Question about nameserver selection mechanism

[Eric Jain]

According to DNS and BIND (5th edition) [http://oreilly.com/catalog/dns5/] 
"BIND chooses between a number of nameservers that are authoritative for 
the same zone by selecting the nameserver with the lowest round-trip time."

Can anyone point me towards more details about how BIND gathers and 
maintains these statistics? Are all nameservers queried right away, or is 
at most one nameserver queried for each lookup request (which would mean 
that the results would have to be kept in some kind of database)?