That sounds to me like avian pox. You can get more information on the Cornell web site: http://www.birds.cornell.edu/pfw/AboutBirdsandFeeding/DiseasedBirds.htm. Warning: The photos aren't pretty!

Greg Butcher
Director of Bird Conservation
National Audubon Society
1150 Connecticut Avenue NW, Suite 600
Washington DC 20036
Tel.: 202-861-2242, ext. 3034
Fax: 202-861-4290


Protecting our Great Natural Heritage through the Important Bird Areas program,<http://www.audubon.org/bird/iba>, 
and the WatchList, <http://www.audubon.org/bird/watchlist>.