Nelson’s Sparrow

Nelson’s Sparrows were initially named the Nelson’s Sharp-tailed Sparrow. These birds were named after the American Naturalist, Edward William Nelson. He was responsible for conducting field surveys for the U.S. Department of Agriculture and was the chief of the Bureau of Biological Survey from 1916 to 1927.

About Nelson’s Sparrows

Nelson’s Sparrows are categorized into three subspecies based on their primary location. Birds from Yukon to Minnesota are labeled “A. n. nelsoni”, birds from Hudson Bay to James Bay are labeled “A. n. alter”, and birds from Canadian Maritimes to Massachusetts are labeled “A. n. subvirgata”.

Interestingly, the A. n. nelsoni and A. n. alter subspecies are genetically very similar, the subvirgata are not. This has led some researchers to speculate that the A. n. subvirgata or “Atlantic” or “Acadian” form is actually a different species.

They are unsociable birds who prefer leading solitary lives. However, an extraordinary fact is that they practice male-dominated polygyny. No pair bonds are formed and a small proportion of males mate with a disproportionately large number of females, sometimes by force. The males do not engage in any nesting or child-rearing activities.

Female Nelson’s Sparrows tend to be skittish and avoid confrontation, choosing instead to dive and hide when faced with predators. Nonetheless, certain female subspecies in New York have turned aggressive towards males when forced to copulate. Nelson’s Sparrows seem especially interesting, don’t they? Let’s learn a little more about them.

● Nelson’s Sparrow Photos, Color Pattern, Song
● Nelson’s Sparrow Size, Eating behavior, Habitat
● Nelson’s Sparrow Range and Migration, Nesting

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Nelson’s Sparrow Color Pattern

Nelson’s Sparrow plumages are bright colors. The rich warm tones of saffron-yellow and brown cover the whole body. They have very distinctive yellow eyebrows and whisker marks that frame their greyish cheeks.

The yellow coloring continues onto the breast, throat, and sides with ample streaking on the breast and sides in the interior subspecies. The definitive basic and alternate plumages have dark brown to olive backs, usually with four distinct lateral grayish or white stripes along them.

The coloring on the Atlantic subspecies is more muted with pale-grey upper backs and pale buff on the undersides. There is pale streaking on their breasts. Juvenile plumages are similar to the adult ones, however, they have a blackish crown with a broad orange-buff median strip and broad supercilium that are not present in adults.

Description and Identification

Nelson’s Sparrows are tiny, short-tailed, pot-bellied sparrows. The easiest way to spot them is to look for the distinctive bright yellow-orange supercilium stripe above the eye and the brownish post-ocular stripe.

The interior species are more brightly colored than their coastal counterparts and have well-defined cinnamon streaks on the flanks. The Atlantic coastal birds have blurry streaks on the flanks.

It is easy to confuse these birds for the Saltmarsh Sparrow, as it is similar in appearance. Nevertheless, the size of the bird can help distinguish the two as Nelson’s Sparrows are generally smaller than Saltmarsh Sparrows. Furthermore, Nelson’s Sparrows also have shorter bills and a well-defined white belly patch that is absent in Saltmarsh Sparrows.

Nelson’s Sparrow Song

The primary song consists of short, raspy trills that sound almost mechanical. The “p-tssssshh-uk” sound they make sounds like when a hot iron is put in a bucket of water. In an open country situation, the wind makes it difficult for the sound to travel long distances.

Nelson’s Sparrow males tend to sing frequently at night but stop singing when another male approaches. However, unlike many other birds, they don’t use singing to guard their territory. Surprisingly, these birds do not have well-defined territories as any individual male’s territory will broadly overlap with those of other males.

Additionally, males may use a flight song during a parabolic flight display to entice females. This song is more complex than the primary song. Nelson’s Sparrow females only utter alarm calls (“chic, tic”) when dependent young are present. The tic-twitter alarm may be made when an aerial predator approaches, but usually, the bird dives into cover.

Nelson’s Sparrow Size

Nelson’s Sparrows are relatively small birds. They weigh between 19 and 21 grams. Interestingly, the size of the three subspecies varies slightly. The coastal birds on average tend to be larger and have longer bills than the prairie marsh birds.

Moreover, sexual dimorphism is present. Male wing chords range between 54 to 64 millimeters in length, whereas female wing chords range only 52 to 60 millimeters in length. The male tail length is approximately between 46 to 56millimeters, while the female tail length ranges from 43 to 52millimeters.

Nelson’s Sparrow Behavior

Nelson’s Sparrows are usually unfriendly birds who tend to pursue solitary lifestyles, especially during the breeding season. However, there’s a promiscuous relationship between the sexes. Females may temporarily solicit copulation, through crouching and raising their bills and tails from the nest, during the breeding season.

It is intriguing to learn that the mating system on a whole is male-dominated polygyny. A small proportion of males will regularly mate with a disproportionate number of females, sometimes despite their resistance. During the breeding season, males are aggressive and frequently fight over females. Many males may chase a single female in circular flight paths. The males do not seek nesting territories and copulate with females in their nests. In addition,
they do not undertake child-rearing and they don’t make any pair bonds.

However, during the post-breeding and migratory seasons, the birds may form loose feeding groups of 10 to 40 birds. Nevertheless, the stability and daily composition of these groups remain unknown.

Nelson’s Sparrow Diet

Nelson’s Sparrows forage by hopping on the ground or making rapid, short flights. They occasionally hover a few feet above the marsh vegetation before quickly dropping back down.

