Mockingbird feeding wrong babies?

OK, my first question is what kind of little bird is this ? Seems to build it's nest in my planters all the time and a bit skiddish. Next and more puzzling is why would the mockingbird (who has a nest around the corner) fight this bird off from feeding it's young and then to my disbelief feed the very same babies her(him)self? I was super amazed at this behavior as it continued for days. Her own nest was empty but she would also visit it often. Thanks all

Thanks Fred, what's he doin in FL? LOL