Inverted hummingbird explained

Our hummingbird feeder has been thriving with activity this week, as migrants head south from the north woods.  So I was interested in this observation from the Wingnut (bird-watchers) blog at the StarTribune:

The Hanging Hummingbird has returned. For the third September in a row, on the same feeder on the same Lutsen shoreline property, a female Ruby-throated Hummingbird is displaying what I consider odd behavior. It hangs upsidedown from one of the perches on a sugar-water feeder here...

Sunday morning it hung by one claw on one foot, what looked like a precarious position. I thought perhaps the bird was weak or injured. The hanger returned that evening, hanging for perhaps 30 minutes. It was close to dark when it released its grip and flew away. Monday morning, at 8 a.m., there was the Hanging Hummingbird one more time, firmly gripping the perch with all toes on both feet, eyes open, no problem evident. It hung there while a second hummingbird fed, the feeder returning more than once.

More details at the link.  He requested help, and posted a reply the following day.  Ponder the problem for a while, if you wish.  The explanation is below the fold...
The hanging hummingbirds? Most likely juveniles too weak, too depleted of energy to hold themselves upright at the feeder. Nancy Newfield, who feeds, bands, and studies hummingbirds from her Louisiana home sent an email to answer my question....

“The Ruby-throated Hummingbird appears to be a youngster. Many recent fledglings embark upon a rigorous migration before developing their full strength,” she wrote. “During migration a certain percentage of them will seriously deplete their energy reserves (fat), and become weakened, at least temporarily.”..

The meadows between Lutsen and Grand Marais are filled with blooming wild flowers right now, but few that offer the nectar cup the hummingbirds seek. Migration takes a toll on birds of all ages. “Especially during fall migration,” Ms. Newfield wrote. “These energy-deficient youngsters are the most vulnerable. They’re less able to force their way to a feeder, and are much more vulnerable to predators.”..

So, why didn’t the exhausted bird simply drop from the perch instead of hanging there like an ornament? Ms. Newfield explained that when birds perch their feet automatically lock onto the perch. We humans must make a conscious effort to tighten a fist or curl our toes. It’s the opposite for birds. They must make the effort to release their grip. The bird would fall only when it became so weakened that it lost even reflexive muscle control.