Eventually we all encounter molting or bald birds at our feeders, however these are normal signs of a bird renewing its plumage and don't indicate severe illness. Additionally, natural deformities like discolored feathers, overgrown bills, crooked talons and the like aren't signs of illness. Some combination of wet, crusty or swollen eyes, matted feathers, rapid or trouble breathing, inability to fly properly, and/or sitting still - even when approached (instead of alert), are potential signs that a bird may be sick. If you witness a potentially sick bird at your feeder, once it has fed please clean your feeder thoroughly to minimize contagion to other birds. If you see several sick birds, take down all your feeders for at least a week to give the birds a chance to disperse. Note: Only veterinarians or federally licensed wildlife rehabilitators can legally treat wild birds. If you see a bird that appears to be compromised in some way, perhaps due to sickness or injury, do not try to care for the bird yourself.