Googled "when birds sing" attached to the article is a graph of the highest times people are seen for influenza like illnesses? Weird!
https://www.wired.com/2014/03/birds-sing-morning/
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sing to attract mates
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sing to protect their territory to prove strength
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sing in the morning because when in darkness they are not foraging or socializing so what else are they to do?
Brainstorming on that last one….Birds sing in darkness i.e. when they are caught in their evil, i.e. when they hit bottom?