No, not likely, nor would I suggest
possible. Nestling birds don't moult (other than first down, for those
that have down). They are just growing new feathers. Inasmuch as the
word moult relates to losing feathers, which normally occurs with new
ones growing. No point having brand new feathers fall out, until the
first normal moult occurs (which is usually at one year old). Are you
sure you were looking at a nestling, rather than an adult? Most likely
is feathers could be out of arrangement or simply broken.
The feathers of Darters are unusual.
The contour feathers appear to be very small. Wing and tail feathers
are normal but contour feathers look almost like fur, rather than
feathers. So that the base of the tail and wing feathers are not
covered by the usual smooth streamlined pattern in other birds (have a
look at the photo in the Reader's Digest book or HANZAB).
Oops, sorry. Wrong adjective. These darters are immature, not young,
not nestlings. They are fully-grown and fully fledged, but the very
pale heads and necks indicate immature birds. Could these be going
through the first normal moult you mention? Does that happen around
this time of year? Or is there another explanation for immature darters
sitting immobile on branches suspended over the water and refusing to
fly away even when approached (one I approached eventually did dive
into the water)? I'm used to seeing adult birds sitting on rocks at the
water's edge with their wings hung out to dry, but these birds are
behaving quite differently.