Saltmarsh Sparrow is weakly differentiated geographically within its narrow, latitudinally restricted, and fragmented coastal range. Populations from south-central New Jersey northward are paler and less-strongly patterned than southern populations (see Subspecies for descriptions of two named taxa). The general ground-colors are grayer to the north and blacker to the south. A zone of intergradation is evident around Tuckerton, New Jersey, and perhaps extends southward to southern Delaware (27). In southern marshes where blacker phenotypes of the species prevail on the Delmarva Peninsula, suitable marshes are meadow-like, and sparrows are locally prevalent, at least at Chincoteague, Accomack County, Virginia. Perhaps not coincidentally, Black Needlerush (Juncus roemerianus) reaches its northern limit of distribution around Chesapeake Bay in Maryland and Virginia where this form of Saltmarsh Sparrow is distinctly dark (USA National Phenology Network). Interestingly, in an example of apparent convergence, a dark-streaked subspecies of Seaside Sparrow, A. m. macgillivraii, occurs just south of the range of Saltmarsh Sparrow in a region where this rush becomes a dominant graminoid type in coastal marshes (28).
Despite variation in dispersal ability, species such as Saltmarsh Sparrow that are habitat specialists and area sensitive often are influenced by natural or anthropogenic fragmentation of habitat (29, 30). Geographic variation in fine-scale genetic population structure appears to be one consequence in the species. Five geographic groups between Long Island, New York, and southeastern Maine (Scarborough Marsh), each have distinctive allelic profiles for 10 microsatellite loci (31, 32). This variation is consistent with an evolutionary pattern of isolation by distance between the groups. But, one subcoastal (inland) population in New Hampshire was isolated from coastal populations nearby and also exhibited strong differentiation, even though it was only about 15 km from the coast. Significant dispersal isolation from other populations coupled with strong adult site tenacity seem to be key factors in the species promoting incipient genetic divergence (32).
Two subspecies are traditionally recognized, although molecular data do not support a clear distinction (see below). We list these subspecies pending a formal assessment of their ranking status. Saltmarsh Sparrow was formally introduced to the ornithological world by Gmelin in 1788 as Oriolus caudacutus, based on a non-binominal account prepared earlier by Pennant (33) and reproduced and illustrated by Latham (34) before Pennant’s description appeared (also see 35). Latham credited Pennant with both the description and illustration, although Pennant’s treatment did not include the illustration.
- A. c. caudacuta (Gmelin, 1788). Northern migratory race, breeds from coastal Maine (see Distribution, Migration and Habitat: Breeding Range) south to south-central New Jersey (type locality: in Noveboraco = New York), with some large populations in Long Island, New York (36), and coastal Connecticut and Rhode Island (37, 38, 39, 40, 41). Color pattern above and below is species-typical, but birds in New England and New York are characterized by clear ground-color hues of olive-brown on back and scapular feathers, a moderately well-defined olive-grayish mid-coronal stripe with black streaks mostly confined to the brown lateral crown stripes, and a distinct black, subterminal line that frames the inside of the pale tertial fringes, and dark brown or blackish-brown ventral stripes. Grayish to blackish-brown streaks also occur in the post-ocular portion of the supercilium or eyebrow.
- A. c. diversa Bishop, 1901. Bishop (42) based his name on an adult female in his collection (type locality: Wanchese, Roanoke Island, North Carolina) taken in early May and on a series of specimens that he collected on Pea Island, North Carolina, in February. We have no evidence that the species has ever bred in North Carolina, and we now know that the birds taken in February were apparently wintering at this location. However, Pea Island continues to be listed with discredited support (Wetmore  contra Montagna ) in the AOU Check-list as the southern breeding location of the species (45; see Distribution, Migration and Habitat: Breeding Range). Large areas of coastal marsh containing the species remain within John B. Forsythe National Wildlife Refuge in southeastern New Jersey (46). Pattern generally darker than in nominate race. Background hues of back and scapulars are strongly dusky, the blackish crown streaking tends to be heavy and intrudes into the weakly defined mid-coronal stripe from the dark lateral crown stripes, the tertial centers become black thus obscuring the frame-line typical of nominate birds, and the ventral stripes also are heavy and blackish. Post-ocular streaking in the eyebrow tends to be conspicuous and strongly blackish. Diagnostic plumage attributes of this subspecies are based on extreme variants represented chiefly on the south end of a short latitudinal cline across its stated range, while a few dusky individuals have been found as far north as Connecticut and New York (CSE, JSG). Two specimens taken in New York were sufficiently dusky above to be indistinguishable from those collected in Tuckerton, New Jersey, where duskiness first becomes apparent at a population level (27; JSG, unpublished data). Although extreme variants are recognizable and may be useful geographic markers, many intermediate specimens are problematic and result in uncertain determinations of specimens taken off the breeding grounds in winter (47). Recent molecular data do not support recognizing distinct subspecies in the species (48: 855, 25). We accept the view that north-south geographic variation in A. caudacuta is a clinal pattern of recent origin (25) and recommend that the species be regarded as monotypic.
