Considering Other Animal Parts To Cast

I have a collection of skulls which I thought would be useful to look at when experimenting with casting. I wanted to try and attempt to cast the skulls, and get replicas of these skulls which I could then use within my project. After talks with Ang she advised me to use the smaller skull which was a birds skull. This was due to the skull being less complicated with less areas that would get the cast stuck in them. She advised me to fill in all the holes with clay before casting. However, she did say that it is likely that the skull would be damaged through the casting process. I decided against casting these skulls in the end as they are quite precious to me and hard to come by. I didn't want to damage them through casting and there was no guarantee that the cast would be successful. 

Rabbit Skull: 

Bird Skull: