John and others,
One of the best ways to get this info is to use eBird to find recent locations where this bird has been seen (http://www.ebird.org). Click on the "Explore Data" tab, then select "Range and Point Maps". From there you can select a species, a time span, and a location or just zoom in like on a Google map. For your specific request, you can check out the link below to find the data for Marsh Wrens in Colorado.
I have limited the scope of the map to approximately the counties you had mentioned, and limited the search time span to between June and August for better results for this time of year.
The red
balloons/markers are for sightings within the last 30 days from today, the blue balloons are anything before that. By clicking on the balloon you can get more info about sightings from that location.
Hope that helps you John, and hopefully others can take advantage of the wonderful data source that is eBird. Good luck on your search.
Bryan Guarente
Instructional
Designer
The COMET Program
University Corporation for Atmospheric Research
Boulder, CO