To add to George's thoughts on the topic, there's a neat aspect of avian
life history that, albeit on the science heavy end of birding, is worth
considering when thinking about early migrants at this time of year. Given
the extreme physiological demands (e.g., migration, etc.) that birds are
faced with during the non-breeding seasons, the testis of males shrink to
about the size of the ballpoint in a pen (ouch!).  By the onset of the
breeding season they will have swelled to 400x that size...but this takes
time so, later breeding species have lower testosterone than, say, a
local breeder who's just about to start.  And since song is tied to
testosterone levels in most species, most migrants don't achieve the
volume, consistency, or even pattern of song that we're familiar with 
until
when we would normally expect to see them.  Those early breeding sparrows,
like Juncos, Towhees, Field, and Fox Sparrows are a great example since
their testosterone levels are getting up there enough for them to be
singing - but not quite as "well" as our local Cardinals who are about at
the onset of breeding.  Their "subsongs" and "plastic songs" are rarely
(if ever) included in audio sets so the sounds can sound completely
'atypical' for one species and more reminiscent of another; thereby
warranting the heightened role of visuals at this time mentioned by
George.

Cheers,

Dave

---------------------------------------------------------
Dave Ziolkowski Jr.
USGS PWRC, Laurel
dziolkowski at usgs dot gov