Ammodramus henslowii (Audubon, 1829)
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Basis for Listing
The Henslow's sparrow breeds across the northeastern United States, from northwestern Minnesota east to New York and south to Oklahoma and North Carolina. Across the country, the number of Henslow's sparrows declined by over 68% between 1966 and 1991 (Herkert 1994). In Minnesota, Henslow's sparrows have been a widespread but relatively uncommon summer resident throughout the southern two thirds of the state. The species requires uncultivated grasslands and old fields with standing, dead vegetation and a substantial litter layer. Areas used one year may be abandoned the next year if the grass has become too long or too short. Therefore, the Henslow's sparrow's distribution in the state is sporadic and the extent of its former range is difficult to delineate.
The Henslow's sparrow is an inconspicuous, secretive bird whose quiet, short, two-syllable song is easily overlooked. This sparrow's most distinctive feature is its large, relatively flat, olive-colored head with dark stripes. Its streaked chest, rufous-tinged wings, and short tail are also diagnostic.
Henslow's sparrows prefer uncultivated grasslands and old fields with stalks for singing perches and a substantial litter layer. Litter depth, vegetation height, and the number of standing, dead herbaceous stems are important components of occupied areas (Herkert 1994). Grasslands over 100 ha (247 ac.) are preferable, but smaller areas of suitable habitat are also used by this species.
Biology / Life History
Henslow's sparrows are believed to overwinter primarily in Florida, Georgia, Louisiana, and Texas. They have been described as breeding in loose colonies, in which several pairs occupy a small grassland (Hanson 1994), but individual nesting is not uncommon. Males utilize tall forbs as singing perches and vantage points from which to defend their territories, which are approximately 1 ha (2.5 ac.) in size. Females build nests at the base of grass clumps, and often construct runways through the leaf litter for use in escaping predators. Females lay an average of 4-5 eggs, and incubate them until they hatch in about 11 days. The young grow quickly and are fed by both the male and female. There is usually time for a pair to raise two broods in a season. Brown-headed cowbird (Molothrus ater) parasitism of Henslow's sparrow nests is believed to occur infrequently, as the nests are very difficult to find. More significant causes of nest failure include trampling by cattle and predation by snakes and small mammals (Herkert 1994). Henslow's sparrows forage in the leaf litter for insects, spiders, small mollusks, and seeds.
Conservation / Management
The grasslands inhabited by Henslow's sparrows require management to maintain their attractiveness to the species. Because of their need for tall vegetation and a substantial litter layer, Henslow's sparrows do not occupy heavily grazed areas. Fire management may be necessary at times to restore or improve grassland habitat for this species, but few birds will use these areas during the first several years after burning. Mowing is compatible with habitat use by Henslow's sparrows if vegetation is allowed to grow to an acceptable height and density by the next breeding season. These management practices, and possibly light grazing, can be combined on a rotational basis to maintain suitable habitat (Herkert 1994). Because Henslow's sparrows are primarily insectivorous, they may be adversely affected by pesticides (Pruitt 1996).
Conservation Efforts in Minnesota
A 1988-89 habitat evaluation of 23 sites where Henslow's sparrows had been sighted in previous years revealed that one-third of the sampled sites were no longer suitable because of urban development and agricultural use (Hanson 1994). For a number of years, Great River Bluffs State Park (formerly O.L. Kipp State Park) supported a stable population of Henslow's sparrows. This population was intensively studied between 1987 and 1989 and was found to consist of 19-23 adults. From 1996 to 2006 the number of singing males at Great River Bluffs State Park fluctuated between 1 and 28 singing males (S. Fritcher, DNR, pers comm.). A management plan for Henslow's sparrows was drafted in 1996 (Bolin 1996). Some of the habitat maintenance activities recommended in this plan have been implemented, including prescribed burns, removal of woody vegetation, and fertilizing brome grass to increase vigor. Since 1996, observations of Henslow's sparrows have increased, with breeding season records coming from counties scattered throughout western, central, and southern Minnesota. Of particular interest is the consistent presence of this species in several large tracts of native prairie in western Minnesota. Increased monitoring of these western populations is needed to assess the status of Henslow's sparrows at these sites.