Palm Warbler is a very interesting species to look at in these occurrence maps, particularly because it consists of two distinct populations. These two populations are sometimes referred to as Yellow Palm Warbler (the eastern subspecies) and Western Palm Warbler. The differential movements of these two populations could not be more obvious on the animation above and the differences in migration timing between the two subspecies are no less marked than differences in migration timing between many species!
In the winter, both subspecies winter in the southeastern United States, but it is the Yellow Palm that winters farther west, occurring in grassy southeastern pine forests west to east Texas. It occurs in the piney portions of northern Florida only and is the one that is less likely to winter well to the north along the Atlantic Coast. Western Palm Warbler, by contrast, winters throughout Florida, occurring in scrubby and beach habitats also the way to the Florida Keys as well as in the Bahamas and Greater Antilles. When small numbers of Palm Warblers are found on Christmas Counts from Virginia north, the vast majority prove to be Western Palms; this subspecies is also most likely to occur as a vagrant to the West.
In spring migration, there is an obvious early movement along the East coast that precedes the inland movement by a couple weeks or more. Starting in early April, there is a northeastward push that brings Palm Warblers to New England by about 20 April. This represents the spring movement of Yellow Palm Warbler, since Western Palm is almost unknown in New England in spring. That is followed by a surge to the northwestward, involving Western Palms. This movement occurs along a broader front as this is the larger population. You can envision these birds moving from the Caribbean to Canadian breeding areas north of the Great Lakes. We know these are all Western Palms, since Yellow Palm is generally not found west of the Appalachians in spring. Watch for the ‘X’ formed by the northward migrations of the two Palm Warbler subspecies.
In fall, the picture is more muddled. While Yellow Palm remains primarily an East Coast form (occurring largely east of the Appalachians), Western Palm can be extremely common along the length of the East coast as well. Western Palm is an earlier spring migrant, with the vanguard of migrants reaching the lower 48 in late August. Significant September movements can occur, especially mid to late September. Yellow Palm, by contrast, migrates in October and even in northern areas the species is almost never seen away from breeding grounds earlier than late September. These two separate fall pulses exist on the map (especially in a later presence of Palm Warblers along the East coast), but are much less obvious than in spring. To really see the differences in the fall pattern between the subspecies we would need to look at the data separately for the two subspecies. Fortunately eBird does collect data on subspecies, so this is theoretically possible. If you know what subspecies of Palm Warbler you are seeing, by all means report it as such. If the two forms are even split as species, you will be glad to have your data separated out!