Your link mentions them weighing "a quarter of a ton". Now, idk what "ton" theyre using there, but a quarter of a US ton would be "only" 226 kg. That's ~500lbs.
I'd like to remind everyone that a reality TV-show called "My 600-lb life" exists.
So regarding hypothetical horror scenarios, I'm not too fussed about the overgrown seagull as much as I am about the feathered 4000-8000kg monster with a skull almost the size of an adult woman running at me at 70km/h.