Nelson's Sparrow Life History

Habitat

Habitat Marshes

Nelson’s Sparrow is far more varied in its choice of nesting habitats than its close relative, Saltmarsh Sparrow. Birds that breed in the interior of the continent breed in freshwater marshes, from the taiga into the prairie potholes, which hold a wide range of plant species, including cattails and other rushes, cordgrass, sedges, reeds, rivergrass, and foxtail barley. Individuals that breed around Hudson Bay use sedge bogs above the high tide line, with reeds, blue joint, red fescue, sweetgrass, buck bean, field horsetail, and Baltic rush, as well as willow and dwarf birch. The form that breeds along the Atlantic Coast of Canada and New England nests in tidal wetlands similar to those favored by Saltmarsh Sparrow, with saltmeadow cordgrass, saltgrass, and saltmarsh rush, but may also use farm fields and wet grasslands next to rivers, where sloughgrass and scaly sedge are key plants. Migrating Nelson’s Sparrows normally choose sites similar to breeding habitat, though they have been found in many types of wetlands, including bogs. In the nonbreeding season, wintering birds are found in tidal salt and brackish marshes near the major ocean and gulf coastlines.

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Food

Food Insects

Nelson’s Sparrows eat adult and larval insects, spiders and other arthropods, and amphipods, as well as seeds of grasses and wild rice during the nesting season. Their winter diet is predominantly seeds, with lesser quantities of animal matter. They feed mostly on the ground within marsh vegetation or around patches of wrack (dead vegetation deposited by high water) but also along the edges of ditches, mudflats, and pools. Like Saltmarsh Sparrows, they take a wide variety of prey, including beetles, crickets, grasshoppers, moths, ants, wasps, flies, leafhoppers, earwigs, beach fleas, wolf spiders, fiddler crabs, small mollusks, and nemertean worms. Seeds in the diet include cordgrass, wild rice, smartweed, panicgrass, lambsquarter, clover, and dandelion. Seeds are sometimes taken on the ground but often stripped from the seedhead while perching in the vegetation.

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Nesting

Nest Placement

Nest Ground

Nests are set in the grass, 2–10 inches above the ground, supported at the sides by stalks of vegetation and sometimes supported below by the “thatch”—a mat of bent-over cordgrass that is a common feature in saltmarshes.

Nest Description

The female chooses the nest site and builds the nest. Males may occasionally carry nesting material to a female. The base of the nest is composed of coarse, dead grass; this is lined with finer grasses taken from near the nest. The female approaches the nest secretively, usually along small pathways under the cordgrass. Sometimes, females construct multiple nests but lay eggs only in one. Nests may be cup shaped or domed; they average 2.6 inches tall by 3.7 inches across, with the interior cup 2.5 inches across and 1.9 inches deep.

Nesting Facts

Clutch Size:2-6 eggs
Number of Broods:1-2 broods
Egg Length:0.7-0.8 in (1.7-2.1 cm)
Egg Width:0.5-0.6 in (1.3-1.5 cm)
Nestling Period:8-11 days
Egg Description:

Greenish, covered with dark speckles.

Condition at Hatching:

Helpless.

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Behavior

Behavior Ground Forager

Nelson’s Sparrows typically forage and rest on or near the ground, making only short flights. However, displaying males make fast, direct flights of several hundred yards or more, skimming just over the tops of the marsh vegetation. Females occasionally make “lookout” flights, hovering momentarily a few feet above the marsh grass before dropping back down. Males sometimes sing from a conspicuous perch but more often from within marsh vegetation. They sing frequently at night but stop singing when another male approaches. Singing males are technically not territorial, as their territories broadly overlap those of other males—also true of Saltmarsh Sparrow. However, unlike Saltmarsh, male Nelson’s Sparrows do frequently fight with and chase off other males when a female is present. They also follow her, appearing to guard her from other males—even though there is no evidence of lasting pair bonds in this species. Male Nelson’s do not participate in nest site selection, nest building, or rearing of young. Scientists term this breeding system a “male-dominance polygyny,” in which a small proportion of males mate with a disproportionate share of females, sometimes despite resistance from the female. LeConte’s Sparrows sometimes chase breeding Nelson’s Sparrows where their ranges overlap, and likewise Savannah Sparrows show aggression toward nesting Nelson’s Sparrows in Maine. After breeding, small flocks of Nelson’s Sparrows are often seen feeding on seeds of cordgrass, especially in autumn. In the coastal marshes where both Saltmarsh and Nelson’s are found wintering, Nelson’s feed more in the upper (supratidal) parts of the marsh, mostly on seeds, while Saltmarsh feed lower in the tidal parts of the marsh, taking mostly insects and other arthropods.

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Conservation

Conservation Low Concern

Nelson’s Sparrow numbers have remained stable or slightly increased between 1966 and 2015, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 1 million and ranks the species a 12 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. The distinctive “Atlantic” or “Acadian” form of Nelson’s Sparrow has a tiny population, perhaps only 25,000 birds, and almost all of these nest in tidal marshes, which are gravely threatened by sea level rise. Historically, the destruction of both coastal marshland and of inland wetlands resulted in massive loss of habitat for this habitat specialist across its range. The stable or slightly increasing BBS trend over the last half-century probably reflects the recent success of wetland protection and restoration in Canada and the U.S., efforts that were originally designed with waterfowl populations in mind. Agricultural activities such as mowing, draining, plowing, burning, and spraying for insects can harm birds that nest in farm fields.

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Credits

Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

Smith, F. M. (2012). Photo Essay: Subspecies of Saltmarsh Sparrow and Nelson’s Sparrow. North American Birds 65:368–377.

Walsh, J., I. J. Lovette, V. Winder, C. S. Elphick, B. J. Olsen, G. Shriver, and A. I. Kovach. (2017). Subspecies delineation amid phenotypic, geographic and genetic discordance in a songbird. Molecular Ecology 26 (5):1242-1255. doi: 10.1111/mec.14010.

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