I made my annual visit to Tri-Valley Wildlife Area (well, some years 2-3 visits) in Muskingum County, Ohio and this species was high on my list. I knew I would see it, as the large dry meadows there support many. But, as always, I first HEARD the sparrow and therein lies the rub. If one is not clued to their song, the chances of seeing a Henslow's sparrow plummet.
The song is quite charismatic, in my view, but could easily be overlooked. Indeed, to someone unfamiliar with it, the short aria might well be thought to be a cricket or some other insect. It's an explosive little chirp that is said to last about 3/5th's of a second. I'd say that's about the right duration. My Canon 5D IV, somewhat turbocharged by its auxiliary battery pack, can crank off about 8 frames a second. If I hold the trigger down on burst mode for the duration of the Henslow's song, I'll get maybe three or four images of the bird is some phase of delivering the song.
Data from the Ohio Breeding Bird Atlas II (field work from 2006-11) suggests there are about 11,500 singing males, and that figure probably has not changed significantly since. The majority of the birds now occur on old "reclaimed" strip mine lands in southeastern Ohio. Tri-Valley Wildlife Area is such a site, and in total there could be as much as 250,000 acres of reclamation grasslands in the southeastern hill country. While strip mining was - and is - one of the greatest ecological catastrophes perpetrated in North America, the use of post-mining habitats by grassland species such as Henslow's sparrow has been an unexpected positive effect.