Maggie's Farm

They are the smallest of our warblers, but their call (at the link) is distinctive even if you can't see them foraging high in the forest canopy.

A factoid about these birds is that they breed in southern forests (in Spanish moss nests) and in northern forests (where conifers have Old Man's Beard lichen), but pass over a band across the middle of the US where there are neither of those tree parasites. 

In warbler migration season (right now, in the US) you will quite likely hear one in the woods in the morning.

With sound,  Parula Warbler 

I wish my memory for warbler songs were better. I forget them every time May comes along.

In the US and Canada during these weeks, go outside in the morning where there are brush areas and some tall Oak trees, and listen to the migrating warblers singing. Experts don't bother with binoculars. Warblers do not really warble, but they do sing.