Original description: Ammodromus caudacutus var. nelsoni Allen 1875
Taxonomic history in AOU/AOS Check-list
AOU 1 (1886): Nelson’s Sparrow, Ammodramus caudacutus nelsoni
AOU 2 (1895): Nelson’s Sparrow, Ammodramus caudacutus nelsoni; Acadian Sharp-tailed Sparrow, Ammodramus caudacutus subvirgatus
AOU 3 (1910): Nelson’s Sparrow, Passerherbulus nelsoni nelsoni; Acadian Sharp-tailed Sparrow, Passerherbulus nelsoni subvirgatus
AOU 4 (1931): Acadian Sparrow, Ammospiza caudacuta subvirgata; Nelson’s Sparrow, Ammospiza caudacuta nelsoni
AOU 5 (1957): Sharp-tailed Sparrow, Ammospiza caudacuta subvirgata, Ammospiza caudacuta altera, Ammospiza caudacuta nelsoni
AOU 6 (1983): Sharp-tailed Sparrow, Ammodramus caudacutus
AOU 7 (1998): Nelson’s Sharp-tailed Sparrow, Ammodramus nelsoni
IUCN Conservation Status: Of least concern
Like all birds dependent on grasslands or wetlands for breeding or wintering, this species is susceptible to habitat loss as those landscapes are degraded or destroyed by agriculture, urbanization, and natural and manmade changes in water levels. In the Canadian Maritimes, suitable breeding habitat for subvirgata—probably the least numerous subspecies—has been reduced by at least 50% since European settlement.
Birds breeding in the interior of the continent can be affected by drought, flooding, grazing, and agricultural development. Coastal breeders, including those around Hudson and James Bays, are sensitive to rising sea levels, as are all birds on the wintering grounds. At all seasons, the challenge of detecting these secretive birds in their marsh habitats makes it difficult to assess population levels and to draw conservation conclusions. In 2008, the US Fish and Wildlife Service listed the nominate subspecies of the Nelson Sparrow as a species of conservation concern in ten of the Bird Conservation Regions where it occurs as a breeder, migrant, or winter resident.
Behavior: Like other passerellid species of grassland and marsh, Nelson Sparrows are shy and extremely unobtrusive, spending most of their time on the ground beneath vegetation, half-flying, half-running across open patches, head lowered. Feeding birds walk or hop over and sometimes under spartina mats or climb grass stalks in search of small invertebrates and their eggs; though they do eat some seeds, Nelson Sparrows are among the most carnivorous of sparrows at all seasons.
Confronted by a potential predator or other disturbance, Nelson Sparrows run a few feet, then freeze on the ground, apparently hoping to go unnoticed. They usually wait until nearly stepped on to fly, when they take off with fast and irregular wingbeats and fly low over the grass, dropping twenty-five or thirty feet away to scamper away beneath the grass; flushed birds often fly straight away, then turn sharply before landing. Patient observers may be rewarded some minutes later by seeing the bird climb slowly up a grass stem to inspect its surroundings. Sustained flight is infrequently seen, but tends to be straighter, with stronger wingbeats, than the flopping actions of flushed birds.
Advertising males deliver their song from a perch atop marsh grass or the stalk of a dried forb.
Voice: On the breeding grounds, song is a helpful way to distinguish Saltmarsh Sparrows from Nelson Sparrows inhabiting the same marshes. The Nelson sings a fast, two-parted buzz, both parts loud and broadly modulated. The first element of the song is higher pitched, with a distinctive metallic jangling quality; the second element is lower pitched and more shirring or sibilant, though still decidedly a buzz. Some songs begin with a very brief lower-pitched buzz, barely separated from the remaining elements; other songs are followed by two or three fairly faint, high-pitched ticking notes. In overall pattern, the main two-parted song can recall that of the Seaside Sparrow, but it is faster, higher pitched, and more silvery, with a bright, breathy quality lacking in the Seaside Sparrow’s more rattling buzzes. This song, krrZZHHsshhtt, has no parallel in the vocal repertoire of the Saltmarsh Sparrow, making it a diagnostic feature in areas where both may be present.
This song also makes up part of the Nelson Sparrow’s flight display, in which the male ascends to a height of as much as 50 feet while uttering a variable series of three to ten irregularly spaced ticking or squeaking notes; the buzzing song follows these notes as the bird begins its descent. Twice over a period of ten years, Greenlaw observed Saltmarsh Sparrows uttering similar ticking notes (but without the species-specific buzz of the Nelson Sparrow) in an apparent flight display.
The Nelson Sparrow lacks the “whisper” song given by Saltmarsh Sparrows.
Nelson Sparrows in migration and winter call only infrequently, a very high-pitched, thin teek without noticeable attack or decay; it can be given in a fast series by birds under stress, and this call is among the notes introducing the terminal buzz of the flight song.
