Beyond what's already been discussed about diet and housing, you can sway your odds at getting longer-lived birds by your selection of breeds. Obviously that doesn't help you if you're wanting your current birds to live longer, but for future reference in selecting chicks, it does make a difference. In general, birds who have been bred toward high productivity of meat or eggs (i.e. Cornish cross, hatchery White Leghorns, ISA Browns, etc.) do not live as long. Mutts and crosses seem to have longer lifespans on average compared to purebred birds, the Cornish cross and other high-production hybrids being the obvious exceptions here. From my experience and many accounts I have read, bantams tend to live a bit longer than large fowl as well. These all are general statements, of course; nothing is guaranteed. All you can do is keep them safe, comfortable, and healthy, and hope for the best.
