Once common on the tallgrass prairies of the Midwest and wet grasslands of eastern North America, the Henslow's Sparrow is a remarkably inconspicuous grassland bird. It prefers habitats with tall, dense vegetation and thick litter, whereas the allied Grasshopper Sparrow (Ammodramus savannarum) and widespread Savannah Sparrow (Passerculus sandwichensis) usually select drier sites with shorter and less dense vegetation. Henslow's Sparrow is often difficult to detect because it sings from inconspicuous perches on low forbs, shrubs, or grasses. Even when singing from an exposed perch, it is easy to overlook this sparrow's dry, thin, insect-like song, accurately described as a “feeble hiccup” (1). In 1881, C. M. Jones described the Henslow's Sparrow's song: “The musical performance of this bird has very little to commend it. When the muse inspires his breast he mounts to the top of a weed or some other object that raises him just above the grass. There he sits demurely until the spirit moves, when he suddenly throws up his head and with an appearance of much effort, jerks out his monosyllabic ‘tsip,' apparently with great satisfaction. Then, having relieved himself he drops his head and waits patiently for his little cup to fill again. Somehow I cannot watch him while thus engaged, without a feeling of pity for a creature so constituted that he can be satisfied with such a performance” (2).
Henslow's Sparrow populations have declined over the last half-century, and this species has been identified as a high priority for grassland bird conservation in eastern and midwestern North America (3). This species' population decline appears to be attributable to the loss, draining, and degradation of grassland habitats throughout its range. More recently, conversion of hay fields and pastures to row crops and other intensively managed forage crops, such as alfalfa, have contributed to the population decline, an estimated decline of 1.5% per year from 1966–2015 (4). Since the early 1990s, local population increases in some Midwestern states appear to be associated with the creation of grassland habitat by the Conservation Reserve Program (CRP). These local increases have been sufficient to reverse the range-wide population trend (2005–2015), although populations have failed to recover in many states (4).
Pioneering studies by Hyde (5) and Robins (6, 7) still provide some of the best information on this species' behavior, ecology, and natural history. More recent work has provided important new information regarding this species' nesting biology (8, 9, 10), wintering ecology (11, 12), and response to habitat management (13, 14). Despite the recent surge in research attention, many aspects of this species' ecology and breeding biology remain poorly known.