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, insectlike song, accurately described as a “feeble hiccup” ( Sibley 2000 ). In 1881, C. M. Jones described the Henslow's Sparrow's song in the following way: “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” ( Jones 1881 ).
Henslow's Sparrow populations have declined over the last half-century, and this species has recently been identified as the highest priority for grassland bird conservation in eastern and midwestern North America ( Herkert et al. 1996 , Pashley 1996 ). This species' long-term 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 continued population decline, estimated to be -7.5% annually over the last 3 decades (1966–2000), which is the steepest decline for any species of grassland bird in North America ( Sauer et al. 2001 ). Very recent (after 1990), local population increases appear to be associated with the creation of undisturbed grassland habitat by the Conservation Reserve Program (CRP). However, it does not appear that these local increases have been sufficient to offset overall population declines ( Sauer et al. 2001 ).
Pioneering studies by Hyde ( Hyde 1939 ) and Robins ( Robins 1971a , Robins 1971b ) 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 ( Winter 1999a , Reinking et al. 2000 , Moss 2001 ), winter ecology (Plentovich et al. Plentovich et al. 1998a , Plentovich et al. 1999 ), and response to habitat management ( Zimmerman 1988 , Herkert 1994b ). Despite the recent surge in research attention, many aspects of this species' ecology and breeding biology remain poorly known.