Grasshopper Sparrow breeds from April to August and migrates south in October and November. It winters commonly in Mexico and more scarcely in the southern U.S., with the greatest concentration in Texas. Its spring migration in April and May brings these birds suddenly back to their breeding areas.
Grasshopper Sparrow, like many other grassland breeders, has a somewhat patchy distribution. Although suitable grassland habitat is widespread in the Great Plains and upper Midwest, elsewhere it is a very patchy habitat. Forested areas, like the Rocky Mountains, northern Maine, the Black Hills of South Dakota, and the Sierra Nevada show up as a black spots on the map. conversely, areas such as the Delmarva Peninsula, the Central Valley of California, or eastern Washington, show up as bright. All of these areas have agriculture and hayfields that are suitable for Grasshopper Sparrows. At finer scale, this map shows even more detailed granularity in grasslands. Even individual faint pixels in western Massachusetts correspond to airport grasslands that are known areas for the species.
As a secretive grassland species, the period of singing is the peak of this species’ detectability. As with Wood Thrush and Northern Cardinal, these maps do not correct for detectability, so some of the apparent changes just relate to periods when Grasshopper Sparrow go silent. Thus, like Wood Thrush, the fall migration is hard to detect on these maps. Fall Grasshoppers Sparrows are so much harder to detect than singing summer birds, that it appears that the species just disappears when in fact thousands are passing southward in September and October. Ideally, future versions of these maps will be able to incorporate species-specific detectability variables and will start measuring abundance, not just occurrence (probability of detecting the species).