Warblers are small birds which eat mainly insects and spiders. They migrate through much of the continental United States in spring and fall, although a few breed in Colorado in the summer. All warblers move to the southern extremes of North America or beyond for the winter. Adult male warblers in spring and summer are the most identifiable of these birds, between their plumage and song. Females, immature males, and winter males may be very difficult to identify.

https://en.wikipedia.org/wiki/American_yellow_warbler
Author: Mdf. Creative Commons Attribution-Share Alike 3.0 Unported.

• Common warbler which, unlike most warblers, breeds across much of the continental United States including Colorado
• Male's breeding plumage is a bright yellow on the front with rusty streaks on breast; greenish tinge on back; brown in the wings and tail
• Females look similar, but not quite as bright yellow, and have plain breasts
• Song is high-pitched and sweet-sounding

https://commons.wikimedia.org/wiki/File:Yellow-Rumped_Warbler.jpg
Author: Ingrid Taylar.  Creative Commons Attribution 2.0 Generic.

• Probably the most commonly seen warbler across the continental United States during spring migration
• Note the yellow rump, which from a distance resembles a pat of butter
• Formerly considered two species, Myrtle (Eastern) and Audubon's (Western)