Domestic and feral cats kill billions of birds annually; keeping pet cats indoors or in enclosed gardens is the most effective intervention available to individual owners.
Cat predation on wild birds represents a welfare harm at enormous scale. Individual birds killed by cats experience the terror of pursuit and the pain of predation. At the population level, sustained cat predation depletes local breeding bird populations, particularly in areas with dense cat populations. The solution available to individual pet owners is clear and without significant cost: keeping cats indoors, or providing enclosed outdoor runs, eliminates both the welfare harm to birds and the additional safety risks to cats from outdoor hazards.