i wrote a piece about the hormonal milieu that underlie autumnal singing in
birds (although i don't specifically discuss the photoperiodic aspects.) it
might answer a few of your questions:

https://medium.com/ at GrrlScientist/why-do-songbirds-sing-in-the-autumn-grrlscientist-2017eeec7e03#.gktfbcc30



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