There are two other bird calls that sound similar to Worm-eating Warbler.  They are Chipping Sparrow and Pine Warbler.  If you hear recordings of the three, they are somewhat different, but in the field, only hearing one of them, it can be challenging.  The key is habitat, which you mentioned in your post, using the phrase "steep, wooded, hills."   That tells me that it is likely Worm-eating Warbler, which prefers that habitat as it migrates through our area, which is right now.  Chipping sparrows tend to occur in any park like areas that have mowed grass, and Pine Warblers will most always be within site of Pine trees.  Be careful when you hear the call in mixed habitats.  I have been on Red Bridge Road in Christian County where I heard the call that I thought was Worm-eating Warbler due to the steep, wooded hilly habitat, but located the bird, which was a Chipping Sparrow.  There was a private residence with a mowed yard across the road.  There is an area of Barry County on Butler Hollow Road where Worm-eating is regularly seen, but I have to be careful because at the top of the ridge, within site, is a cluster of pine trees.  When in doubt, find the bird and make sure. 
Jerry Williams
Republic, Greene County, Mo.

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