Some silkies with vaulted heads have holes in there skulls.  I have got to admit that when I first read that "Silkies have holes in their skulls"  I was skeptical.  I didn't think that there was any way they could survive if this was true.  What I found out is not only do they have holes in their skulls but some have very little bony structure to their skulls at all.  You can see a few examples of birds that I have posted on the photos page. 

Just a warning though;  These are real photos of chickens heads and are slightly graphic. 

Click here to go to the photos page. 

Related Links:

http://www.browneggblueegg.com/Article/HoleInHead.html