Yes it can be given orally. If your roo is under about 4 pounds I'd only give him 1/4cc orally once a day for 5 days. If he's over 4 pounds, 1/2cc orally once a day for 5 days. The reason you administer for 5 days is because it takes longer to absorb into their system, alot is excreted, not like injecting once a day for 3 days where it goes directly into the respiratory system.
Orally, you should see improvement about the third or fourth day. If it's really bad and you dont see any improvement at all, you can give it orally up to the seventh day, then stop completely.