Diet and Foraging

Observations of both captive and free-living flocks suggest a direct role for the adrenal hormone corticosterone in promotion of feeding in individuals under nutritional stress (Astheimer et al. 1992 Astheimer, L. B., W. A. Buttemer and J. C. Wingfield. (1992). Interactions of corticosterone with feeding, activity and metabolism in passerine birds. Ornis Scandinavica 23:355-365. Close ).

Main Foods Taken

Seeds of a variety of annual plants, notably composites and grasses, and small seeds of various trees, including an assortment of conifers and deciduous species such as alder (Alnus spp.) and birch (Betula spp.). Also known to eat tender buds of various deciduous trees, as well as the soft stems and leaves of forbs. Actively forages for insects and spiders.

Microhabitat For Foraging

Primarily in the canopy of relatively open conifer or coniferous-deciduous hardwood forests (Todd 1940 Todd, W. E. C. (1940). The Birds of Western Pennsylvania. University of Pittsburgh Press, Pittsburgh, PA, USA. Close ). However, also uses these trees planted in rural or suburban areas, parks, and cemeteries. In trees, begins foraging usually near the top; less commonly at middle levels or in brush. At various times of year will forage on lawns; in meadows, weedy fields, or grasslands; along roadsides; or in scrub and chaparral. Readily uses artificial feeders.

Food Capture And Consumption

Often forages in flocks, even when nesting (Granlund 1994 Granlund, J. (1994). "Pine Siskin Carduelis pinus." In The birds of Michigan., edited by G. A. McPeek and R. J. Adams, 335-337. Bloomington, IN: Indiana Univ. Press. Close ). Feeds primarily during the day, but seen feeding under illumination from floodlight at 02:30 on cold (–30°C) Dec night in Quebec (Prescott 1985a Prescott, D. R. C. (1985a). Feeding at night by wintering Pine Siskins. Journal of Field Ornithology 56:419. Close ), raising possibility of moonlight foraging by energy-challenged individuals.

Feeding habits resemble those of redpolls and crossbills more than those of goldfinches (Bagg and Eliot 1937b Bagg, A. C., and S. A. Eliot (1937). Birds of the Connecticut Valley in Massachusetts. Thomas Todd Co., Boston, MA, USA. Close ). Often feeds on the ground like redpolls, but has an affinity for cone seeds and may forage by hanging upside down on the tips of conifers like crossbills. Slender bill limits siskins in dealing with seeds that have tough shells. May circumvent this limitation at feeders by using fragments of thick-shelled sunflower seeds left by larger carduelines (Davis 1926 Davis, E. R. (1926). Friendly siskins. Bird-Lore 28:381-388. Close , Balph and Balph 1979 Balph, D. F. and M. H. Balph. (1979). Behavioral flexibility of Pine Siskins in mixed species foraging groups. Condor 81:211-212. Close ).

Versatile in feeding mechanisms used. Not only removes, cracks, and ingests seeds from sources ranging from cones to seed heads of various forbs to Australian gum nuts (Eucalyptus spp.), but also gleans small arthropods from leaves, branch tips, and other structures (Rodgers 1937 Rodgers, T. L. (1937). Behavior of the Pine Siskin. Condor 39:143-149. Close , Siebenheller and Siebenheller 1987 Siebenheller, B. and N. Siebenheller. (1987). Possible effects of artificial feeding on nest-site selection by Pine Siskins. Chat 51:57-58. Close ). Also takes insects from the air (Rodgers 1937 Rodgers, T. L. (1937). Behavior of the Pine Siskin. Condor 39:143-149. Close ). Eats buds of both conifers and deciduous trees (Kemsies 1948 Kemsies, E. (1948). Northern Pine Siskin in Hamilton County, Ohio. Auk 65:146. Close , Miller 1955c Miller, A. H. (1955). The avifauna of the Sierra del Carmen of Coahuila, Mexico. Condor 57(3): 154–178. Close ), as well as flowers, soft leaves, and stems of forbs, including garden plants (McCabe and McCabe 1929 McCabe, T. T. and E. B. McCabe. (1929). Economic status of Pine Siskin. Condor 31:126-127. Close , Forbush 1929 Forbush, E. H. (1929). Birds of Massachusetts and Other New England States, Part 3: Land Birds from Sparrows to Thrushes. Massachusetts Department of Agriculture, Boston, MA, USA. Close ). Reputed to eat young needle tips of conifers, but apparently seeking insects (Rodgers 1937 Rodgers, T. L. (1937). Behavior of the Pine Siskin. Condor 39:143-149. Close ).

Well adapted to clinging or hanging from cones, seed heads, or terminal branches. Shows distinctive yellow wing-patches while leaning over alder cones to pick out seeds (Bailey 1928b Bailey, F. M. (1928). Birds of New Mexico. New Mexico Department of Game and Fish, Santa Fe, NM, USA. Close ). Dandelion (Taraxicum spp.) seeds attract birds to feed on lawns. Observed hopping on white seed heads of these plants, bending them to ground where seeds could readily be extracted (Shaub 1951 Shaub, B. M. (1951). A study of the behavior and population of Pine Siskins at Northampton, Mass., February-May, 1947. Bird-Banding 22:71-79. Close ). May use sapsucker borings as a source of sap (Batts 1953 Batts, Jr., H. L. (1953). Siskin and goldfinch feeding at sapsucker tree. Wilson Bulletin 65:198. Close ). Consumption of nectar from Australian gum blossoms also suggested (Rodgers 1937 Rodgers, T. L. (1937). Behavior of the Pine Siskin. Condor 39:143-149. Close ).

