Are just what's needed for a bird feeding its young (4,3,4)
I believe the answer is:
fill the bill
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for fill the bill that I've seen before include "Be exactly what is needed" , "meet requirements?" , "feed hungry pelican?" , "What a hungry pelican must do" , "For a pelican, fish might" .)