Thanks for the reference to the 1962 nesting in Franklin County; it provides more information that might shed some light on this mystery.
The authors (Glines and Trautman) discussed the simultaneous dropping of all flight feathers by the common gallinule and full recovery of flying ability by October 15.  They were unable to find records of a similar flightless interval in the Purple Gallinule, but presented evidence to support this theory.
Has it now been verified that Purple Gallinules also shed their flight feathers in this fashion?  I would not be surprised, but something should exist in the scientific literature by now to confirm it.  I hope.
So, here is my hypothesis to explain the normal northern limit of the breeding range of the migratory Purple Gallinule:
The flight feathers could be molted in response to the shortening of day length in late summer, probably in August in Ohio, probably in July farther south.  In more northern locations, days are longer before the equinox (September 21 or 22), and shorter after it.  Therefore, to the north of the normal breeding range, flight feathers would be lost and re-grown at dates later in the year, while freezing temperatures and the diminishing of the food supply would occur earlier in the year.  At some point, the birds would be expected to starve or freeze before they were able to fly south.  While this might not affect the fledglings, the breeders would be removed from the population if they nested too far north.  Since breeding "success" is generally measured by the survival of the offspring, the untimely demise of the parents might not have been noted before.
Evidence to test this hypothesis might be found in Argentina, at the southern limit of the species' range, where the same constraints should operate even though the seasons are reversed.  However, Cornell's range map
http://www.allaboutbirds.org/guide/Purple_Gallinule/lifehistory
indicates that migratory Purple Gallinules are only found in southeastern North America.  The species has also become naturalized in parts of Europe, and might be migratory there.
What would be most helpful, and probably does not exist (does it?!?) is an extensive record of what dates this species was observed to breed, cease flying, resume flying, and depart on migration for various locations in temperate zones far from the equator.

If anyone has another hypothesis, or a simpler method of testing this one, I'd love to read it.

Dr. Bob Barrett
Geography and Planning
University of Akron
________________________________________
From: Bill Whan [[log in to unmask]]
Sent: Friday, July 16, 2010 8:29 AM
To: Barrett,Robert P
Cc: [log in to unmask]
Subject: Re: [Ohio-birds] Clearfork Reservoir Rarities, Gallinules. (7/15) Ben Warner

As we discussed (see my post here on 5/17/10 and
http://www.crh.noaa.gov/lmk/?n=flood_050210_how_it_happened ) the region
experienced a large incursion of purple gallinules as a result of
violent weather. Somehow a male and a female found one another, with the
normal result; single birds have occurred north well into Canada in
other years. No doubt these birds will return to their normal climes in
a more leisurely way once the young are able to fly; as far as we know,
vagrants always have. Ohio held the world record for the northernmost
nest of this species based on a nesting here in Franklin County ( for
that story, see
http://elibrary.unm.edu/sora/Auk/v081n02/p0224-p0226.pdf  , where a
discussion of molt of flight feathers seems to determine when they can
depart), and now holds the new one. These rallids are not strong fliers
and easily blown off-course; before all the birds counted this year,
there were 13 state records since 1980.
Bill Whan
Columbus

Barrett,Robert P wrote:
> Exciting news, no doubt about it. But I have to wonder, what will
> they do when it turns cold? While it's possible for a pair of
> Gallinules to produce young in Ohio, can we expect the young to breed
> in their native wetland at Clearfork?  Will they fly south in the
> fall and stay there, or might they return in the spring?  Will the
> young even be strong enough to fly south in October?  Or does this
> species not migrate (willingly) far from the Gulf Coast?  If not, why
> not?  What could tell them that they have gone far enough, and that
> heading farther north is not in their favor?  I'm thinking that they
> must be responding to some sort of signal or indicator (cypress
> trees?  Alligators?) and wondering what it could be.  There must be
> some good reason why we don't normally encounter Purple Gallinules in
> Ohio, and this makes me very curious about it.
>
> Dr. Bob Barrett Geography and Planning University of Akron
> ________________________________________ From: Ohio birds
> [[log in to unmask]] On Behalf Of [log in to unmask]
> [[log in to unmask]] Sent: Thursday, July 15, 2010 10:53 PM To:
> [log in to unmask] Subject: [Ohio-birds] Clearfork
> Reservoir Rarities, Gallinules. (7/15) Ben Warner
>
> This is exciting news! As predicted and prophesied by some this
> spring during the Purple Gallinule invasion, it seems some Purple
> Gallinules have found love at the northwest marshes of the Clearfork
> Reservoir! I went there this evening with Dave and Emily Slager
> (8-9:30pm) and re-found the little family. I only saw one adult but
> did see at least 4 different black, fuzzy, baby Gallinules following
> the adult around! Other observers have located and counted at least
> youngsters and both adults! Really cute little black fuzzballs. Even
> heard the adult vocalize. IF you go there to see them Please be
> respectful of the habitat and the rare birds and do not disturb the
> birds (no recording playback) or the marsh and stay right there along
> the roadside. The birds are a bit tricky to see through the
> spatterdock lilly pads and burr reed and cattails etc. so be patient.
> here is a google map of the area in Richland Co. near Lexington Ohio.
>
>
>
> Link:
> <http://maps.google.com/?ie=UTF8&ll=40.725625,-82.654878&spn=0.004464,0.009602&t=h&z=17>
>
>
>
> The birds are fairly faithful to the area north of the road, in the
> far western reaches of the marsh, and often are seen in the lower
> weedy areas in the middle of the Lilies. There is a couple pull-offs
> right before you cross the small bridge on Orweiler Road. Its not
> very busy. Other birds there this evening were Common Moorhen, Swamp
> Sparrows, Green Heron, Wood Duck, Cedar Waxwing, Eastern Blue Bird,
> and hundreds of roosting Grackles and Red-winged Blackbirds. Cool
> stuff! And the babies were found by local Amish young men, nice work!
> Here is a photo Someone Else took (Bruce Glick) of the adult and some
> babies.
> http://www.facebook.com/photo.php?pid=31290960&l=ec20ac4ffe&id=164901416
>  Cheers! - Ben Warner
>
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