Doesn't take much to acclimate even wild ducks to people with food. There is a spot I drive past on my way to the usual boat ramp for tidal hunts. Small brook in the middle of town that, due to groundwater inputs, stays warm and unfrozen all winter. Of course the ducks find it whenever other freshwater starts to freeze up. Right in the middle of a residential neighborhood and across the street from a college, and students and homeowners alike toss food to them from whenever they show up until they leave. We sometimes see 50+ blacks and mallards there quacking for popcorn after we've spent 3 hours hunting their relatives down on the salt who flare if they see a head turn.