My understanding is Dawn Dishwashing Detergent is great for degreasing birds that are contaminated in oil spills, etc. It IS MILD, & will remove grease, oil etc.
BUT, with Show Chickens you don't want to remove the natural oils on the feathers, as they lose their shine & lustre.
Many people who show chooks use a Show Shine Spray on their birds on the day of the show.
I prefer to use a mild Baby shampoo that removes dirt & poop from their feathers, & I feed my birds a heavy dose of Black Oil Sunflower Seeds starting 3 weeks before a show, & wash them 2 days prior to showing.
To each their own, I'm just offering what I do to wash my chooks.