For the past 10 days have been observing a female cardinal with a completely bald head. She otherwise looks and seems healthy and active. I've never seen this before, but apparently it is not entirely uncommon, especially with Northern Cardinals and Blue Jays. Found one interesting article. http://www.acsu.buffalo.edu/~insrisg/nature/nw98/baldbirds.html . Haven't yet followed the couple of links to other articles at the end of the one posted.