It's hard to say. With minimal human contact, I would think that it's possible they could fly off for the winter and return later. Predators are a real possibility, however. We have a fox that is living in an overgrown hedge about 200 yards from my yard and coop. This summer, I had 14 adult mallards. By November, we were down to 2 because of the fox. Foxes are sneaky, brave, and have no problems camping out near a readily available food source, even if it is what us humans would think is uncomfortably close to human activity. My kids play outside all the time, and they see this brazen fox sneaking around our property at least once a week (however, this hedge in which the fox lives is NOT our property--hence the reason I haven't been able to do much to eliminate the threat). I am, however, currently exploring options for permanent but humane removal.