New avian disease
There appears to be some kind of bird plague afflicting the eastern United States. Not much is known about it other than that it is often fatal and it seems to cause blindness. It has not yet made it to Florida, but I have no doubt that it will show its head soon. I mistakenly thought that a Common Grackle at my feeder had it, but it appears that it was just Avian Pox, a different disease that causes tumor-like growths.
What is known
Not much is currently known about the disease. It is not caused by any of the major known bird diseases. It is also not known if it is caused by an organism/virus or an environmental factor, like pollution. It does not seem to be transmissible to humans, but it is not known it if it is transmissible from bird to bird. It appears to cause some kind of build-up around the bird's eyes.
I live in the Eastern US, what should I do?
If you live in an effected area you should take down your feeders for the time being. It is unfortunate but with the mortality rates that have been reported it is probably the best course of action. As of the time this was written, the current states that seem to be effected are Virginia, Maryland, Delaware, New Jersey, Pennsylvania, West Virginia, Kentucky, Ohio, and Indiana.