Saltmarsh Sparrow and Nelson’s Sparrow (A. nelsoni) are recent taxonomic splits from an earlier single-species ranking of the two taxa (collectively referred to here as “sharp-tailed sparrows”). Saltmarsh Sparrow is unique among passerines in North America and elsewhere in being an obligate tidal-marsh specialist. It chiefly breeds in dense, supratidal Spartina patens-Juncus salt meadows, but in some marshes, it readily uses Smooth Cordgrass (S. alterniflora) in the upper intertidal, which it often shares with its close relative, the Seaside Sparrow (A. maritimus), in more southern areas. Its breeding range is narrowly linear along the north-central Atlantic coast of the United States, in localized and discontinuous populations, where it extends from Knox County, Maine, south to Chesapeake Bay and the Delmarva Peninsula of Maryland and northern Virginia. From northern Massachusetts north, its range overlaps with that of its more northerly sister relative, Nelson’s Sparrow, with which it interbreeds and has previously been considered conspecific under the collective name “sharp-tailed sparrow.” In the breeding season, adults forage on mud in moderately open to dense graminoid stands, in dense stands where they also clamber into the herbage column, and along ditch or tidal channels or edges of marsh pools, and on open mud at low tide. Nestling diets are comprised entirely of arthropods taken from the ground and vegetation.
Certainly the most notable, perhaps even unique, characteristic of this species is its breeding system. It occupies large home ranges without space defense, and exhibits female-only parental care and promiscuous mating that results in nearly every brood exhibiting mixed parentage. Males spend much time roaming their home range and seeking females. Although females solicit coition from males when receptive, males chase, intercept, and attempt to forcibly mount females at any time in the nest cycle despite resistance. Some mountings are successful and others are not. Overt aggression between males is rare. Evidence suggests that males do not know the locations of most conspecific nests. If this is so, they often may not know—unless a recent flood event destroys many nests—when the sexual receptivity ‘window’ is for individual females they encounter. Still, the significance of roaming and mounting behavior remains unknown, but it may play a role in female choice based on male quality when she solicits copulation, or in a form of sperm competition among males.
Periodic tidal flooding in some salt marshes is the chief source of nest mortality, especially in northern populations of this species. Selection arising from such events has molded several adaptations that mitigate flooding risk, including nest placement, addition of a canopy above the cup of most nests that prevents some eggs from floating out of flooded nests, and rapid post-flood renesting. In extreme cases of flooding, loss of vulnerable nests early in the breeding season often results in synchronization of subsequent nests to a tidal cycle in marshes subject to a monthly pattern of a single highest (spring) tide. In one unaltered marsh on New York in which sparrows were exposed to moderate flooding and predation risks, this uniparental species demonstrated an ability to converge on nest survival and offspring provisioning capability of a biparental care relative co-occurring in the same marsh. Many marshes within the range of the species, especially in New England, are subject to greater risks of flooding, and variable levels of nest depredation, so nest survival varies significantly depending on local circumstances and perhaps even landscape factors. Although nest depredation decreases with increasing latitude, nest flooding does not vary with latitude. Failure of nests due to predation tends to exceed losses to flooding in the southern parts of the species range.
The occurrence of the Saltmarsh Sparrow in small populations occupying discontinuous marsh patches in a limited coastal range, combined with historical losses of coastal marshes, led to the species being listed as a species of high conservation concern. Subsequent range-wide surveys suggested that the global population declined by about 9% per year between the 1990s and early 2010s, reducing the global population size by 75% to 50,000–60,000. Estimates of population growth, based on detailed demographic data, match this rate of decline, and projections accounting for sea-level rise suggest possible extinction by mid-century without management interventions.