[mou] Is this normal behavior for redbreasted nuthatch?

[Steve Patterson]

I know these birds feed upside down on trees, but have never seen this 
behavior on my feeders before.  The bird  hung upside down on the feeder, 
doing nothing.  When I approached (I thought it might be hurt) it 
reluctantly flew to a nearby tree.  Warming in the sun, maybe?  Curious what 
people think.  Photo here:
http://www.pbase.com/spatterson/image/92033445