Major Food Items

The following based on McAtee (McAtee 1926b McAtee, W. L. (1926b). The relation of birds to woodlots in New York State. Roosevelt Wildl. Bull. 4. Close ) and Palmer (Palmer 1968 Palmer, R. S. (1968). "Spinus pinus (Wilson) Pine Siskin." In Life histories of North American cardinals, grosbeaks, buntings, towhees, finches, sparrows, and allies., edited by Jr O. L. Austin, 424-447. U.S. Natl. Mus. Bull No. 237 (1). Close ). Seeds of a variety of grasses and forbs, particularly those of composites, including dandelions, chickweed (Stellaria media), sunflowers (Helianthus spp.), and ragweed (Ambrosia). Also small seeds of various trees; alder, birch, and assortment of conifers—northern white cedar (Thuja occidentalis), American larch or tamarack (Larix laricina), Canada hemlock (Tsuga canadensis), spruce (Colorado spruce [Picea pungens] and others), various pines (short-leaf pine [Pinus echinata], ponderosa pine [P. ponderosa], and others)—are especially favored.

Also feeds on seeds of sweet gum (Liquidambar straciflua), Australian gum, elm (Ulmus spp.), and maple (Acer spp.). Known to eat buds and catkins of willows (Salix sp.); buds of elms and maples; berries of honeysuckle (Lonicera spp.) and eastern redcedar (Juniperus virginiana); and soft stems and leaves of various forbs, occasionally including young garden vegetables (Forbush 1929 Forbush, E. H. (1929). Birds of Massachusetts and Other New England States, Part 3: Land Birds from Sparrows to Thrushes. Massachusetts Department of Agriculture, Boston, MA, USA. Close , McCabe and McCabe 1929 McCabe, T. T. and E. B. McCabe. (1929). Economic status of Pine Siskin. Condor 31:126-127. Close ). Fed on cones of Australian pine (Casuarina equisetifolia) in Florida Keys.

Actively forages for insects and spiders, which are a minor but significant portion of the diet. Animals consumed include weevils; aphids; scale insects; chalcid larvae; sawfly larvae (Callirhythis bicornis); as well as larvae, pupae, or, occasionally, fresh egg masses of spruce budworm (Choristoneura fumiferana); larvae of western spruce budworm (C. occidentalis); and larvae of winter moth (Operophtera brumata; Torgersen and Campbell 1982 Torgersen, T. R. and R. W. Campbell. (1982). Some effects of avian predators on the western spruce budworm in north central Washington. Environmental Entomology 11:429-431. Close , Jennings and Crawford 1983 Jennings, D. T. and H. S. Crawford. (1983). Pine Siskin preys on egg masses of the spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae). Canadian Entomology 115:439-440. Close , Roland et al. 1986 Roland, J., S. J. Hannon and M. A. Smith. (1986). Foraging pattern of Pine Siskins and its influence on winter moth survival in an apple orchard. Oecologia 69:47-52. Close ). Avidly consumes items provided at feeders, especially Niger thistle (Guizotia abyssinica) and sunflower seeds. Also eats suet occasionally (Palmer 1968 Palmer, R. S. (1968). "Spinus pinus (Wilson) Pine Siskin." In Life histories of North American cardinals, grosbeaks, buntings, towhees, finches, sparrows, and allies., edited by Jr O. L. Austin, 424-447. U.S. Natl. Mus. Bull No. 237 (1). Close ).

Quantitative Analysis

Anecdotal nature of information on food preferences prevents detailed quantitative analysis of diet, a task that will undoubtedly be complicated by nomadic tendencies of Pine Siskins. McAtee (McAtee 1926b McAtee, W. L. (1926b). The relation of birds to woodlots in New York State. Roosevelt Wildl. Bull. 4. Close ) concluded that one-sixth of the total food of siskins is of animal (i.e., arthropod) origin.

Primarily a visual feeder; use of food items limited by bill morphology, as in other cardueline finches (Newton 1972 Newton, I. (1972). Finches. Taplinger, Engelwood-Cliffs, NJ, USA. Close ). Roles of taste and olfaction in food selection unknown. Not known to cache food, relying instead during winter on daily regimen of fat accumulation and use (see Nutrition and Energetics, below). Foraging effectiveness, maximization of energy reserves, and transport of nutrients to nestlings are enhanced by ability to store seeds in anterior portion of digestive tract (presumably in a rudimentary crop, i.e., a distensible esophagus, similar to that described for American Goldfinch [Spinus tristis]; Middleton 1993 Middleton, A. L. A. 1993. American Goldfinch (Carduelis tristis). In The Birds of North America, No. 80 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington D.C.: The American Ornithologists' Union. Close ). “Crop” contents of as much as 1.1–1.5 g found in siskins in Seattle, WA, area. Net energy increment of latter amount of sunflower seeds ingested at end of day might support heat production of a postabsorptive siskin resting at –20°C for approximately 5.5 h of a winter night's fast (WRD).

