On May 30th, 2009 I made the trip to Clarion, PA to see Henslow's Sparrows at the reclaimed strip mines at Piney Tract IBA, where I'd read there may be 100 pairs.
Henslow's Sparrows have a short, insect-like song that has been famously described as "one of the poorest of vocal efforts of any bird" by Roger Tory Peterson. Each male sings a single song that is made up of about five to seven notes. I recorded five different males; two had essentially identical songs, another two differed only by an extra introductory note by one of them, and the fifth had a very different song. So there were three distinctly different songs in a sample of five.
These birds were recorded in the afternoon (Henslow's Sparrows also sing at night), and they mostly sang from the small bushes that dotted the grasslands:
Henslow's Sparrow breeding habitat at Piney Tract IBA
Note: when observing Henslow's Sparrows, the green tinge of the head is not obvious in a lot of views, but they are readily told from Grasshopper Sparrows by the necklace across their breasts.
These two had nearly identical songs, the first and last notes being slightly diferent:
The next two differed by only by an additional introductory note by one bird:
The next had a song that is noticeably different even by ear.
It would be interesting to know how many different songs there are among this population. As can be seen on the sonagrams, it is only our inability to decipher the song's complexity that makes it a "poor vocal effort".
Piney Tract also had many Savannah and Grasshopper Sparrows, Bobolinks and Eastern Meadowlarks, and I was fortunate enough to see a male Northern Harrier performing its somersaulting flight display. A place that was well worth the 300 mile trip from Philadelphia.