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The Scarecrow Song - also known as Scarecrow Song's in the original - is an optional but essential song for those wanting to see everything in Zelda: Ocarina of Time.
Learned by conversing with - you guessed it, some scarecrows - it's a singular song in that you compose the notes required to play it.
Once you have learned it in Zelda: Ocarina of Time, you can then summon a scarecrow at certain points in the game to help you reach otherwise inaccessible areas, allowing you to source collectables and other goodies.
How to learn the Scarecrow Song in Zelda: Ocarina of Time
You won't fully learn the Scarecrow Song until you become Adult Link, but you have to lay the groundwork as Child Link first.
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First, visit Lake Hylia as a child. There are a few prerequisites to this: you have to have access to Zora Domain, which first requires completing Dodongo Cavern dungeon. From there, enter the underwater tunnel opposite the waterfall in Zora Domain; you have to access this as part of getting Princess Ruto’s letter.
In Lake Hylia, swim to the closest shore, where to the right are two scarecrows.
Pull your ocarina out in front of the lower one (Bonorru), and play an original song. It cannot be a single repeating note, but two alternating ones should work - whatever you can remember for later.
Now return here as Adult Link. Present Bonorru your ocarina, then play the song you composed. You'll then unlock the Scarecrow Song in full.
From here, whenever you see a small gust of floating straw in certain out-of-reach locations - including several of the game's dungeons - Pierre will be summoned to help you access them with the use of the Hookshot. Again, all entirely optional - but for completionists looking to collect everything, it’s a must.