Because of absence of quantitative dietary data for any population, nutritional statements concerning this species are not feasible. Winter fattening is pronounced; siskins at Ann Arbor, MI, in Dec and Jan were about 1.5 times fatter than Common Redpolls and American Goldfinches obtained at the same site, despite the similarity of these species in lean body mass (Dawson and Marsh 1986 Dawson, W. R. and R. L. Marsh. (1986). Winter fattening in the American Goldfinch and the possible role of temperature in its regulation. Physiological Zoology 59:357-368. Close ).

Pine Siskin shares the cardueline proclivity for salt consumption (Bennetts and Hutto 1985 Bennetts, R. E. and R. L. Hutto. (1985). Attraction of social fringillids to mineral salts: an experimental study. Journal of Field Ornithology 56:187-189. Close ), suggesting some mineral requirement is not satisfied by its food. May ingest ashes, blue clay, road salt, powdered andesite rock, newly set Portland cement, etc. (McCabe and McCabe 1928c McCabe, T. T. and E. B. McCabe. (1928c). The plumage of the Pine Siskin. Condor 30:221-227. Close , Bennetts and Hutto 1985 Bennetts, R. E. and R. L. Hutto. (1985). Attraction of social fringillids to mineral salts: an experimental study. Journal of Field Ornithology 56:187-189. Close , Messineo 1986 Messineo, D. (1986). The 1985 nesting of Pine Siskin, Red Crossbill and White-winged Crossbill in Chenango County, NY. Kingbird 35:233-236. Close ). McCabe and McCabe's (McCabe and McCabe 1928c McCabe, T. T. and E. B. McCabe. (1928c). The plumage of the Pine Siskin. Condor 30:221-227. Close ) suggestion that mineral ingestion eliminates deficiencies developed over the preceding winter is inconsistent with avid consumption by immature birds that have not yet been through such a season.

Nocturnal basal metabolic rate (BMR) of Pine Siskin (mean body mass 13.8 g ± 1.5 SD, n = 17) in winter is 4.52 ml O₂/(g·h), 40% above predicted value for 13.8 g passerine (Dawson and Carey 1976 Dawson, W. R., and C. Carey (1976). Seasonal acclimatization to temperature in cardueline finches I. Insulative and metabolic adjustments. Journal of Comparative Physiology 112:317–333. Close ). Zone of thermal neutrality in winter is approximately 15° to ≥35°C. Regression equation for relation of oxygen consumption (VO₂ in ml/g·h), an indicator of heat production, to ambient temperature (Ta °C) below thermal neutrality is: VO₂ = 7.46 – 0.19Ta (n = 37, r = 0.96).

Can sustain metabolic rates nearly 5 times the BMR under cold challenges as severe as –60° to –70°C. Approximately half the birds tested in Feb–early Mar maintained body temperatures >34°C over at least 3 h at these Ta, indicating robust powers of cold resistance for so small a bird (Dawson and Carey 1976 Dawson, W. R., and C. Carey (1976). Seasonal acclimatization to temperature in cardueline finches I. Insulative and metabolic adjustments. Journal of Comparative Physiology 112:317–333. Close ). However, not immune to adverse weather. Populations in fir (Abies concolor and A. magnifica) forests of Sierra Nevada, CA, were relatively low during summer of 1983, following record snowfall of the 1982–1983 El Niño winter (Hejl et al. 1988 Hejl, S. J., J. Verner and R. P. Balda. (1988). Weather and bird populations in true fir forests of the Sierra Nevada, California. Condor 90:561-574. Close ).

Drinking movements not well described, but probably similar to those of the American Goldfinch, which obtains mouthful of water, closes its bill, raises its head, and swallows (Middleton 1993 Middleton, A. L. A. 1993. American Goldfinch (Carduelis tristis). In The Birds of North America, No. 80 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington D.C.: The American Ornithologists' Union. Close ). Dales and Bennett (Dales and Bennett 1929 Dales, M. and W. W. Bennett. (1929). Nesting of the Pine Siskin in Iowa with remarks on regurgitative feeding. Wilson Bulletin 41:74-77. Close ) alluded to the raising of the head by drinking Pine Siskins in their description of the feeding of a female on her nest by her mate. Also observed foraging for insects (Rodgers 1937 Rodgers, T. L. (1937). Behavior of the Pine Siskin. Condor 39:143-149. Close ), which could provide water. Nesting pair in Iowa drank from a basin of water provided them, which they also used for bathing (Hayward and Stephens 1914 Hayward, W. J. and T. C. Stephens. (1914). The Pine Siskin breeding in Iowa. Wilson Bulletin 26:140-146. Close ). Not known to cast pellets. Available data on defecation concern nestlings (see Breeding: parental care, below).