These warblers winter in western South America.
They are usually our latest warbler to show up in migration, often in late May or even early June.
In Ontario they nest in what Fred Bodsworth called the “land of little sticks” where stunted spruces push out into the tundra near the Hudson Bay lowlands.
Winter is slow to release its grip in this country so there is no hurry for the Blackpolls to arrive there.
The trees are generally in full leaf by the time these warblers reach southern Ontario, so the observer must listen to their high-pitched, staccato, crescendo, then diminuendo, song: “ti, ti, ti, ti, TI, TI, TI, ti, ti, ti,”.
When finally located the spring male with its black cap, white cheeks and underparts, with black streaks along the sides and on its greenish back, is easily identified: the female is green above and greenish white below, with white wing bars and white undertail coverts, streaked with black above and below.
Fall birds have sometimes been confused with fall Bay-breasted Warblers but are much more streaked above and below than that species and have white undertail coverts, not buffy as in the Bay-breasted (which usually shows a hint of the brownish sides even in fall). (The pale legs of the Blackpoll are sometimes used to separate them from the darker-legged Bay-breasted Warblers).
Dr. J. Murray Speirs