With the split of the eastern and western versions of the Winter Wren it appears that there is some discussion regarding the possibility that our area may host a few Pacific Wrens from time to time. While the Sibley guides have put Missouri squarely in the middle of the Winter Wren territory (http://www.sibleyguides.com/2010/08/new-draft-range-maps-for-winter-wren/) others are suggesting that there may be some overlap and occasional interloping Pacific Wrens in the east. See Bill Pulliam's essay here http://bbill.blogspot.com/2010/08/winter-vs-pacific-wren-vocalizations.html. .

It appears that the primary visual mark to differentiate the two is throat color (rusty red for Pacific, mousy brown for Winter) but that might be somewhat variable.  It has been suggested that the most effective means of determining if a wren is Pacific or Winter is by its call and song.  See these articles http://earbirding.com/blog/archives/774 and  http://earbirding.com/blog/archives/826.  The complexity of his argument is a bit beyond me right now but suggests that there is a tonal quality difference in the call not unlike the pip notes between Hairy and Downy Woodpeckers (my comparison).

Scott Laurent
Kansas City, MO (Jackson County)

The Audubon Society of Missouri's Wild Bird Discussion Forum
ASM Website: http://mobirds.org/