The primary diet during the nesting season consists of cordgrass, dandelion, and wild rice seeds, as well as spiders and other arthropods, adult and larval insects, and amphipods. During the winter months, there is an increased reliance on the vegetal matter.

Nelson’s Sparrows also hunt for prey such as moths, ants, leafhoppers, and fiddler crabs. The seeds they eat are occasionally picked up from the ground, but often they are stripped from seed heads while perching on the marsh vegetation.

Nelson’s Sparrow Habitat

Nelson’s Sparrows have a more varied habitat than the Saltmarsh Sparrow. Nevertheless, all three subspecies choose marshland that has a variety of vegetation.

The inland subspecies (A. n. nelsoni) choose freshwater marshes as their primary habitat. The prairie potholes hold a wide range of plant species such as cattails, cordgrass, reeds, river grass, and foxtail barley.

The A. n. alter subspecies use the sedge bogs above the high tide line. These regions have vegetation like sweetgrass, buckbean, baltic rush, and blue joint. The Atlantic subspecies favor tidal wetlands to be their primary habitat, similar to Saltwater Sparrows. The main plants in these wetlands are salt marsh rush, salt meadow cordgrass, and scaly sedges.

Despite the differences in the breeding season habitat, intriguingly, all three subspecies choose similar habitats for overwintering. These wintering habitats include tidal salt and brackish marshes near the major ocean and gulf coastlines.

Range and Migration

Even though all three subspecies are native to the North American continent and only migrate at night, the three follow different migratory patterns.

The western subspecies of the Nelson’s Sparrow (A. n. nelsoni) have the most widespread migration out of the three subspecies. During the spring migration season, birds wintering in eastern Texas fly up to North and South Dakota and the Canadian Prairies. Birds wintering in Georgia fly along the south of the Appalachians. The fall migration lasts longer. A majority of the subspecies fly south through the Mississippi River basin directly to the Gulf Coast.

The A. n. alter subspecies migrates north along the coast to reach places like New York and Toronto in the spring season. Occasionally they move west to reach the states of Michigan and Ohio. The birds overwinter in regions similar to the A. n. nelsoni subspecies and fly along the western side of the Appalachian mountain trail to reach the gulf coast.

The A.n. subvirgata subspecies migrates in a south-to-north fashion along the coastline. They arrive at the breeding grounds by early June. They depart these breeding grounds by the first week of December to overwinter in coastal marshes in southern regions.

Nelson’s Sparrow Lifecycle

Nelson’s Sparrow clutch sizes range from 2-6 eggs per brood, with 1-2 broods per season. Both the egg incubation period lasts 1-3 days and the nesting period can last anywhere between 8 to 11 days. The eggs appear short, elliptical, and greenish with distinct dark speckles. The size of the egg doesn’t vary by subspecies and is generally between 17 to 20 millimeters long.

Nestlings leave the nest between 8 to 11 days after hatching. After leaving they mostly run on the ground, but many begin flying after 3 to 5 days. The juveniles gain independence approximately 1 month after hatching and form a loose flock for the migratory season. In the wild, males live for a maximum of 10 years whereas females live for a maximum of 6 years.

Nesting

A surprising feature of this species, that sets it apart from other birds, is that the males do not select the nesting site or help in the construction of the nest. In extremely rare circumstances, males may help females by carrying construction materials to the nest site.

The female constructs the base of the nest with course dead grass and lines them with soft fine grass available near the nest. These nests can be cup or dome-shaped and measure 2.6 inches tall by 3.7 inches across. The interior cup measures 2.5 inches across and 1.9 inches deep. The nest is constructed at a height to ensure that it is protected from flooding.

Female Nelson’s Sparrows choose secluded and secretive areas to build their nest. They enter and leave the nest under the thick cover of tall cordgrass to ensure that they’re not spotted. Females often construct multiple nests but only use one of them. These preventive tactics may be employed to prevent aggressive copulation attempts from new males.

Anatomy of a Nelson’s Sparrow

Nelson’s Sparrows are plump little birds that are known for their pot-bellies and short tails. Other identifying features of their anatomy include a round-topped head and a spiky bill. Furthermore, the crown is stark grey and bordered with dark brown edges.

Nelson’s Sparrows’ plumages consist of nine functional primaries and nine secondaries that include three tertials and twelve rectrices. All the subspecies generally have brown irises.

The bill is generally pinkish in hatchlings, but it becomes darker with age. In fully grown adults, the upper mandible is darkly colored and paler towards the base. Like the coloring on the bill, the legs and feet of hatchlings are pinkish but turn dark. Adults have dark olive-brown feet. It is crucial to note that the wingspans and lengths of female birds are shorter than males on average.

Final Thoughts

Nelson’s Sparrows are fascinating little birds who live in varied marshlands throughout the North American continent. Males can turn aggressive when competing for a single female but normally try to avoid any confrontation with predators by silently retreating under vegetation cover. Females do the same, especially when protecting the nest.
Observing their unique and shocking mating ritual can seem strange but it has allowed the species to survive despite the decline in their habitat.

Both natural changes and human intervention have contributed to the sharp decline in feasible primary habitats for these birds. For example, by 1920, it was estimated that 80% of all salt marsh had been converted to agricultural land in the upper Bay of Fundy. Even though these birds can adapt to a variety of habitats, inconsistent rainfall has led to
destroyed nests and an increased mortality rate.

Ornithology

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