Duck Call
One of my neighbors keeps a duck in his yard that I hear when taking a walk. This rooftop crow was making a particular call nearby in alteration with the duck (barely audible in this video). I've not heard these short frequent caws from a crow before, so it definitely seems like he's intentionally imitating the duck.
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Pretty normal crow noises, I have a bunch near my house and I hear calls like that all the time. My 1 1/2 yo kid thinks all birds say caw caw.
Most of the calls in my neighborhood tend to be shorter bursts—territorial or assembly calls. The greater frequency calls tend to be reserved for when a hawk or owl is around and needs to be mobbed once backup arrives. The fact that the similar sounding duck quacking is interspersed seems to indicate imitation to me.