Henslow's Sparrows stage at Peck Farm on the way south

Two Henslow's Sparrows were reported in the tallgrass prairie at Peck Farm Park in Geneva on September 7, part of a handful of early-September checklists from the Geneva Park District's 385-acre park, in an area generally described as the western edge of the morainal country.^1, 2 Henslow's Sparrow (Centronyx henslowii) is a bird more people have read about than seen. It spends nearly all its time on the ground inside dense grass, walking and hopping rather than flying. When flushed it flies low for a few yards and drops back into cover. Its song is a dry two-syllable "tsi-lick," delivered from a low grass stem, often called the weakest song of any North American songbird.

Anyone trying to pick one out from the other small sparrows in the same grass should look at the head. Henslow's is stocky and short-tailed with a flat-looking, oversized head washed olive-green on the face and nape, and the breast is streaked dark. Grasshopper Sparrow, its closest lookalike, is paler and buffier with a clean unstreaked breast. Savannah Sparrow shows crisp streaking on white underparts and a yellow patch in front of the eye that neither of the others has.

What the birds at Peck Farm are doing is fueling up. Henslow's Sparrow breeds in Illinois.³ It needs large tracts of grass left unmowed and unburned long enough to build a thick mat of dead stems and litter, and the state's breeders have mostly left by late August. Birds still in northern Illinois in September are almost certainly migrants passing through at night toward wintering grounds on the southeastern coastal plain. In September they are still eating mostly insects, crickets and short-horned grasshoppers above all, plus beetles and caterpillars. The shift to a seed diet of wiregrass, sedge, ragweed and smartweed comes in winter, farther south.

The litter layer is the whole story for this bird. It nests low in the grass under standing dead stems from prior years, and it will not use a field that was burned or hayed the same season. That puts it in tension with prairie management, because tallgrass prairie needs fire to keep shrubs and trees out. Studies of burned grasslands show Henslow's numbers dip the first summer after a burn, peak in the second and third years as the thatch rebuilds, and fall off again by the fourth as the stand gets too rank. The answer managers have settled on is to burn in rotation, a third of the ground at a time, so some patch is always in the two-to-three-year sweet spot. The Geneva Park District manages roughly 300 acres of prairie across its properties and burns about 100 acres every 18 months, which is that rotation in practice.

Rangewide, the species lost enormous ground through the twentieth century as prairie and hayfield went under the plow, and estimates from Partners in Flight put its global population at roughly 410,000 birds, though figures vary by source. Since the early 1990s, idle grassland set aside under the Conservation Reserve Program has driven local increases across parts of the Midwest. Some Bird Conservation Network surveys in the Chicago region have suggested an increase over the past fifteen years, though the trend is not consistent across all survey sites and sizeable populations remain confined to large grasslands. The prairie at Peck Farm is far smaller than that. It is a place a migrant can drop into for a few days on the way south, and that depends on the grass being there, thick and unburned that season, when the bird arrives.