Yellow-rumped "Audubon's" Warbler , Yellow -rumped "Myrtle" Warbler

Yellow-rumped Warblers were both visible and audible on many of the trails that I hiked while in Colorado. The photo (Above, 7-8-10) was taken on a hike up Deer Mt. in Rocky Mountain National Park. Note the yellow throat, which is one of the main differences between the Western ("Audubon's Warbler") and Eastern ("Myrtle Warbler") populations. 

The Eastern population, also known as the Yellow-rumped Myrtle, has a white throat that wraps around below a darker cheek. As you can see above, the Western Warbler's yellow throat does not extend to its cheek. The photo of the Myrtle (Below, 5-17-09) was taken in Rock Cut State Park in Northern Illinois.  It may have been passing through on its migration further North into Canada or the northern regions of Wisconsin and Michigan. The "Myrtle" will spend summers in the Northeast as well (New York, New Hampshire, Vermont, and Maine)

Below (6-26-09) is another Audubon's Warbler (or is it a Myrtle?), taken on the beautiful Alta-Meadow Trail in Sequoia Nat. Park, California. It appears to be a young bird as you can see the yellow throat feathers just coming in. ...Or as I read in the National Audubon Society Field Guide to North American Birds, the two species (or at least were once thought to be two species) hybridize "freely" where their two zones overlap. Perhaps this is a hybrid? Note the yellow and white throat feathers.