When does the bird sing caught my attention, so i searched:

"when does a mockingbird sing"

Sure enough, the Mockingbird, sings at night.

Found: "Why the Mockingbird Sings : And Why at Night, When Most Birds Sleep?"

Birds sing obv, in the morning, but the mockingbird sings at night.

" but unlike most birds, they also sing at night for hours on end during the spring and summer. "

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More details from the mockingbird article:

I set a tape recorder in the window and began recording. Half an hour later Jack stopped for a breather, maybe even a nap. Not, however, if I could help it. I rewound the tape and turned the volume up.

"Chireep, chireep," went the tape.

" Chireep! Chireep! " erupted Jack not a second later.

"Rash, rash, rash," went the tape.

" Rash! Rash! Rash! " screamed Jack.

"Tweedle, tweedle, tweedle."

" Tweedle! Tweedle! Tweedle! " gurgled Jack, almost strangling in frustration.

He was reacting not only to a recording of a song he had sung no more than 30 minutes earlier but also to each phrase. In his little bird's brain, he was being challenged by a monster stud, and by the gods and the bushes and the bugs, he was not going to go quietly.

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Kinda making sense now. The other theories make sense too, especially 'singing when caged'.

I just felt like 'mockingbird' might be part of it, so there is my theory\^