Editor’s Note (December 2016): Maps, rich media, and text have been updated to reflect a taxonomic split for this species. This species account is still being edited and may contain content from an earlier version of the account.
Saltmarsh Sparrow (Ammodramus caudacutus) and Nelson’s Sparrow (A. nelsoni) are the result of a recent taxonomic split of the “Sharp-tailed Sparrow”. The Saltmarsh Sparrow is an obligate tidal-marsh specialist, and as such, is unique among passerines in North America and elsewhere. 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 coastal Maine, south to Chesapeake Bay and the Delmarva Peninsula of Maryland and northern Virginia. To the north of northern Massachusetts, its range overlaps with that of its more northerly sister relative, Nelson’s Sparrow, with which it interbreeds. In the breeding season, adults forage on the mud in moderately open graminoid stands, in dense stands where they also clamber into the vegetation column, and along ditch or tidal channels, or edges of marsh pools, on open mud at low tide.
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 which 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. 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 many, perhaps most, salt marshes is the chief source of nest mortality in this species. Selection arising from such events has molded several adaptations that mitigate flooding risk, including nest placement, nest-repair and egg retrieval behaviors, and rapid post-flood renesting. Flooding 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 Long Island, New York, where Saltmarsh Sparrows were exposed to moderate flooding and predation risks, this uniparental species demonstrated an ability to converge on the reproductive success 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 depredation, so nest success varies significantly depending on local circumstances and perhaps even landscape factors.
The occurrence of Saltmarsh Sparrows in small populations occupying discontinuous marsh patches in a limited coastal range, combined with historical losses of coastal marsh, initially led to its classification as a species of high conservation concern. Subsequent range-wide surveys suggest that the global population declined by around 9% per year between the 1990s and early 2010s, reducing the population size by approximately 75% to 50,000–60,000. Population growth estimates, based on detailed demographic data, match this rate of decline, and projections accounting for sea-level rise suggest that without management intervention, extinction by mid-century is likely.