CMBO Associate Naturalist Kathy Horn pointed out to me the other day that Villas WMA, besides the draw of its Red-headeded Woodpeckers, is an excellent place to practice identifying trilled bird songs, with Chipping Sparrows, Pine Warblers, Dark-eyed Juncoes, and (in the back) Swamp Sparrows all present. Chipping Sparrows will become a lot thicker there in the next couple weeks, while juncos have already begun fading away to the north.
CMBO devotes a fair amount of the Cape May School of Birding to identifying birds by sound, including several workshops devoted to just that. In particular, check out Birding By Ear - South (May 1-2), Warblers by Sight and Sound (May 3-4 or May 5-6) and Birding by Ear - North (May 30-31); more info here.
A trill, by the way, is a very rapid series of notes, usually too fast to count easily, and usually staying on the same pitch for most of the song but often rising or falling at the end. Chipping Sparrow's song is kind of the trill reference standard. Trills can have different qualities – some are musical, some are dry, some are buzzy, etc. - and also vary in volume and length. In North America, it's pretty much only certain sparrows and warblers that sing simple trills, although some wrens have trills or trill-like calls, and a few other birds sing songs with mutliple trills on different pitches and/or combined with other sounds.
One important trick in learning bird songs is to group species by similar songs in your mind and on the tapes or cd's you practice with. The "Birding by Ear" cd's do a reasonable job of this, but in this digital age it is fairly simple to load bird song cd's onto computers or i-pods and then create play lists in whatever order you want. For trillers in our region, I'd put together such a play list in the following order: