Jamieson, Fausset and Brown on Ezra 8:16-20

16-20. then sent I for Eliezer ... with commandment unto Iddo the chief--Ezra sent this deputation, either by virtue of authority which by his priestly character he had over the Levites, or of the royal commission with which he was invested. The deputation was despatched to Iddo, who was a prince or chief of the Nethinims--for the Persian government allowed the Hebrews during their exile to retain their ecclesiastical government by their own chiefs, as well as to enjoy the privilege of free worship. Iddo's influence procured and brought to the camp at Ahava thirty-eight Levites, and two hundred twenty Nethinims, the descendants of the Gibeonites, who performed the servile duties of the temple.