Purple Martin Post Fledging Behavior

When I asked this question last week on the Purple Martin Conservation Assoc. (PMCA) page,

One of the most-often-repeated discussions that Mr. Freeze and I have every year about this time when the fledges are spreading their wings and taking flight is, "why do the adults (sometimes their own parents) often push them down when they start to fly?" There always seems to be one or two that are 'bumping' the new fledge or seemingly trying to ground him.
We speculate a lot that they're trying to get the new fledge to practice his evasive maneuvers right out of the gate or just trying to keep him from going too high, especially since hawks are in the area.
Are there any more-educated guesses on this practice by the adults? Is it just a matter of, "only the strong that survive the initial flights, will survive migration, so they're being tested"?

This was the answer provided by Louise Chambers, PMCA:

Here is an older publication about post-fledging behavior; comments about harassment in the later pages (384-385): http://sora.unm.edu/.../wilson/v090n03/p0376-p0385.pdf.

I thoroughly enjoyed the article and thought my readers might enjoy it as well.