Centronyx henslowii

ADULTS: Characterized by large flat head, large gray bill, and short tail. The head, nape, and most of the central crown stripe are olive-colored, with the wings extensively dark chestnut. The breast is finely streaked. When flushed, the bird flies low and jerkily, with a twisting motion of the tail. Otherwise, it is shy and secretive, with its presence most often revealed by its song (Peterson 1980, National Geographic Society 1987). The sexes have similar appearances. A cloacal protuberance (male) and brood patch (female) are reliable indicators of sex for living birds in the hand during the period from May through September (Pyle et al. 1987).

JUVENILES: Juveniles are clay-colored above and streaked on the head and back with black. Below, a faint yellow with tinges of buff on the chin and throat. The sides of the throat are typically unstreaked although occasional streaking may occur (Roberts 1949).

EGGS: The eggs are approximately 18.3 x 14.4 mm in size (Graber 1968) and are white with spots or blotches of brown, mostly at the larger end.

NESTS: Nests can be either open or domed and they are located from 0-50 cm above the litter (Hyde 1939, Robins 1971, Flanigan 1975). Those nests that are off the ground are attached to grass or forb stalks. Hyde (1939) describes a "typical" domed nest in southern Michigan as being located at the base of a clump of grass with dead grass from the clump forming an arched roof over the nest. The single entrance is located at an oblique angle on the side of the nest. Occasionally a nest is placed in a depression in the ground (Johnsgard 1979), but most are at least two cm above the substrate. The nest is loosely woven with dead grass and lined with finer grasses and hair.

VOCALIZATIONS: The song is distinctive and diagnostic: a short, quiet "see-lick," accented on the second syllable (Peterson 1980, National Geographic Society 1987). Sometimes sings on quiet nights, Given this bird's secretive nature, an ability to identify its song is essential for reliable census and survey work.

The striped, olive-colored head and reddish wings together are diagnostic.

BREEDING: Open fields and meadows with grass interspersed with weeds or shrubby vegetation, especially in damp or low-lying areas, adjacent to salt marsh in some areas. Uses unmowed hayfields (abandoned if cut). Found in a variety of habitats that contain tall, dense grass and herbaceous vegetation (Smith 1968, 1992). Hyde (1939) describes a variety of preferred habitats: upland weedy hayfields or pastures without shrubs, wet meadows, drier areas of saltmarshes, grassy fields, and sedgy hillsides with recently planted pine seedlings (PINUS spp.). Graber (1968) found that their habitat was usually quite dense from 30-61 cm off the ground and reported them to be "adapted" to unmowed hayfields. In New York, Peterson (1983) found them in large, ungrazed fields, often on hilltops, with a variety of moisture regimes and no woody invasion. They are not typically associated with grazed areas (Peterson 1983, Zimmerman 1988), although they can survive quite well in pastures that are only lightly (Skinner 1975) or moderately (Smith and Smith 1990) grazed.

Wiens (1969), in his three year study, found four territories in the first and third years and none in the second year. He found that territories had a low percent cover of forbs, dense vegetation, a high effective vegetation height, little bare ground or low vegetation, and no trees, posts, or fence lines.

In eastern part of range, are reported in "...moist upland meadows not under the plow, grown up to clumps of ferns, tall meadow rue (THALICTRUM spp.), and scattered shrubbery" in Vermont (Kibbe and Laughlin 1985); "...in small swales, meadows or other moist grassy lands...in fields of heavy timothy or clover" in Massachusetts (Forbush 1929); and "...in grassy fields and meadows with scattered bushes and herbaceous plants, both in wet and dry situations" in New York (Bull 1974). In addition to the above types of habitats, occurrences have been reported in wet meadows vegetated by sedges (CAREX spp.), rushes (SCIRPUS spp.) and grasses, in fields of seedling pines, and in drier upland portions of saltmarshes in the Northeast (Craig 1979).

Along the Atlantic Coast, the birds probably nested on the edges of saltmarshes before the arrival of Europeans (Hyde 1939, Craig 1979). In West Virginia, Hall (1983) reports that Henslow's sparrows "...prefer fields with growths of orchard grass [DACTYLIS GLOMERATA] or a rank weedy growth." Birds in Maryland have occurred chiefly in broomsedge (CAREX SCOPARIA) fields and weedy sedge meadows, and occasionally in hayfields (Stewart and Robbins 1958). Near Louisville, Kentucky, "...exclusively in or near fields largely or wholly composed of the orchard grass much raised locally as a crop, and has preferred the crop fields to patches of untended grass" (Mengel 1965). In Kentucky, Mengel (1965) noted that the sparrow often occurred in association with sedge wrens (CISTOTHORUS PLATENSIS) and in dry, upland sites that were in marked contrast to the marshy, meadow habitats considered typical habitat by Hyde (1939).

In the Midwest and Great Plains regions, formerly bred in tallgrass prairie interspersed with forbs and shrubs. Where tallgrass prairie persists, J. Zimmerman (pers. comm.) reports that they still use it for nesting habitat in Kansas and Missouri. At present, typical breeding habitat includes neglected grassy fields, pastures and meadows with scattered shrubby vegetation, or hayfields with dense cover, usually in damp or low-lying areas (Whitney et al. 1978, Johnsgard 1979, AOU 1983). Bajema et al. (2001) document extensive use of grasslands on reclaimed coal mines in southwestern Indiana, and estimated a density of 0.16 males per hectare, suggesting an overall population of 'a few thousand' in the mine grasslands of southwestern Indiana.

