Jane has given the answer I think you're looking for, but there are actually two Hank Williams songs that mention the whip-poor-will or whippoorwill, which is a bird, a kind of nightjar found in Central and North America. It is named after its haunting call and is a cultural symbol associated with death and mourning. There is a useful article on it in Wikipedia, with a long list of songs that mention this bird: http://en.wikipedia.org/wiki/Whip-poor-will

There are copyright issues about posting complete lyrics, but, as Jane says, they can be found very easily on numerous websites just by googling either the names of the songs or, as she suggests, "Hank Williams" and "whippoorwill" (with two Os!).

Hank Williams Sr (1923-1953) recorded both these songs in 1949 (see http://hankwilliamsdiscography.com/index.html ). "I'm So Lonesome I Could Cry" begins:

Hear that lonesome whippoorwill,
He sounds too blue to fly.
The midnight train is whining low,
I'm so lonesome I could cry.

In "Alone and Forsaken" it is mentioned in the second verse:

The roses have faded, there's frost at my door
The birds in the morning don't sing any more
The grass in the valley is starting to die
And out in the darkness the whippoorwills cry."

So the song you're thinking of is probably "I'm So Lonesome", where it comes in line 1, and this is the more famous of the two songs. Indeed, it is no. 111 in Rolling Stone magazine's list of the 500 Greatest Songs of All Time and has been covered by many artists. See http://en.wikipedia.org/wiki/I'm_So_Lonesome_I_Could_Cry