"SINGING"

This is a Red-winged Blackbird (Agelaius phoeniceus); male.  The female is a dusky brown color.
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One of the most common birds we have in this area they leave in the fall when the snows arrive and return in March...typically on the 15th +/- 2 days.  This year they were almost two weeks early.  The males arrive a few weeks before the females and they stake out their territories and defend them against all interlopers and that includes humans.
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Although it sounds like "singing" to a human it's surely just anthropomorphic. He's vocalizing to others of his species letting them know that this is his territory and others better keep their tail feathers out or pay the price.
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You've likely noticed that the amount of "singing" gets less and less as the summer progresses and families have been raised.  No need to "sing" when territory is no longer an issue.
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BEST IN LARGE.