Ammodramus nelsoni Allen, 1875
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Ammodramus caudacutus, Ammodramus caudacutus nelsoni, Ammospiza caudacuta
Basis for Listing
The Nelson's sparrow has three disjunct populations. One occurs on the north Atlantic Coast from Maine to Quebec, another at Hudson Bay, and a third in the interior of North America from Minnesota and North Dakota northwest to Alberta and southern Mackenzie, Canada. The species is relatively secretive, and its inconspicuous song and inconsistent, often nocturnal singing habits, make it challenging to assess the distribution and population status of this species in Minnesota. It was formerly thought to be a relatively rare and local species, largely restricted to the northwestern corner of the state. Recent surveys, however, have found the species' range extends well into north-central Minnesota. Nonetheless, the Nelson's sparrow has very narrow habitat requirement and even slight changes in water levels in wetlands can impact this species. The Nelson's sparrow and its habitat are therefore sensitive to drainage, impoundments, and other disturbance and habitat conversion activities. For these reasons, it was listed as a special concern species in Minnesota in 1984.
The Nelson's sparrow is relatively brightly colored, with a broad, orangish eyebrow and grayish ear patch and crown. The finely streaked upper breast and sides are rich buff in coloration, contrasting with the white belly. Due to its secretive nature, the Nelson's sparrow is often best detected and identified by its distinctive song, a wheezy, gasping sputter which resembles the sound of hot metal being placed in water. The species often sings at night, but vocalizes at dusk and dawn, as well. Nelson's sparrows can be easily confused with the more common Le Conte's sparrow (Ammodramus leconteii), which often occurs in the same habitat, and also sings at night. The Nelson's sparrow however, has an unstreaked gray nape and bolder, whitish streaks on its back, compared with the fine chestnut streaks on the nape, and less distinct, straw-colored streaks on the back of the Le Conte's sparrow.
The Nelson's sparrow is primarily a bird of sedge wetlands, and it is often found in the same areas and habitat as the yellow rail (Coturnicops noveboracensis), another Minnesota special concern species. However, Nelson's sparrows may be slightly more tolerant of a wider range of water levels, and perhaps slightly coarser vegetation than yellow rails. The Nelson's sparrow breeds in sedge- or grass-dominated wetlands, particularly wet prairie, rich fens with narrow-leaved sedges, such as fen wiregrass sedge (Carex lasiocarpa ssp. americana), and wet meadows with wide-leaved sedges and grasses, such as lake sedge (C. lacustris) and bluejoint grass (Calamagrostis canadensis), and avoids cattail-dominated marshes. Within these habitats, water depths between 2-30 cm (1-10 in.) are optimal. Appropriate water depths are especially important to Nelson's sparrows during the breeding season. Sites with ideal vegetational structure but unsuitable water conditions (either too wet or too dry) will not likely contain this species. A litter layer of dead, procumbent sedge or grass, is also important.
Biology / Life History
The Nelson's sparrow's main wintering grounds are coastal marshes on the Gulf of Mexico and the Atlantic Coast from North Carolina to Florida (Greenlaw and Rising 1994).The species is present in its breeding habitat from approximately May through September (Janssen 1987). It is not known whether the Nelson's sparrow nests colonially or if it simply appears so because of patchy local distribution of habitat. It is possible that the species is actually territorial (Greenlaw and Rising 1994). Nests are built on the ground in a matted bed of sedges or grasses. Clutch size is typically 3-5 eggs. Major food items are insects, spiders, and other small invertebrates (Greenlaw and Rising 1994).
Conservation / Management
The primary threat to the Nelson's sparrow is habitat loss due to human destruction and conversion of grassland and marsh habitat to agriculture (Greenlaw and Rising 1994). The species is usually sparsely distributed, even in vast expanses of habitat, because water depths typically vary within a particular area. Because of this variability in water depth, as well as seasonal fluctuations, it is important to protect large areas of habitat to ensure that optimal water conditions are present in at least a few locations within a particular wetland. Prescribed burning might be considered as a means of perpetuating suitable habitat and preventing succession to more woody vegetation.
Conservation Efforts in Minnesota
Assessing the distribution and population status of the Nelson's sparrow has been one goal of the Minnesota Biological Survey (MBS). MBS has conducted bird surveys in much of the potential range of this species in Minnesota, resulting in the documentation of more than 90 records of Nelson's sparrows in 16 counties from 1987-2008. Breeding season observations were documented by other surveyors in an additional four counties over the same time period. Increased knowledge of Nelson's sparrows in Minnesota is an important step in their protection in the state.