By mid-October, Palm Warblers have left their Canadian breeding grounds and are settled in the West Indies and along the Gulf and southeastern US coasts where they will spend the winter. They make winter birding fun in these areas, where it is easy to spot their characteristic tail bobbing and to listen for their cheerful song.
Palm Warblers fall into two subspecies, one known to have a pale belly and the other with a yellow belly. Pale-bellied birds (Western Palm Warbler, Setophaga palmarum palmarum) generally breed west of Ottawa, while yellow-bellied individuals (Yellow Palm Warbler, Setophaga palmarum hypochrysea) breed to the east. However, their ranges overlap and there is a hybrid zone where they come into contact.
A recent study of the two Palm Warbler subspecies used genetics to more precisely map their breeding grounds and the large hybrid zone found between them. They also characterized how genes flow between the two subspecies, and used this information to determine that the Palm Warbler population likely split in the mid-Pleistocene when separated by an icesheet.
If you host Palm Warblers, look at their bellies... Western, Yellow or hybrid?