Saltmarsh Sparrow

Ammospiza caudacuta

Order:
Passeriformes
Family:
Passerellidae
Sections

Diet and Foraging

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Figure 6. Saltmarsh Sparrow with Fundulus sp.

Saltmarsh Sparrow with likely Marsh Killifish (Fundulidae: cf. Fundulus confluentus), Volusia County, Florida, 26 January 2012. Image by Ken Schmidt.

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Figure 7. Saltmarsh Sparrow with Sailfin Molly (Poecilia latipinna).

Saltmarsh Sparrow with Sailfin Molly (Poeciliidae: Poecilia latipinna), Volusia County, Florida, 26 January 2012. Image by Ken Schmidt.

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Saltmarsh Sparrow with Seaside Dragonlet (Erythrodiplax berenice).
© Doug Hitchcox , Maine , United States , 30 June 2014

Feeding

Main Foods Taken

A solitary forager; trophic guild is ground and near-ground (herbaceous) forager, chiefly arthropodivore and granivore depending on season, rarely carnivore. Adults appear to feed in the same microsites and use similar prey capture tactics when foraging for themselves or for dependent young (JSG).

During the breeding period, forages exclusively or almost exclusively on animal matter, featuring amphipods, larval flies, marsh grasshoppers, lycosid spiders, adult and larval moths, but including a larger range of taxa including beetles, homopterans, hemipterans (heteropterans), and small snails; occasionally captures and eats larger prey such as a 5 cm “sandworm” (Annelida: Nereis sp.) extracted from mud (140), or small fish in intertidal pools (see Figure 6 and Figure 7). During postbreeding and autumn periods, opportunistically and heavily dependent on cordgrass seeds from seedheads and on ground at low tide. During the overwintering period, generally consumes animal matter mixed with cordgrass seeds.

Microhabitat for Foraging

During the breeding period, forages on the ground in dense and open stands of grasses (e.g., Spartina alterniflora (Smooth Cordgrass), S. patens (Saltmeadow Cordgrass), Distichlis spicata (Saltgrass), ditch margins, edges of shallow pools and mud, pannes, patches of wrack, and outer intertidal at low tide (JSG). Breeding females often forage close to nest (10), but may fly long distances to feed dependent young (see Breeding: Parental Care). Out of 7 foraging microsites defined in a New York study, 91% of foraging observations were in open mud and edges of muddy pools (35%), new-growth Spartina (23%), residual Spartina (19%), and wrack patches (14%) (141). Among tidal marsh-inhabiting passerines, foraging in pure, dense saltmeadow (supratidal) patches is associated uniquely with Saltmarsh Sparrows and Acadian Nelson’s Sparrows (latter forage in saltmeadow vegetation in New Brunswick; JSG). In this microhabitat and in other patches, they frequently take food from dead vegetation (141). The species in some cases favored patches out of proportion to relative coverage in a sample of quadrats. The chief differences between usage and availability of foraging sites were in the amount of time spent in tall new-growth cordgrass (24% use versus 57% coverage, and in ecotonal mixtures of saltmeadow and medium-height S. alterniflora (33% use versus 6% coverage). Shallow pools covered only 5% of the quadrats, but were used 19% of the time (141). Woolfenden (7) emphasized tendency to forage in densest stands of grass in New Jersey, in contrast to Seaside Sparrow. During postbreeding period before migration in fall, sparrows appear to spend most of their time in ditches and channels and are much less likely to be flushed from the marsh plain than during summer (CSE).

Limited information for the nonbreeding period. If water levels permit, transient and wintering Saltmarsh Sparrows in South Carolina feed in tall, regularly-flooded S. alterniflora stands during both high and low tides. They feed on seeds in seedheads, and on fallen seeds. They also take insects (Hemiptera: Lygaeidae?) from seedheads. During extreme flood tides, they move to landward edges of marshes, where they gather in stands of Juncus roemerianus (Black Needlerush) and medium-height or dwarf S. alterniflora. They sometimes feed on ground in J. roemerianus, and along the interface of Juncus and dwarf S. alterniflora. They take refuge in stands of Myrica cerifera (Wax Myrtle) and Baccharis, which grow on the marsh edge and on sand banks, hummocks, or berms. When the tide recedes, the birds are rarely found in these landward habitats, apparently returning to tall S. alterniflora (WP). They sometimes commute 200–300 m daily between roost sites in Juncus on the marsh edge to strips of tall S. alterniflora bordering a local river in South Carolina (WP). Occasionally capture live, small fish by wading into muddy, intertidal pools in Florida (see above).

