I spent a few hours birding Boulder Creek with my brother this morning. At the very same spot where I found the Magnolia Warbler yesterday, we found a Blackpoll Warbler. We didn't see the Magnolia, though I heard it was seen earlier in the morning. The Blackpoll was feeding with a flock including Townsend's, Yellow-rumped, Virginia's, and Yellow Warblers, Western Tanagers, Plumbeous Vireo and Warbling vireos.
The flock later moved across the creek to the north side near the football practice fields. At that point the blackpoll was quite high in the Willow trees.
Here is a link to a photo of the Magnolia Warbler with a geotag of the location:
http://www.pbase.com/birdercellist/image/160134218
Good birding,
Thomas Heinrich
Boulder, CO
Nyc...@aol.com