Movement Migration Subspecies of grasshopper sparrows differ in their
migratory behavior. The birds breeding in most of the eastern US and southern Canada (
A. s. pratensis) migrate latitudinally up to several thousand kms each year, spending winters along the
Gulf of Mexico and the Caribbean. The birds breeding through most of the Western part of their North American range (
A. s. perpallidus) winter from Texas west to California, and south into the highland desert grasslands of northern and central Mexico.
A. s. ammolegus, which breeds in the desert grasslands of SE Arizona, SW New Mexico, and adjoining states in northern Mexico, appears to be a short-distance, partial migrant, with some birds remaining resident year-round, and others likely moving further south in winter. Birds of multiple breeding populations and subspecies winter together in SE Arizona. Grasshopper sparrows breeding outside of North America, including those living in the Caribbean, Central America, as well as the Florida grasshopper sparrow are not known to migrate. Males typically migrate north in the spring a week or two earlier than do females. Data from
light level geolocators indicate that males are present on the breeding grounds from April to October, Individuals migrated an average of ~2,500 km over ~30 days. However, relatively few movement tracks for this species exist so far, potentially due to the sparrows' secretive behavior during most of the year other than the breeding season. Few recoveries of banded birds have ever been made, especially at sites other than a birds' initial banding location. In Oklahoma, grasshopper sparrows are one of the species most frequently found dead after colliding with windows, despite rarely being detected in migration.
Dispersal In Maryland, a study of
natal dispersal (the movement from a natal site to the site of first reproduction) revealed that most fledglings remained within their natal habitat during the summer months after fledging, and most recaptured fledglings were encountered within a couple hundred meters of their natal nests. Although distances recaptured from the nest increased with time, the average fledgling was recaptured only 346 meters from its nest and recaptured 33 days after fledging. The longest distance detected was 1.6 kilometers, which was accomplished in less than 20 days. Grasshopper sparrows are unusual in their particularly high rates of breeding dispersal (i.e., one-way movements between successive breeding sites). Like some other grassland-dependent species, their dispersal movements lead them to be called semi-nomadic, as they opportunistically take advantage of suitable habitats as they appear on the landscape. Return rates of adult grasshopper sparrows to their previous breeding sites (site fidelity) differs widely between populations. Site fidelity is far higher in eastern parts of the breeding range, with >50% of adult males returning to breed in subsequent seasons at a site in Connecticut and over 70% in Maryland. In contrast, 0% of adult males in Nebraska returned to breed in subsequent years, 8.9% returned in Montana, In northeastern Kansas, individual birds also commonly disperse within seasons between nesting attempts. Between 30 and 75% of birds move over 100 m within season, and individuals were detected defending new territories or nesting up to 8.9 km from areas they occupied earlier in the season. Little is known about the movements of birds during winter, but inter-annual variation in abundance in northern Mexican grasslands is positively related to related to broad-scale variation in vegetation, rainfall the previous summer, and plant productivity. Grasshopper sparrows build inconspicuous, dome-shaped nests on the ground, typically very well hidden amongst
grasses and
forbs. The nests have a small side opening and are usually made of a mix of dead and live grasses. Nests are built on the breeding pair's territory in areas away from shrubs and trees.
Offspring Females typically lay between 4–5 eggs per
clutch. Grasshopper sparrow eggs are smooth and oval-shaped. They are a cream-white color, with reddish-brown speckling that is concentrated towards the larger end of the egg. and feeding young, but this behavior is apparently not ubiquitous. Upon leaving the nest, the offspring often remain in the area. Parents provide post-fledging
care, but the duration and level of care is unknown.
Brood parasitism Grasshopper sparrows are a common host of the
brown-headed cowbird, a
brood parasite which lays their eggs in the nests of other species. Cowbirds can be detrimental to host species because they often remove host eggs and nestlings or destroy nests to initiate another nesting attempt, allowing for future parasitism. However, they do not necessarily reduce the chances of nests successfully fledging young. The rate of brood parasitism by brown-headed cowbirds varies across the grasshopper sparrow's range and depends on habitat characteristics such as the amount of woody vegetation and whether the land is grazed. In surveyed populations, 2–65% of nests were parasitized by cowbirds. Snakes, small mammals such as rats, larger mammals such as skunks, armadillos, hogs, and opossums, and even ants have been observed eating eggs and nestlings. Other causes of nest failure include flooding of the nest cup due to rainfall, trampling by livestock or humans, and abandonment by the parents. In Florida, grasshopper sparrows may be eaten by some growth stage of invasive snakes such as
Burmese pythons,
Reticulated pythons,
Southern African rock pythons,
Central African rock pythons,
boa constrictors,
yellow anacondas,
Bolivian anacondas,
dark-spotted anacondas, and
green anacondas. ==Vocalizations==