You can set up "unidentified" as the default value of the field. This loads every new record with this text until the user changes it.
In the auto-enter tab of field options, you can simply type "unidentified" in the data box to do this.
You could also use an OnRecordCommit trigger that performs a script to check for and put Unidentified into the field if it is empty when the record is committed if you wish.
The obvious answers, thanks again.