sunflower-heart diet (McGraw et al. 2004).

When orange carotenoid pigments (canthaxanthin) were provisioned in very high quantities in the water source, feces were brightly colored orange by pigments that apparently could not be intestinally absorbed (McGraw et al. 2001).