The Good Shepherd
Second half of 3rd century
Catacomb of Priscilla, Rome
The use of a shepherd with a sheep on his shoulders to symbolize Christ was already traditional when this image was created. It derives from Jesus' parable of the good shepherd (John 10:11-18) and from a dozen or so other passages in the gospels where he presents himself as a shepherd. The birds in the trees and the approaching animals are taken from the iconography of Orpheus, who like Christ went into the underworld on a mission of salvation.
The Wikimedia page for this image identifies the birds as doves and the trees as olives.
More of Christ as the Good Shepherd
Source: Wikimedia Commons