Birds have a very good memory of the places where they find an abundance of food. Once they find a good spot they will come back frequently to check out the goodies.
When you put the food out there is bound to be one sparrow in the vicinity who spots you and comes down, this is a sign for the other sparrows who will soon follow suit. More than one sparrow/finch feeding together is indicative of a food supply and soon attracts other birds, especially if they have found food at that place before. As has been said, gulls are scavengers as well as predators, and beside scanning for food, will watch each other to see if one lands, indicating possible food. If they have found food in P.Greenwood's garden before, one of them will be keeping an eye on the place, the other gulls rely on it.