Baytril shots vs. oral: Because of circumstances beyond our control, we had to take Winona to a vet we don't particularly like. He handled her roughly and without much respect. You could tell he thought a chicken's life didn't have much value. But he did the duty he was paid for. One thing he did was give us a vial of the antibiotic Baytril and told us to give two subcutaneous (under the skin) shots a day for a week. This was a big mistake. It gave Winona bruises all over her abdomen at the injection sites and caused a big lump to form. We thought these things might be our fault because we weren't that savvy at giving shots. But we decided to get a second opinion on Winona because we disliked the vet so much, and we went to the vet we absolutely love for our dogs — and she happens to be a bird specialist. She knew just how to handle Winona in order to minimize stress, and she said that she would never prescribe Baytril shots because it's too caustic for their thin skin. She gave us liquid Baytril, which we squirted in her mouth and things went much better and she healed quickly.
Baytril is also available in pill form, which is easy to drop into a chicken's mouth. But they probably have to be cut up to be appropriate for your chicken's weight.