Good questions, Valerie (in New York City).
Depending on your location in North America, you may begin seeing warblers dispersing and migrating as early as late June, though July and August are the more common months to start seeing them away from their nesting areas.
For example, in the Midwest and East, Yellow Warbler, Tennessee Warbler, and Louisiana Waterthrush are well known to be early migrants. (I remember a few years ago reading of a bander who already captured on a returning Louisiana Waterthrush (below photo) on July 4th to its wintering grounds in the Bahamas.)
In the West, Orange-crowned Warbler often leave nesting grounds by early June. Some populations in coast California areas disperse upslope to the Sierra Nevada mountain range (and its foothill areas) to feed amid temporary "staging ground" areas before truly migrating in August and September.
Where I live in the San Francisco Bay area, Orange-crowned Warblers are still present amid their nesting grounds some cases, but the vast majority have already left their nesting areas — including some that have left for the Sierra.
Dawdling Orange-crowned individuals remain here, however, such as the males I heard singing today. It's possible these late-stayers may be re-nesting and, as a result, are still hosting recently hatched newborns.
Other nesting warbler species in my area — Common Yellowthroat, Black-throated Gray, Yellow-rumped, Wilson's, and Hermit — have also completed their nesting cycle, or will surely do so in most cases by no later than July.
That's why I'm now often forced to drive long distances to find these warblers in the mountains where songbird nesting remains vibrant.
Happy Birding to all, Daniel
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