Different flies have different size maggots. Does it move on its own? Someone asked if it is a preen gland. There are pictures on the internet that show the preen gland that you can look at to see if that is what it is.
Location on the chicken would help to solve that too.
I don't know how to link to an article here, but if you search this string it will take you to an article.
What to do when your flock is struck by FLYSTRIKE!