Detailed description and measurements drawn from standard reference works
Adult Ammospiza nelsoni nelsoni: Tail feathers rusty brown with very thin slightly paler edges and blackish shafts. Upper tail coverts and rump rump feathers with large black centers and tawny brown edges and tips. Ground color of back and scapulars gray-brown, the feathers with large black centers and broad, well-defined whitish edges aligning into contrasting stripes. Short primaries gray-brown with narrow grayer edges, secondaries browner; tertials black with yellow-tinged white edges and tips. Greater coverts black toward tip, brown-gray at base, with dull brownish edges and tips forming faint scalloped wing bar in fresh plumage. Median coverts brown-gray with blackish shaft streaks broadening toward tip; edges and tips browner. Marginal coverts of underwing yellowish. Nape gray with olive tone.
Undertail coverts and vent yellowish, with faint blackish shaft streaks. Belly and lower breast clear white, abruptly set off from orange buff flanks and upper breast. Flanks, leg feathering, sides of breast, and breast variably deep orange buff with variable streaking. In fresh plumage, flank streaks broad and distinct, with a decided rusty tinge. Scattered breast streaks sootier and finer, nearly absent in many individuals. Throat and chin buffy whitish, blending into deeper orange buff of upper breast and separated from deep orange jaw stripe by inconspicuous narrow, often incomplete blackish lateral throat stripe. Jaw stripe extends below and behind ear coverts. Crown largely black with narrow dark gray median stripe; median stripe usually with internal black streaking. Ear coverts gray with irregular yellow or orange spotting; bordered below by very faint dark gray whisker, above by incomplete blackish eye line, widening to the rear. Broad buffy orange supercilium continues across lore to join broad jaw stripe, forming bright yellow-orange ear surround. Faint whitish eye ring, strongest below and at rear of eye.
Bill pinkish gray with dark gray culmen. Tarsi and toes gray.
Juvenile Ammospiza nelsoni nelsoni: Tail feathers black with olive edges. Upper tail coverts and rump buffy with black shaft streaks. Back and scapulars bright brown with coarse black shaft streaks creating broad black stripes. Primaries and secondaries sooty gray with paler gray to buffy edges; black-centered tertials with orange buff edges. Greater and median coverts with dark centers, bright brown edges, and inconspicuous whitish tips. Nape clear buffy brown.
Undertail coverts, vent, and belly yellowish white, blending into deeper orange or buffy flanks, breast, and throat. Sides of breast variably marked with fine blackish streaks, sometimes continuing to center of breast. Entire face, including ear coverts, buffy orange, with thin dark whisker and partial eye line. Crown blackish with variable grayish yellow median stripe.
Bill pinkish with darker culmen. Tarsi and toes gray-pink.
Length 116-118 mm (4.6 inches)
Wing chord 54-55 mm (2.1 inches)
Tail 46-48 mm (1.8-1.9 inches)
Adult Ammospiza nelsoni subvirgata: Tail feathers grayish with darker shafts. Upper tail coverts and rump with dusky streaks. Ground color of back and scapulars gray-brown with olive tinge, the feathers with large brownish centers and narrow, rather diffuse whitish edges aligning into inconspicuous stripes. Short primaries brown with narrow green-gray edges, secondaries browner; tertials dark gray with yellow-tinged white edges and tips. Greater and median coverts dark gray toward tip, brown-gray at base. Marginal coverts of underwing pale yellowish. Nape gray with pale yellowish brown tone.
Undertail coverts and vent yellowish, with faint gray shaft streaks. Belly and lower breast dull white, abruptly set off from pale buff flanks and upper breast. Flanks, leg feathering, sides of breast, and breast buff with variable streaking. In fresh plumage, light grayish flank streaks. Scattered grayish breast streaks blurry and diffuse, nearly absent in many individuals. Throat and chin yellowish brown, blending into buff of upper breast and separated from buffy jaw stripe by inconspicuous narrow, often incomplete blackish lateral throat stripe. Jaw stripe extends below and behind ear coverts. Crown largely black with fairly broad gray median stripe, usually without internal black streaking. Gray ear coverts bordered below by very faint dark gray whisker, above by incomplete blackish eye line, widening to the rear. Broad pale buffy supercilium continues across pale whitish lore to join broad jaw stripe, forming dull yellowish ear surround. Faint whitish eye ring, strongest below and at rear of eye.
Bill bluish gray with dark gray culmen, pinker beneath. Tarsi and toes grayish pink.
Juvenile Ammospiza nelsoni subvirgata: Tail feathers black with buffy edges. Upper tail coverts and rump chestnut-buff with black shaft streaks. Back and scapulars chestnut brown with coarse black shaft streaks creating broad black stripes. Primaries and secondaries black with paler gray to buffy edges; black-centered tertials with chestnut edges. Greater and median coverts with black centers, bright chestnut edges, and inconspicuous buffy tips. Nape clear buffy brown.
Undertail coverts, vent, and belly yellowish buff, blending into darky buffy flanks, breast, and throat. Sides of breast sparsely marked with fine blackish streaks. Entire face, including ear coverts, buffy, with thin dark whisker and partial eye line. Crown blackish with variable grayish buffy median stripe.
Length 124-125 mm (4.9 inches)
Wing chord 56-58 mm (2.2-2.3 inches)
Tail 49-50 mm (1.9-2.0 inches)