Food Habits

Feeding

Main Foods Taken

Winter: primarily small seeds and fruits, insects when available. Summer: primarily insects, supplemented with greens and fruit. Young are fed insects (see Breeding: Parental Care). Late summer and fall diet reverts mainly to fruits and seeds.

Microhabitat For Foraging

During migration and winter, forages primarily on the ground. Frequents small openings in dense thickets where leaf litter covers ground or edges of clearings and hedgerows near dense cover. In experiments with uniformly distributed food, birds depleted sources nearest cover before moving to more open sources; tendency similar when food is in patches. Results suggest that White-throated Sparrows reduce risk of predation at expense of foraging efficiency (Schneider 1984).

In summer individuals forage on vegetation as well as on ground, frequenting small bushes or lower portions of coniferous trees, foraging along upper surfaces of horizontal branches. Males often feed between singing bouts, hence some foraging occurs near treetops.

Food Capture And Consumption

Forages in loose flocks during migration and winter, singly or in pairs during summer. Typically clears away leaf litter with 1–4 rapid kicking movements of both legs. For analysis of this double-footed kick see Hailman (1973, 1974, 1984). As strong scratchers able to find deeply buried seeds, White-throated Sparrows are adapted to forage near woody vegetation where the litter is deep. This ability may influence habitat selection and winter survival (Whalen and Watts 2000). Also flips leaves by using a side-to-side or forward motion of head. May “pounce” on active insects. Pulpy parts of fruits are eaten after manipulation of skins and large seeds. In vegetation, systematically gleans along stems and through leaves or needles, especially near branch nodes or tips. May fly from an exposed branch to capture aerial insects or insects on ground or lower vegetation. Food consumed as found (unless fed to young). Wings, head, and thorax usually removed from large winged insects.

In winter flocks, dominance and individual spacing influence access to food (see Behavior: spacing [Territoriality and Dominance]). Dominant birds supplant subordinates from food patches close to cover, the latter taking nearest patch not occupied by a bird of higher rank (Schneider 1984). High- and low-ranking birds equally likely to find novel sources, but once found, food is taken over by high-ranking birds. Low-ranking individuals that find a new source feed frenetically or visit the source discontinuously. These tactics may increase birds’ food intake before they are supplanted (Wiley 1991). Search, detection, and harvest rates of foraging White-throats analyzed by Getty and Pulliam (1993). Tuttle et al. (1990) investigated food choices in relation to foraging theory in controlled laboratory experiments.

Diet

Major Food Items

Winter diet mostly seeds. Wild food includes grass and weed seeds such as ragweeds (Ambrosia spp.) and smartweeds (Polygonum spp.), also fruits of sumac (Rhus spp.), grape (Vitis spp.), highbush cranberry (Vibernum trilobum), mountain ash (Sorbus spp.) and rose hips (Rosa multiflora). In Texas, this sparrow prefers seeds of the genera Ambrosia, Aristida, Parietaria and Viola (Worthington et al. 2004). Feeds opportunistically on insects. Common visitor to feeding stations where favorite food is millet (Panicum miliaceum).

In early spring, feeds on oak buds (Quercus spp.); buds and blossoms of apple (Malus spp.), maple (Acer spp.), and beech (Fagus grandifolia); and young seeds of elms (Ulmus spp.) and maples (Lowther and Falls 1968). In early May in Algonquin Provincial Park, Ontario, eats young birch (Betula spp.) and hazel (Corylus cornuta) catkins (JGK). Captive birds eat young grass, clover (Trifolium repens), and an assortment of unidentified sprouts in spring (JGK). As insects become more available, they form larger proportion of diet. Stomachs of males captured in early May (Algonquin Park) contained approximately 60% vegetable material; in late May they contained about 90% insects (JGK). Principal insects include adult Odonata, Hymenoptera, and Heteroptera and adult and larval Coleoptera, Diptera, and Lepidoptera. Also consumes a variety of spiders, millipedes, centipedes, and snails. Young are fed arthropods. Tuttle (1993) sampled territories of WS and TS males (see Introduction) finding no difference in type or availability of insect food.

