Rumor has it that Chestnut-sided was a very rare bird in the times of John James Audubon, and that he may have only seen the species once! This is hard to fathom today, as it is a common breeding species of cutover forests and open glades throughout the Northeast.
As a specialist of regenerating forest, it may indeed have had very low populations in the 1700s when eastern forests were largely healthy and mature, while second-growth was comparatively rare. As the primary forest was harvested over the past couple centuries (there is essentially no virgin forest left anywhere east of the Rocky Mountains), the populations of Chestnut-sided Warbler surely got a boost. With ongoing lumbering and lots more edge forest than previously, Chestnut-sided Warbler remains fairly common today.
The various eastern-breeding North American warblers follow three primary migration routes: an eastern Caribbean/Florida route; trans-Gulf (roughly from Gulf Coast states to the Yucatan Peninsula); or circum-Gulf (around the Gulf of Mexico and through mainland east Mexico). Chestnut-sided breeds in the East winters primarily in eastern Central America. Its spring passage is around and over the Gulf of Mexico rather than down the Florida Peninsula. Compare the spring migration of Chestnut-sided Warbler (a classic trans-Gulf migrant) to that of Blackpoll Warbler (which migrates north through the eastern Caribbean). However, Chestnut-sided is especially interesting in that its fall migration is significantly more easterly (as discussed in Garrett and Dunn’s 1997 A Field Guide to the Warblers of North America). Notice how the fall movement of this species tends to focus on the eastern Gulf states and Florida. Some other species may show shifts in the routes of spring and fall migration, so watch for this pattern in other STEM maps.