What does the gossip stone message about "crows that fly near the town walls" mean?
Solution 1:
The crow is called a Guay.
The solitary Guay that flies around Clock Town in Majora's Mask will ignore Link until he plays one of the four Temple songs, at which point it will begin dropping progressively more valuable Rupees.
You can read all about these guys here, although there isn't much: Guay
Solution 2:
If you play one of the Temple songs (Sonata of Awakening, Goron Lullaby, New Wave Bossa Nova, Elegy of Emptiness), it will begin dropping Rupees.