When Vermont did the breeding bird atlas (2003-2007), June 1 was the safe date for many species, particularily for neo-tropical migrants. "Possible" and "Probable" designation were not acceptable. Breeding had to be confirmed. Of course, there were earlier safe dates, depending on the species.Those "safe dates" were pushed earlier for nearly all species during the course of the atlas survey.
Ten years later, I don't remember how that applied to different species, but my sense it that breeding is generally becoming earlier as Spring becomes earlier.
On May 15, I posted a photo of a Baltimore Oriole nest whose construction was nearly complete. The female was working on the nest (but not when I had my camera aimed, alas).
Here are a few more "Confirmed" breeding photos ...
American Redstart, female, May 21, Hinsdale setbacks, checking size, shape, and fit of the nest she was building ...
|Black-throated Blue Warbler, female|
|Black-throated Blue Warbler|
|Yellow-rumped Warbler, female|