Birds of a feather flock together for real. Flocks of blackbirds usually include crows, common grackles (sometimes but rarely boat-tail grackles), red-winged blackbirds, cowbirds, and sometimes, but not usually starlings (they usually do their own things). The birds in the picture look like grackles to me (long tails). Can't view the video here but will look when I get home.

Crows are incredible birds. The are predators when the opportunity presents itself. Opportunistic feeders is the best label. They were probably killing the duck because it was already ill. Definately not nice to see but that is nature. Unfortunately when you interrupt something like that the duck gets away to spend the rest of the day (or perhaps longer) to die and the crow's babies do not get any food. I believe that unless you plan on rescuing the duck and caring for it, nature should take its course. Nature is harsh.

Crows gather in big flocks at a meeting places and "discuss" the goings on in the area, then they disperse and do their thing. They use tools and have been shown to think out situations from beginning to end result. When kept in captivity they can mimic human speech and other sounds. I know one fromt he Bird Sanctuary named Jeffery who is fabulous. He is my "boyfriend" and when I walk into his enclosure he says "Whoa!" in a sexy voice, then asks for head scratches. There was another who has since passed on called Josephine and when I would approach her I always said "How's my good girl!" After a few weeks of this, when she saw me she would say "Goo Girl!" I think that was her name for me. . .