Food Capture and Consumption

Performs food searches on the ground by walking, occasionally interrupted with single or a few hops (JSG). Individuals spend most of their foraging time on the ground, using their beak to deftly brush stems aside in dense stands of grass (10), and clamber into the grass column when residual stems are present and clumped (JSG). Most food taken while searching substrates within reach from the ground, while gleaning new-growth or residual grass, surfaces of mud and water, and sodden litter (45% of attack maneuvers observed), or while gleaning or probing into mud (35%). They also glean as they walk and hop on wrack (algae and eelgrass) patches and other flotsam washed ashore by tides and stranded on open mud or on the tops of erect grasses (14%) (141). Other attack maneuvers include vertical leaps at grass stems or blades over the head, and chases and lunges on the ground. No sallying or hawking observed by us or reported in the literature. Species frequently employs bilateral scratching with both feet simultaneously (142) in loose litter in captivity (flight cage, aviary), but behavior observed only once in wild by authors (28 February 2010, Pinellas County, Florida, on thin fibrous litter in intertidal under mangrove edge; JSG)—perhaps because marsh litter often compacted and wet and difficult to move in that manner.

Larger adult and larval insects are bitten repeatedly before eating; not known whether prey is subdued by beating against the ground and shaking. Seeds of cordgrass consumed by taking them from mud at low tide where they fall from plants, and directly from seedheads. On seedheads, birds perch sideways on grass culm, and pluck seeds one at a time with a twisting, pulling motion of head and bill (JSG). Generally, sparrows are flexible about where they seek food, scanning substrate surfaces and investigating holes between stems at plant bases and under wrack (10). May tug at pieces of vegetative debris on ground (often ineffectually), but not known to use closed beak to “sweep” litter aside. Eats only millet from commercial mixed seed supplied in captivity. Seed held near tip of bill and hulled by biting and turning it (with aid of tongue?) with rapid movement of jaws; seed coat ejected and kernel eaten.

Diet

Major Food Items

Stomach contents (Office of Migratory Bird Management, U.S. Fish and Wildlife Service [USFWS], from old Biological Survey data [143]) documented relatively few arthropod groups dominating adult diet (Table 2). Most important food items for birds taken from Massachusetts to Delaware (May to August), were adult and larval insects, amphipods, and spiders. Overall, breeding period diet appears to be opportunistic and broad-based across insect and other arthropod groups associated with tidal marshes. Adults and their nestlings consumed similar major prey groups (including amphipods) (Table 2 versus Breeding: Parental Care: Feeding). Although a dietary generalist, some specialization is evident in the species, in comparison to its relative the Seaside Sparrow, perhaps related to their different evolutionary histories (49), with amphipod prevalence reflecting its tendency to obtain invertebrates in supratidal, coastal salt meadows (141). Generalizations provided by Martin et al. (144) apply to both Saltmarsh Sparrow and Nelson’s Sparrow before the two species were separated, so Greenlaw and Rising (127) reanalyzed the USFWS samples to distinguish between northern and southern populations of sharp-tailed sparrows. This information is updated here to apply to Saltmarsh Sparrow in Table 2.

Quantitative Analyses

Twenty stomachs of Saltmarsh Sparrow taken during the breeding period (May–August) along the north-central Atlantic coast from Delaware to Massachusetts contained 92% (relative total volume) animal matter and no plant material. Twelve stomachs taken from Delaware to Rhode Island during the postbreeding period (September–October) contained 70% animal matter and 30% plant matter (143; Table 2). Animal matter during the breeding period constituted 44% insects of all life stages, 24% amphipods (Orchestia grillus), and 21% spiders (Lycosidae, Dictynidae, and Araneidae were the most prevalent in Spartina grasses on Long Island; 145), and 4% small mollusks (Table 2).

The breakdown of insects by order during the breeding period (relative percent volume among insects only, with mention of some notable families) in the USFWS sample was Coleoptera (all adults, 21%; Elateridae, Carabidae, Coccinellidae, Curculionidae, Staphylinidae), Orthoptera (few eggs, mostly adult 20%; Acrididae, Gryllidae, Tettigoniidae), Diptera (adult and larvae 20%; Chironomidae), Hemiptera (adults 18%; Lygaeidae, Pentatomidae), Lepidoptera (some adults, mostly larvae 15%; Noctuidae most prevalent), Homoptera (all adult 2%; Cicadellidae, Delphacidae, Fulgoridae), Hymenoptera (all adults 2%; Braconidae), and other insects (2%) (Table 2). This viewpoint is sensitive to variation in local feeding behavior of individuals just before they were collected, reflecting numbers and sizes of prey consumed. An alternative perspective is the relative frequency of occurrence in stomachs across the total sample, reflecting consumption in a population or species. For all food categories, relative frequencies of occurrence in stomachs during breeding in descending order are spiders (21%), Coleoptera (15%), amphipods (13%), Diptera, Orthoptera, and Hemiptera (8% each), and Lepidoptera (7%); remaining categories < 5% each. These stomach samples do not mention adult or larval dipterans in the family Stratiomyidae, which were important in food brought to nestlings in New York (141), and almost certainly were used by adults. Importance of order Araneae in the diet of breeding sparrows in southern populations (A. c. diversa) mentioned by Montagna (27); chief contents of gizzards examined were “blackish spiders” prevalent in marsh grasses.

