Adding sheathing under a concrete tile roof
If this roof tile is the Spanish terra cotta style(curved or waved shaped) than  plug up the openings at the eaves. What I have done is: 1) cram rolled-up sections of poultry netting into the cavities and mix a batch of mortar keep it fairly dry and trowel it into openings. or 2) get a length of 2-3 inch galvanized roof flashing and snip the outline of each eave opening. Than push it into the eave opening.