I looked in my book The Atlas of Breeding Birds of Vermont (Laughlin and Kibbe, 1985) and found this quote in the Eastern Phoebe species account by William Norse and Christopher Fichtel:
"The last phoebes have departed in the fall by the third week of October." This same full species account is available as a PDF file online at http://www.vtecostudies.org/vbba/pdf/VBBA1/eastern%20phoebe.pdf
I also looked at a copy of one of my field cards from the Vermont Breeding Bird Atlas of 2003-2007 and see that the ending "safe date" for counting "Possible" or "Probable" Eastern Phoebes as breeding birds was August 15.
Confirmed breeding birds definitely occur outside of safe dates. Today, on September 15, we're just about halfway between the 8/15 ending safe date and the October departure date mentioned above. Seems as if your late brood is well within the norm!
PS We had two or three migrant phoebes chasing each other about the yard yesterday. Nice to see them!
From: Keri Charles <[log in to unmask]>
To: [log in to unmask]
Sent: Saturday, September 15, 2012 10:56 AM
Subject: [VTBIRD] Eastern Phoebe Nesting Season Question
In our backyard today we have a group of three of four young Eastern
Phoebes flying awkwardly about and bickering with one another, with adult
bird supervision. Does anyone know what is typically the tail end of their
breeding season? Looking online I could only find info on when they start.
Here's a photo of one of the youngsters:
Catbirds were also plentiful and social today Despite a low overall number
of birds the yard was nonetheless loud and busy!