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Main Foods Taken
Winter: seeds, adult insects, spiders, decapods, amphipods, mollusks. Breeding season: adult and larval insects, spiders and spider egg cases, amphipods.
Microhabitat For Foraging
Feeds mainly in open stands of grass, shallow pools, and pannes. At Gulf Hammock, FL, fed most frequently in smooth cordgrass (Post et al. 1983). In contrast, the dominant cover -- needle-rush – was infrequently used, probably because of low prey availability. Saltgrass was little used for feeding, perhaps because birds have difficulty traversing the dense matrix of stems. In contrast, glasswort, which had about the same prey density as saltgrass but is more open near ground, was a frequent feeding site. Probably due to greater prey availability, much foraging was near tidal creeks, either on edges or in bordering cordgrass (Post et al. 1983).
In NY State, birds (n = 693 observations) foraged in shallow pools 26% of the time, compared to this microhabitat’s 5% coverage of study area. The use/coverage ratio of pannes was 26/20; new-growth S. alterniflora: 24/57; mixed grasses (medium-height alterniflora and salt meadow grasses: 21/6; Phragmites: 2/8; wrack: 1/2; salt meadow grasses: 1/3 (Post and Greenlaw 2006).
Food Capture And Consumption
Long, slender bill is adapted to probing mud; strong, large foot adapted for clinging to upright stems of grass and for running over mud (Audubon 1831, 1838, Trotter 1891, Stone 1937). Most common foraging mode is walking on ground, gleaning arthropods from surrounding vegetation, usually by attacking prey that can be reached by extending neck or by lunging short distances. Individuals walking on ground also probe and peck mud or whatever surface they are walking on. May chase elusive items. Rate of movement on ground about 2.8 m/min; average duration of stop 4.2 s (Cody 1968).
Less frequently, birds forage above ground, slowly hopping or climbing through grass, gleaning as they move. Birds perched above ground also glean surrounding vegetation or snap at items in air; rarely hover or flycatch. In New York, gleaning vegetation accounted for 35% of 282 foraging movements, whereas gleaning or probing mud was used 38% of time, gleaning wrack 11%, gleaning or probing water 10%, hovering or flycatching 4%, and chasing or lunging 2% (Post et al. 1983). Double-scratch employed in wrack (Enders 1970). In fall often forage in loose groups that congregate in patches of tall cordgrass. Birds perch on seed heads and remove individual seeds directly from plant. Seeds also gleaned from ground. Spiders and seed-eating bugs (Lygaeidae) are captured in seed heads.
Major Food Items
Stomach contents (Office of Migratory Bird Management, U.S. Fish and Wildlife Service) show relatively few arthropod groups used. For birds taken in May-October, most important groups were fly larvae, adult and larval moths (primarily noctuids), spiders (wolf spiders), bugs (leafhoppers), and Orthoptera (short-horned grasshoppers and crickets). Compared to Saltmarsh Sparrows collected at same time in NY State, few amphipods were taken; otherwise, diets of two species similar (Post and Greenlaw 2006).
Six stomachs collected in late summer–early fall in New Jersey had large amounts of crabs (Uca, 36% of volume) and snails (Melampus, 24%). Proportionately more seeds taken in fall and winter than in summer. Most important seeds from 13 stomachs were S. alterniflora (25–40% of total sample), saltbush (Atriplex 5–10%), smartweed (Polygonum 2–5%); bristlegrass (Setaria 2–5%) (Martin et al. 1951).
At Grande Isle, LA, diet (n = 16; 6-9 Jun) mainly Orthoptera (33% of volume), moth larvae (26%), and spiders (18 %). Although items not identified to family, most were probably from standing vegetation rather than from ground.
At Cape Sable, FL, most important food items (n = 15, Mar–Apr): beetles (35.5% of volume), spiders (14.5%), Heteroptera (bugs; 9.9%) and Hymenoptera (8.3%). Numerically most important groups were ground-inhabiting taxa such as ground beetles (Carabidae), variegated mud-loving beetles (Heteroceridae), and checkered beetles (Cleridae). Several stomachs contained amphipods and mollusk fragments. Sweep-net samples of vegetation in mirabilis nesting habitat suggest that most prey items found in stomachs probably taken from ground rather than vegetation (Werner 1975).
Food Selection And Storage
In NY State, prey fed nestlings reflected random choice of most available items, regardless of species or size; changes in prey abundance were tracked over the nesting season. No statistical differences between nestling diet and available food, or between the diets of Seaside and Saltmarsh sparrows in the same marsh. Adults foraging for nestlings showed a significant preference for adult Lepidoptera (Noctuidae), sub-adult (pupae and larvae) tabanid flies (Tabanidae), sub-adult soldier flies (Stratiomyidae), and isopods. These 5 groups composed > 80% of dietary volume. In general, adults selected larger but less abundant prey such as flies and moths, and avoided smaller items such as plant bugs (Miridae) (Post and Greenlaw 2006).
Nutrition And Energetics
Captive birds maintained on mixture of seeds (canary seeds, thistle, red and white millet). Propagate in captivity while maintained on commercial soft-billed bird mixtures, supplemented by crickets (Post and Antonio 1981).
Males on large activity spaces (> 8000 m2) on ditched salt marshes in NY State (Long Island) may expend more energy patrolling than do those defending smaller (< 2000 m2) territories on unaltered marshes. Males in ditched marshes moved (in flight and on ground) 7.7 m/min, as compared with 3.5 m/min for males in unditched marshes (Post 1974).
Neither sex appears to lose body mass as result of energy expenditure related to parental care. Weights during breeding season (15 May–15 Aug) and after breeding season (16 Aug–15 Oct) did not differ at Oak Beach, NY (Post and Greenlaw 1982).
Metabolism And Temperature Regulation
Handheld, just captured A. m. nigrescens had mean cloacal temperature of 41.8°C (n = 6, range 39.8–43.0). Panting occurred when body temperature was 41.5°C (C. H. Trost, pers. comm.).
Drinking, Pellet-Casting, And Defecation
From Poulson (1969). Can drink water with about the same salinity as seawater, but in preference tests birds discriminated against salt solutions in favor of distilled water. After acclimation, captives maintained weight while provided ad libitum 0.4 M NaCl. During acute dehydration, 3 birds lost 2.9% of body mass/d. Acclimated birds maintained on 0.4 M NaCl regularly able to produce cloacal urine with concentrations of 450 mEq Cl-/l, a concentration 3.5 times that of their blood plasma and slightly higher than maximum of brackish water in area where birds captured. Adults defecate on ground, apparently randomly; males also defecate from exposed perches during pauses in singing; defecation interspersed with tail-wagging.