USFWS data indicated virtually exclusive dependence on arthropods during the breeding period, and a more mixed diet of arthropods and seeds at other times of the year. Seasonal shift begins in late summer with seed-set on breeding grounds, and extends to wintering range (see 146). Spartina alterniflora (Smooth Cordgrass) seeds are taken by postbreeding local birds from seedheads and on the mud before fall migration, and by flocks of fall migrants; local, postbreeding birds move from inner sections of large salt marshes to outer intertidal zone where tall S. alterniflora is setting seed heavily in late summer and fall (JSG, WP, Long Island, New York).

Almost no details available on diet of birds in nonbreeding range. Seeds of “salt marsh grass” [apparently S. alterniflora] are a chief component in South Carolina. A. T. Wayne reported use of a species of maritime moth present in the spring near Charleston before birds depart (137). First reported case of carnivory in species of small, intertidal fish captured alive and eaten in Florida in winter (see above).

Food Selection and Storage

This species' long, relatively shallow bill at base (49) is associated with strong dependence on arthropods in breeding diet (146). Attenuated bills are forceps-like and adapted to picking arthropod prey from environmental substrates (147, 148, 149). Current evidence suggests predominant opportunism in taking arthropod prey. Still, prey-types such as ground-inhabiting amphipods and larval soldier flies, and adult moths and grasshoppers in the vegetation, were selected more often than availability for delivery to nestlings (141). No information on olfaction or sense of taste in relation to food selection. No evidence of food storage.

Nutrition and Energetics

Little information available on energetics. Females were judged to be in poorer condition than males during both breeding and overwintering periods, based on a scaled mass index that used tarsus to account for body size, yet females carried more fat on average during the breeding season than males (26). Body mass during breeding season (15 May–15 August) and after the breeding season (16 August–15 October) on Long Island, New York, were statistically indistinguishable between sexes (150; JSG). Both sexes also had poorer index scores in winter than in summer, despite carrying more visible fat (26). No differences in survival were detected in connection with these condition differences; however, see Demography: Life Span and Survivorship. No information available on nutrition, apart from observation that captive birds were maintained for several months on a commercial wild-bird seed mix and mealworms (Tenebrionidae: Tenebrio molitor) (JSG), with addition of chick-mash by Poulson (151).

Metabolism and Temperature Regulation

No physiological information. Recent studies reveal importance of some avian bills in dissipating body heat from underlying vascularized tissue below the ramphotheca in the fashion of a thermal radiator (152). A correlational study between bill size in tidal marsh sparrows and maximum summer temperature supports view that relatively large bills in Saltmarsh Sparrow and relatives may play a regulatory, heat-loss role in hot, open habitats (153).

Drinking, Pellet-Casting, and Defecation

A laboratory study of Saltmarsh Sparrow and its closest Atlantic coast relatives (151) provided information on drinking behavior of birds ingesting pure water and saline solutions. No information on kidney morphology or function, or on use of free water in the field, for Saltmarsh Sparrow, and only available for one other tidal marsh relative (Passerculus sandwichensis beldingi) (154). Cephalic salt glands not present in passerines, but other osmoregulatory adaptations to handle electrolytes encountered in tidal marshes may be key to ability of only a few passerine species to live in these habitats (154, 155, 25). Captive subjects can drink water with salinities as high or higher than those typical of tidal marsh habitats they occupy; they are able to maintain body mass up to 0.3 M NaCl, although begin to lose body mass slowly at 0.4 M NaCl solutions (151). Saltmarsh Sparrow in spring increased ad libitum consumption of salt solution up to peak of 0.2 M, well below the maximum level they tolerated. Birds regulated salt ingestion at 0.21–0.24% body mass/d while drinking between 105 and 125% body mass/d. On average, plasma chloride concentration and osmolality, and blood hematocrit, were regulated within narrow ranges up to 0.4 M salt solution ingestion, although measurements became more variable among individuals tested at higher concentrations. In preference tests, birds discriminated against salt solutions in favor of distilled water. Ability to concentrate electrolytes in urine, relative to plasma levels, remained open to question because of the manner in which urine was collected from the cloaca; fluids (and residual fecal material) from the colon can confound cloacal measurements of urine (154). Perhaps most notable result of Poulson’s (151) study was observation that tidal marsh sparrows were able to maintain both body mass and levels of general activity and foraging under laboratory salinity conditions that would be intolerable to even closely-related non-saltmarsh passerines (see also 155). Use of free water in the field is unknown in Saltmarsh Sparrows, but birds likely drink dew from non-salt-secreting plants and rainwater. Yet, they must also ingest electrolytes in their habitats as they take some prey with tissue salinity similar to saline muds in which they live. Given the responses of captives tested on demanding salinity solutions, it is reasonable to predict that Saltmarsh Sparrow (and Seaside Sparrow) consume salty water in the field.

Not known to consume soft fruit, so seed-casting unreported, as is pellet-casting generally. Adults defecate on the ground and from perches as circumstances dictate.

Recommended Citation

Greenlaw, J. S., C. S. Elphick, W. Post, and J. D. Rising (2018). Saltmarsh Sparrow (Ammospiza caudacuta), version 2.1. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.sstspa.02.1