In a brome grass/alfalfa/red clover hayfield in southwestern Michigan, they bred in areas with an intermediate moisture range, a continuous cover of grasses and sedges about 0.8 m high, occasional shrubs, mostly less than 0.9 m high, and accumulated litter (no data on litter depth or percent coverage) (Robins 1971).

Fall and Eliason (1982) located a nest in Hennepin County, Minnesota, at the top of a knoll in an old field. Timothy comprised about 80% of the biomass of the 0.5 m-high vegetation. Except for a few shrubs approximately 0.5 m-high, there was no woody vegetation within 100 m, the vegetation covered about 75% of the soil surface, and there was a complete litter layer up to five cm deep.

Wiens (1969) compared the vegetation structure at nests with the vegetation in unoccupied areas in Fitchburg, Wisconsin. Occupied areas had significantly lower coverage and density of forbs, especially broad-leaved types, and the forb height was significantly greater (occupied average height = 27 cm). In occupied areas, the vertical vegetation density was higher and the litter was deeper and covered a higher percentage of the soil surface (average depth = 4.3 cm, 93% coverage).

Nest is well-hidden in grass, either at base of grass tuft (usually) or to 40 cm up in stems of growing herbage.

NON-BREEDING: In migration and winter also occurs in grassy areas adjacent to pine woods or second-growth woods. No detailed descriptions or studies of the habitat requirements of the winter range are available.

PREDATORS: Potentially important predators include mammals, snakes, and birds of prey. A thirteen-lined ground squirrel (CITELLUS TRIDECEMLINEATUS) was observed taking a young sparrow from its nest in southern Michigan (Robins 1971). Hyde (1939) evicted a blue racer (COLUBER CONSTRICTOR) from a nest in Michigan. Remains have been found in the stomachs of northern harriers (CIRCUS CYANEUS) and sharp-shinned hawks (ACCIPITER STRIATUS) (Hyde 1939, Graber 1968). Skunks (MEPHITIS spp.), weasels (MUSTELA spp.), and raccoons (PROCYON spp.) also may prey on nests, especially those occurring in isolated fragments of suitable habitat.

PARASITISM: There have been very few reports of nests being parasitized by the brown-headed cowbird (MOLOTHRUS ATER) (Hyde 1939, Robins 1971). Considered an infrequent host for the brown-headed cowbird with a total of 11 instances of brood parasitism from Michigan, Illinois, Ohio, Rhode Island, and Maryland (Friedmann 1963). Since nests are very difficult to find, the available data probably are insufficient to support any final conclusions regarding the frequency or intensity of cowbird brood parasitism and its potential effects on populations.

Territory boundaries not well-defined (Ehrlich et al. 1992). Have been reported to nest in loose "colonies" with contiguous territories (Hyde 1939, Wiens 1969, Johnsgard 1979), although Robins (1971) found that most of the territories in his southwestern Michigan study area were separated by buffer zones where no breeding sparrows occurred. The average size of a territory ranges from 0.3 ha in southwestern Michigan (Robins 1971) to 0.6 ha near Madison, Wisconsin (Wiens 1969). Territory size has been reported to increase through the summer (Robins 1971, Johnsgard 1979), although this may reflect movements of adults in response to the wanderings of recently fledged young that still require parental care. Robins (1971) found that the average territory size was smallest and the population density highest in areas with the tallest and densest vegetation.

Conclusions regarding territory size and management strategies based on published information about territory size need to be interpreted with caution, since it is well known that territory size in many songbirds is closely related to the availability of food. When food is easily available, territory size tends to be smaller than when food is scarce. Likewise, although it is widely reported in the literature that Henslow's sparrows are colonial, it does not appear that they are more colonial than other sparrows. The fact that larger numbers tend to occur in more suitable habitat does not necessarily imply that this bird is colonial in the true sense of the word as it usually is applied to herons, gulls, terns, or colonially nesting swallows. The "clumping" may be a secondary effect of the clumped nature of suitable habitat in most situations.

No specific data are available on site fidelity but several authors have commented that local populations tend to be unstable from year to year (Hyde 1939, Wiens 1969, Robins 1971). On the other hand, birds are reported to have bred consistently in some undisturbed, protected areas, like Hayden Prairie in Iowa (Ennis 1959) and Goose Lake Prairie in Illinois (Birkenholz 1983).

Two broods of young per breeding season (Hyde 1939), perhaps three (Robins 1971), are raised. The female does most or all of the nest-building, taking five to six days to complete the process. Clutch size is from three to five eggs. First clutches are normally completed by 20-30 May in the central part of the range (Hyde 1939, Graber 1968). Second nests are initiated in July and August with some extending into September (Robins 1971). Only the female incubates the eggs and broods the young. The incubation period lasts about 11 days and the young stay in the nest nine to ten days. Females make most of the feeding trips during the first four or five days of the nestling period and about 50% of the trips during the latter half (Robins 1971). Young are tended by both parents, leave nest at 9-10 days.

Of the 11 nests found in a southern Michigan brome hayfield, six (54.5%) produced at least one young (Robins 1971). Only one of the 11 nests (9.1%) successfully raised all of the young. Seventeen young were produced from a total of 46 eggs (37.0%).