Birds which lift and move heavy weights?
I believe the answer is:
cranes
'birds' is the definition.
(crane is a kind of bird)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for cranes that I've seen before include "Lifting machines and long-necked birds" , "Lifting machines and wading birds" , "Makes an effort to see" , "Lifting machines or large birds" , "Wading birds with long legs and necks" .)