It shall be the duty of the probation supervisor to investigate all cases referred to him by the court and to make his findings and report thereon in writing to the court with his recommendation. The superior court may require, before imposition of sentence, a presentence investigation and written report in each felony case in which the defendant has entered a plea of guilty or nolo contendere or has been convicted. The probation supervisor shall cause to be delivered to each person placed on probation under his supervision a certified copy of the terms of probation and any change or modification thereof and shall cause the person to be instructed regarding the same. He shall keep informed concerning the conduct, habits, associates, employment, recreation, and whereabouts of the probationer by visits, by requiring reports, or in other ways. He shall make such reports in writing or otherwise as the court may require. He shall use all practicable and proper methods to aid and encourage persons on probation and to bring about improvements in their conduct and condition. He shall keep records on each probationer referred to him.