Look up chestnut-sided warblers in most field guides and the illustrated breeding range will encompass southeastern Canada, the northern Midwest, the Northeastern States and the Appalachian Mountains. However, over the past two decades, this attractive songbird has also been nesting along the Colorado Front Range.
Yesterday morning, while making my daily survey of our Littleton farm, I spotted a warbler flitting about one of our large Siberian elms; due to our persistent mild weather, the tree still retains most of its leaves. After following the bird with my binos for a minute or so, I was able to identify it as a chestnut-sided warbler in fall plumage; its bright olive cap, greenish wingbars, white eye rings and white, unstreaked breast and abdomen led to the identification. These insectivores typically snare insects from the underside of leaves, favoring open, second-growth woodlands.
After a summer in the U.S. or Canada, they head for the Tropics, usually to Central America or northern South America. Why yesterday's visitor was so tardy is hard to say but I suspect our warm autumn weather is the primary culprit; then again, songbirds generally migrate in response to the sunlight cycle, not the daily temperatures. While yellow-rumped warblers are common winter residents throughout much of the U.S., I've never encountered another warbler species so late in the year.