The crows nest on an old time sailing ships was a
barrel shaped unit attached high up on the main mast where a sailor
would climb when on watch for ice or whales. This was the highest
point on the ship and of course afforded the best view so was the best
position to spot whales from.
No your eyes don't deceive you, this particular crow's
nest is not up a mast, but on the deck with a salty old sea dog posing
with his telescope for the camera. The reason for this presumably is
that when the crow's nest/salty old sea dog/telescope are in place atop
a mast it would be all but impossible to achieve this photograph with
the relatively primitive equipment available.