> I could not figure out what the default transaction is used in jaybird
> and how to configure it?

You can configure it when you get connection via FBWrappingDataSource,
there is appropriate property.

> Could be pass it "TRANSACTION_READ_COMMITTED" through database url or
> a property file?

No. If you get connection via DriverManager, it is always read
committed. You can however configure TPB mapping and remap the
TRANSACTION_READ_COMMITTED to isc_tpb_concurrency or isc_tpb_consistency.

Roman