Why Do Birds Sing?

It's spring where Jessi and Squeaks lives, and that means the air is filled with the sounds of birds singing! But have you ever wondered why birds sing and who they're signing to?

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SOURCES:
Ackerman, Jennifer. The Genius of Birds.
http://sites.biology.duke.edu/nowicki/papers/NS04anyas.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3377815/
https://academic.oup.com/beheco/article/20/1/200/215231/The-deterrent-effect-of-bird-song-in-territory
http://www.bioone.org/doi/abs/10.2326/osj.5.5
https://www.allaboutbirds.org/guide/Northern_Cardinal/lifehistory
http://bioweb.biology.utah.edu/yamaguchi/PDF/Yamaguchi%201998%20Condor%20sex%20dim%20learned%20birdsong.pdf
https://www.researchgate.net/profile/Gary_Ritchison/publication/259439030_The_Singing_Behavior_of_Female_Northern_Cardinals/links/0c96052d5740504f85000000.pdf