While filming took place in the actual Mercer house, production could not find an insurance company that would underwrite the project given the extensive value of the antiques. All of the items seen in the movie are therefore replicas with the originals stowed in storage during filming.
For the Christmas party scene at Mercer house, they actually invited a large number of people who had previously been at James Arthur Williams' parties, including several members of his family. These extras praised the movie party because, not only was the atmosphere correct, but Kevin Spacey perfectly played James and his style of schmoozing at parties.
The famous Bird Girl statue shown at the beginning of the movie is not one of the original four. It is a replica.