All:
Field experience has taught me to look (and listen) for Blackburnian Warblers in the tops of Oaks or Walnut Trees. I cannot recall ever seeing a Blackburnian at mid to low level in a tree (unless a bubbler is nearby), always at or near the top  Had one this morning near Martha Lafite Sanctuary in Liberty working the Pin Oaks near the entrance. These Oaks are basically on a hilltop, which seems to be a factor also (although irrelevent in a migrant trap like Tower Grove Park, which has some fine Oaks!) Learning the vocalization is very helpful in locating this species, but it is extremely high-pitched and I'm not sure I always hear them now.
Best,
Doug Willis
Liberty, MO
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The Audubon Society of Missouri's Wild Bird Discussion Forum
ASM Spring Meeting: April 29 - May 1, 2011 in Kansas City, Missouri, http://mobirds.org/Meetings/sprmtg2011.asp