Seaside Sparrow Life History

Habitat

Seaside Sparrows are habitat specialists that spend their entire lives in tidal saltmarshes and brackish marshes, with the exceptions of the Cape Sable subspecies (freshwater marshes of the Everglades) and migrants of the northeastern subspecies (maritima), which migrate to the Southeast to winter and may turn up in odd locations, even in the middle of cities. Some tidal marshes are well away from saltwater, and this species nests up to 13 miles inland in some brackish marshes. Not every patch of tidal marsh holds a pair of Seaside Sparrows. They require nest sites that will not be flooded during highest tides, so some very low-lying marshes are not suitable for nesting. The highest densities of Seaside Sparrows occur in extensive marshes that have a mix of plant species and a mosaic of creeks, which provide openings that offer plenty of prey species. They also use lightly vegetated openings in the marsh called "pannes." Typical plant species found in such habitat include saltmarsh rush, black needlerush, saltmeadow hay, saltgrass, and smooth cordgrass. The sparrows forage in but do not generally nest in lower marsh that is regularly inundated, or in short species such as saltmeadow hay. Smooth cordgrass is especially rich in both seeds and arthropods (insects, spiders, etc.). Other plant species found in Seaside Sparrow habitat are seashore dropseed, marsh elder, and succulents such as glasswort and saltwort. Higher parts of the marsh often have bushes such as wax myrtle or groundsel, and flowering plants such as sea ox-eye. Along the edges of many marshes, the invasive common reed (Phragmites) has become prevalent; Seaside Sparrows do not typically forage in stands of reeds but do sometimes build nests in them. The distinctive Cape Sable subspecies of Seaside Sparrow nests in the Everglades, in freshwater marshes dominated by muhly grass, sparse sawgrass, sand cordgrass, Gulf Coast spikerush, and sea-purslane.

Food

Seaside Sparrows eat seeds, insects, spiders, and other invertebrates. When eating seeds from seedheads, they perch in vegetation, pulling seeds toward them with the bill to strip them off. They do most of their foraging on the ground, using their large feet to balance on the mud as they walk. They may glean animal prey from vegetation or the ground, but their large bill is particularly useful for probing into dense vegetation (for bugs and spiders) and into mud, where tiny crustaceans may hide. They are active foragers, racing around quickly through the vegetation, mouselike, and often darting or lunging after prey. They wade into shallow water to seize prey, even submerging the head on occasion. They also frequently feed in wrack composed mostly of smooth cordgrass. To expose hidden prey, they probe with the bill and “double-scratch” with their large feet, leaping forward and then scratching backward again, much like a towhee. In winter and on migration, small flocks may form where seeds are plentiful, but generally they forage alone. Rarely, Seaside Sparrows catch flying insects in the air. Their known prey items include spiders (and their eggs), tiny crustaceans (decapods, amphipods), mollusks, flies, moths, bugs (especially leafhoppers and milkweed bugs), grasshoppers, crickets, wasps, ants, ground beetles, variegated mud-loving beetles, and checkered beetles. They eat seeds of most of the plants found in the marshes they inhabit, and also saltbush, smartweed, and bristlegrass species.

Nesting

Nest Placement

The female selects a nest site within the male's territory, usually in taller marsh vegetation and 6–12 inches above ground, so that the nest does not become flooded.

Nest Description

Females construct the nest, a tight cup of grasses lined with finer grasses, and often cover it partially with a canopy or dome by pulling in adjacent live marsh plants. Nests average about 3.9 inches across and 2.8 inches tall, with interior cup 2.4 inches across and 2 inches deep.

Nesting Facts

Behavior

Seaside Sparrows breed in springtime, earlier in the southern part of their range than in the northern, where they are migrants. Males sing to claim nesting territories that vary greatly in size from 0.25 acres to over 2 acres. Males partner with a female quickly, usually as soon as one visits the territory. Males display to a visiting female by singing, raising their wings, and giving specific calls, one of which sounds like a whinny. A receptive female gives the same calls in response and in fact may sing (like the male) after mating. Females then begin gathering nest material while making the whinny call, and males often gather material but then drop it.

Females construct the nest as males guard them against approach by other males, at least while the females occupy the territory. If warning displays do not suffice to intimidate opponents, fights between males can be intense, with flying attacks that involve pecking and clawing. Females also rebuff advances of males that are not their mates, especially near the nest. Females alert their mates of the presence of unwelcome males by giving specific alarm calls. They also chase away intruding males, as well as male Saltmarsh Sparrows. Nevertheless, in some populations of the species, more than 10% of the young in a nest are offspring of a male other than the female’s mate.

Both adults incubate the eggs and feed the nestlings. In nonmigratory populations, pairs appear to stay together year-round. In migratory populations, females sometimes repartner with their mate of the previous nesting season. Once fledged, young Seaside Sparrows often form small flocks that forage together, and during the nonbreeding season small flocks sometimes gather where seeds are abundant. At night or during storms, they may gather in loose roosts at the marsh edge, in groundsel, wax myrtle, or eastern redcedar.

Conservation

Because of the Seaside Sparrow's reclusive nature and difficult-to-access habitat, population trends are difficult to estimate with precision. The North American Breeding Bird Survey estimates populations have been stable since 1966, but notes that confidence in this estimate is low. Partners in Flight estimates a global breeding population of 200,000, all of which occur in the U.S. The group rates the species a 15 out of 20 on the Continental Concern Score and includes it on the Yellow Watch List for species with restricted ranges. The Florida Everglades population (subspecies mirabilis, the "Cape Sable" Seaside Sparrow) is federally listed as Endangered. Overall, the species is imperiled by runoff of pesticides and other chemicals, by ditching of marshes (to control insect populations or drain land), and by sea level rise during the era of climate change, which will accelerate loss of tidal marsh habitats.

Credits

Dunne, P. (2006). Pete Dunne's essential field guide companion. Houghton Mifflin Harcourt, New York, USA.

Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2021.1. Patuxent Wildlife Research Center, Bird Banding Laboratory 2021.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Post, W., and J. S. Greenlaw (2009). Seaside Sparrow (Ammodramus maritima), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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