Hairstreak Migration?

[Bruce Grimes]

On Aug. 26, and again on Sept. 9, several observers including myself
were on a high point along the Blue Ridge Mountains (adjoining the
Parkway).  This site is a regular migration watch point, though not
manned as often as some of the more famous sites.  
On both occasions, more than 10 individuals of the Hairstreak group were
seen moving rapidly through the observation field and beyond over the
woods, in a southerly direction.  One was identified as a Red-banded
Hairstreak.  At least three other individuals were seen to have bright
blue upper wing surfaces.  
Although the field is a bit lower than the crest of the ridge in the
woods to the south, the Hairstreaks were not heading toward the crest,
and continued in the same direction over the woods as they were heading
through the field.  
The ridge crest in the woods and beyond was searched, with no
Hairstreaks being seen (although Gemmed Satyrs were present). 
Apparently hill-topping was not the motivation for the behavior.
Other species observed moving south at this site on the same dates
include Monarch, Eastern Comma, Red Admiral, Common Buckeye, and
Cloudless Sulphur.
Have observers previously mentioned seasonal movement of Hairstreak
species?  Is there any literature on this subject? Could there have been
reasons other than "migration" for this movement?

Thanks in advance for any help.

Bruce Grimes
(Remove ns from mail address for reply)