Birds teach their young how to get food (even if its from humans). Suppose you feed your wild birds every day for several years. If you're just handing it to them thats how baby birds will learn to get food. Now one day you die/move away (hopefully the latter of the two). Chances are the birds who did know how to get food from the wild have died. So their young will be dependant on you since they have no idea how to get food from the wild. When you leave the birds will starve.