Nelson’s Sparrow is the sister relative to Saltmarsh Sparrow (49, 50). These 2 species of “sharp-tailed sparrows,” together with their next closest relative, Seaside Sparrow, and basal LeConte’s Sparrow, form a distinct, well-supported, monophyletic clade (51, 50, 52). The phylogenetic relationships between the sharp-tailed sparrows and the 2 other members of its clade, and the history of species limits between the 2 sharp-tailed sparrows, have been uncertain until recently.
In the late 1960s, the close relationship of LeConte’s Sparrow to Seaside Sparrow and sharp-tailed sparrows as then constituted (single-species concept) was finally recognized, after a history of taxonomic uncertainty in generic placements of LeConte’s Sparrow relative to the others (53). Sharp-tailed sparrows as 1 or 2 species, Seaside Sparrow, and LeConte’s Sparrow previously had been variously distributed in 1–3 genera (e.g., 6, 54, 55, 56). Murray (53) regarded LeConte’s Sparrow as the likely closest relative to sharp-tailed sparrows viewed as a single species, but this hypothesis has not been supported by recent genetic evidence (see above).
Sharp-tailed sparrows have been treated with some uncertainty on species limits over the years. During the late 1800s, 1 species and 2 to 3 subspecies were recognized by the A.O.U. in the first 2 editions of its Check-list of North American Birds (57, 58). In 1897, Norton reviewed the status of 2 subspecies (A. c. caudacutus, A. c. subvirgatus) of Ammodramus caudacutus in Maine and, based on plumage distinctness and apparent geographic segregation as known at that time, argued for elevating A. c. nelsoni and A. c. subvirgatus to species rank (A. nelsoni). The AOU agreed (59, 60) until the pendulum of opinion later returned to the single species concept (54, 55). That concept prevailed until new information on song characteristics, singing behavior, and an apparent zone of secondary contact (49), and on genetics and skeletal morphology Rising and Avise (48) became available in the 1990s. Norton’s (61) 2-species view was resurrected (49) and the proposal was accepted by the AOU (62). Finally, in 2018 all 4 marshland sparrows in their well-supported monophyletic clade were returned to the genus Ammospiza, where 2 of 3 marshland species recognized at that rank in mid-century (excepting LeConte’s Sparrow in Passerherbulus) had resided from 1931–1983 (63).
The geographic extent and nature of a secondary contact (hybrid zone) between Saltmarsh Sparrow with its sister species (49, 64, 65), and genetics of hybridization involving sharp-tailed sparrows in New England (48, 66, 1, 67, 68) have received recent attention. A hybrid zone apparently formed sometime after continental ice recession (post-Wisconsin geological time) when populations of Saltmarsh Sparrow along Atlantic coast and more northerly Nelson’s Sparrow spreading eastward from inland refugia contacted one another. Historical specimen evidence suggests that this hybrid zone was initially more restricted than it is today. The initial expansion of the hybrid zone evidently occurred during the 1900s. Specimens taken in the late 1800s and early 1900s indicate that A. caudacuta occurred exclusively in Maine east to Cumberland County near Portland (61, 69; also examined by JSG), while A. nelsoni and intermediate individuals were found in summer east in Sagadahoc County at Popham Beach marsh (70, 27, 71). A. nelsoni in summer was not documented near Portland (Scarborough Marsh) until the late 1900s (49), but no evidence exists that early naturalists took specimens from marshes in summer between Popham Beach west of Portland and Cape Elizabeth just to the south (72, 61, 69, 70, 27). Currently, genetically pure parental phenotypes of both species occur in a series of semi-isolated populations from New Hampshire (Rye, Stratham) north to Weskeag River marshes, Knox County, Maine, but with introgression extending 200 km between north and south ends of the hybrid zone to include northeastern Massachusetts (Hampton to Newburyport) (64, Walsh et al. 2015b, 68). Genetically pure nelsoni and breeding by that species have not yet been confirmed from the latter region (J. Walsh, personal communication), but in the last few years (2015–2016), phenotypic Nelson’s Sparrow have been reported from Parker River National Wildlife Refuge in mid summer (ebird.org, 3–5 males, mid June to July, singing, mist-netted, photographed). How recently the southern portions of the hybrid zone as currently recognized were occupied remains unclear. Molecular and plumage evidence from multiple sites sampled in 1998 and 2013 indicates that the extent of the hybrid zone has doubled and its center shifted 60 km south over this period (73). This change, however, may be caused by the relatively steeper population declines in Saltmarsh Sparrow relative to Nelson’s Sparrow (74), rather than a southward spread of the latter species. However, the range limit northward of the Saltmarsh Sparrow, and perhaps the southward limit for Nelson’s Sparrow at the introgressed Massachusetts end of the hybrid zone, may stabilize at present terminal locations (J. Walsh, personal communication). The frequency of backcross hybrids near each