"Migrating" Cloudless Sulphur will fly fairly high above the ground, probably 8 to 15 ft. +/-...Straight ahead, northbound, rapidly - will not appear to be flying fast but will surprise you when you attempt to chase it...Wingbeats mixed in with some sailing...Will be across a large field in surprisingly short time. Large size, long wings, pale yellow to greenish yellow appearance it flight... 
They are frequently seen in extreme southeastern Massachusetts (Cape Cod and along the coast)...Have been seen fairly regularly in coastal Mass. the past few years...I have myself seen at least 2 specimens in Mass. and one in Rhode Island in recent years, but in all cases in September.
Anyway, we should all keep our eyes open for this species...

From: Vermont Butterfly Survey on behalf of David Hoag
Sent: Sun 8/21/2005 5:53 PM
To: [log in to unmask]
Subject: Re: [VTLEPS] VTLEPS: Spicebush Swallowtail

> The more the merrier. Kevin
>> Unfortunately, no photo - it flew off before
   I could net it.   [snip]   SULPHUR sp. (100+  

22-JULY (not August) Grand Isle
LARGE.   All very pale yellow.   Fast wingbeats.
Larger than the largest C. eurytheme / Orange Sulphur.
Straight steady northbound flight on a south wind.
I saw it approach a hedgerow and ran around the end
of the hedge to catch up with it - just out of reach -
watched it continue north, up and over a cedar swamp.
All the other sulphurs were flying erratically as expected.

Cloudless Sulphur??  Bennington County should have one?! 
Dave Hoag