In late summer and into fall, fruits become an important food source. In addition to those mentioned under winter diet, blueberries, raspberries (Rubus spp.), bunchberries (Cornus canadensis), and dogwood (Cornus amomum and C. racemosa) fruits are eaten. Shows no preference among dogwood fruit related to fat content; prefers undamaged fruit but, if unavailable, will eat decaying fruit (Borowicz 1988a, b). A migrant in a city park surrounded by office towers was observed feeding on the carcass of a conspecific (Lang 1994). Restoration of weight loss on different diets suggests that the adequacy of food available at stopover sites can influence recovery of body reserves in migrants (Pierce and McWilliams 2004). A stable C isotope in breath and plasma revealed short term changes in diet of migrants (Podlesak et al. 2005).

Quantitative Analysis

Food for year consists of 19% animal matter, rest vegetable matter (for details and seasonal variation, see Judd 1901, Lowther and Falls 1968). For food brought to young, see Breeding: Parental Care. See also above (JGK).

Food Selection And Storage

No storage. For selection, see above and Willson (1971).

Nutrition And Energetics

No quantitative studies. Captive birds can be maintained on a diet composed mainly of millet but must also receive greens, insects, a calcium source, and fresh water (JGK). State of nutrition is revealed by feather growth (ptilochronology) and points to likely influence of social status (Jenkins et al. 2001).

Metabolism And Temperature Regulation

Metabolic rate (M) varies inversely with ambient temperature (t) in exercised (M = 28.75–0.48t) and non-exercised birds (M = 23.05–0.38t). Lower limit of temperature tolerance is -29°C with a maximum metabolic rate of 34 kcal/bird/d; with exercise this limit rises to -11°C with same metabolic rate. Free ranging birds in winter show reduction (3.4±1.0°C) in skin temperature at night that is negatively correlated with ambient temperature (Dolby et al. 2004). Upper limit of temperature tolerance is 40°C for nonexercised birds, 36°C for exercised birds; death at high temperatures is caused by inability to increase evaporative cooling. Water, fat, and protein are significantly reduced in birds that die at extreme temperatures (Kontogiannis 1968).

Body weight and fat levels show daily rhythm. Rate of weight increase greatest immediately after onset of light; maximum weight achieved by late afternoon. Nightly weight loss greatest in first hours after dark due to elimination of gut contents; further weight loss due to fat utilization and evaporative water loss (Kontogiannis 1967, Graedel and Loveland 1995). Body lipids peak in late afternoon, liver lipids peak after dark. Daily weight and fat increase varies seasonally (Meier 1977). Daily weight change varies with activity and ambient temperature, lower in exercised birds, and lowest at 22°C (Kontogiannis 1968).

Seasonal variation in body lipids is related to events of the annual cycle, highest before migration and in midwinter, lowest at times of molt. Body mass follows a similar pattern decreasing from a midwinter peak in the coldest part of the winter through the Prealternate molt, rising rapidly to an annual maximum before spring migration, then declining through migration and breeding to reach an annual low with the Prebasic molt in summer and finally increasing to moderate levels in fall reaching a peak in midwinter. This pattern is similar in males and females and does not differ between WS and TS birds (Odum 1949, Weise 1963, Kuenzel and Helms1974, Prescott 1986, Graedel and Loveland 1995). Midwinter lipid reserves are greatest in coldest months, apparently limited in amount, allowing survival for up to 30h in Jan. Many intrinsic and environmental variables influence lipid reserves, especially average temperature in previous 20 d. Response to weather fine-tunes energy reserves in winter (Blem and Shelor 1986).

Hyperphagia preceding fat deposition in spring is controlled by the hypothalamus; ventromedial region inhibitory, lateral regions stimulatory (Kuenzel 1972). Amount of fat and degree of change varies among body tissues, 35–40% subcutaneous. Fat reserves are labile; premigratory fat deposition in males is completed in about 10 d and consumed quickly during and after migration. Deposition is later and not as great in females corresponding to a later onset of migration (Odum and Perkinson 1951). Certain essential fatty acids are retained for reproduction while non-essential fatty acids are released to support migratory flight Mostafa et al. (1994).

See also Migration: Control and Physiology, above.

Drinking, Pellet-Casting, And Defecation

Drinking not observed among wild birds whose territories often lack standing water. May obtain adequate water from food, dew, or rain. Captives drink frequently and cannot be maintained without free water (JGK). Defecate several times per hour during normal activities. No evidence of pellet casting.