end of the zone attenuates, and ecological niche data suggest habitat north and south of the zone is unsuitable (68). Introgression may continue to move some alleles north and south of the hybrid zone. Recent work has examined the genetic structure of the hybrid zone. Environmental variation among marshes evidently influences distribution and frequency of pure species and hybrids across the zone. Pure-bred caudacuta and backcrossed caudacuta hybrids tend to prefer larger (mean 222 ha), wetter marshes near the outer coast with good connectivity to other marshes, relative to pure nelsoni (A. n. subvirgata) and backcrossed nelsoni hybrids, which tend to use smaller (mean 80 ha), more isolated, and drier marshes, often farther from the outer coast (for example, in upper estuaries fringing coastal rivers). This results in scale-dependent variation in genetic structure within the 2 sister species and their backcrossed hybrids among marshes across the hybrid zone, expressed as a spatial mosaic pattern depending on size, position, and associated environmental characteristics of each marsh patch. Still, this is an average pattern with substantial overlap between responses of the 2 species and their backcrossed hybrids to patch marshes (68). Most introgressed birds are backcrossed, however, and few are first generation (F1) hybrids, suggesting that genetic swamping is not occurring (68). A comparison of hybrid and pure parental fitness in the hybrid zone indicates that extrinsic (hatching and fledging rates in relation to tidal flooding) factors influence reproductive success in some hybrid genotypes, and that an intrinsic (survival related to unprotected alleles in the heterogametic female sex) factor results in reduced survival in F1 hybrids (68). Further evidence in Walsh et al. (75) support and extend the previous finding that fitness costs are associated with hybridization. The authors found clear evidence of assortative mating decisions favoring insemination with conspecific males in both species in the hybrid zone, even though intraspecific genetic (allelic) dissimilarity was high in each population. Moreover, they also reported reduced reproductive success between pure and hybrid individuals with only 9% of F1-F2 males in their sample siring offspring while 42% of Saltmarsh Sparrows did. However, within each species, no differences in reproductive success were noted between pure and back-crossed individuals. The authors suspected divergent mating behaviors between the species may play some role in maintaining species boundaries in the overlap zone, where early generation hybrids seem to be relatively uncommon in samples while backcrossing within each species is frequent and evidently without cost.
Apparent or suspected hybridization involving Saltmarsh Sparrow and Seaside Sparrow parents is very rare, but has been known for over a century. The only specimen (MCZ 242478, female) of such an individual, taken in May 1890 near New Haven, Connecticut (76; examined by JSG), appears to be a mosaic of some plumage characters, and similar to one parent or the other, or intermediate, in selected measurements (e.g., narial bill length similar to that of typical Seaside Sparrow from the same region; wing chord within range of Saltmarsh Sparrow males but not of females). Hill (77), who also examined the MSC specimen, reported studying another apparent example in the field in Barnstable, Massachusetts in 1957. Another apparent hybrid caught in Scarborough, Maine, aligns with the hypothesis that hybrids are most likely to occur at the advancing edge of a species range where finding a conspecific mate is harder (CSE, unpublished photographs).
Beecher’s (78) early one-cycle model of speciation in sharp-tailed sparrows assumed that northern elements of ancestral populations, following recession of last continental ice mass, followed pre-isostatic lowlands from the Atlantic coast to what today is the St. Lawrence River and James Bay, and on to northern Great Plains, where isolated and semi-isolated subpopulations diversified. This model fails because of too little time and because it does not account for morphological, social behavioral, and genetic differences between northern and southern populations within the group (49, 48). Greenlaw (49: Figure 1B) developed an alternative 2-cycle, biogeographic model that assumed shared ancestry between early Seaside Sparrow and sharp-tailed sparrows that had speciated along the northern Atlantic coast. Initial split [elaborated here] likely involved populations separated from southern ancestors in tidal marshes from northerly populations along the coast in brackish and freshwater marshes. Members of northern populations spread inland, speciated again to form the northern group that became Nelson’s Sparrow, which returned through range expansion during an interglacial to the north-central Atlantic coast to recontact the original population, which became Saltmarsh Sparrow, where it was associated with northern tidal marshes. This model is supported by estimated timing of split (ca. 600,000 years ago, or mid-Pleistocene, based on mtDNA data) between northern and southern groups (48), and by phylogeny of the clade (52).