Some additional internet research turned up some discussion a couple of 
years ago.  I cut and pasted some of the relevant stuff.  Sorry for any 
goofy blue lines in Quote #1 - I could not get rid of them.

Jim Wilson
Queenstown

QUOTE #1
    In Thomas Hardy's poem "Afterwards", there is a reference to the dewfall 
hawk (in some editions, the dew-fall hawk). Dew-fall is evening, Thomas 
Hardy lived in the West Country, so what bird might that be?   Is it 
necessarily a particular species, or is it possibly poet-speak for any hawk 
which happens to be active in the evening?


Possibly. I rechecked the source and found that I'd been relying too much on 
the internet. The correct hyphenation appears to be "dewfall-hawk" which 
could well be a Hardy coinage (like "delicate-filmed" in the same poem).

I did wonder if the name had been applied to any specific bird, or
alternatively whether there was a hawk that tended to fly just after sunset,
which is I suppose when dewfall will occur on a clear night. If not, then
we will never be certain what he meant. I did think that writing
"the" rather that "a" dewfall-hawk rather indicated a specific bird but you
can never be sure with poetry!

QUOTE #2
 Dew often falls well before sunset. I happen to notice it because I play
crown green bowls, and dew can make a big difference to the speed of the
bowls.

QUOTE #3
 Is it necessarily a particular species, or is it possibly
poet-speak for any hawk which happens to be active in the
evening?