I want to make some practice chickens...how thick does the steel need to be?

Rule 4.1 Targets specifies that smallbore rifle chickens must be no less than 1/4" and no more than 1/2" thick. The foot must be 1"x2".

A suggestion: If you're planning on welding them to swingers, make them as thick as possible.

If you use A36 steel 3/8" thick will suffice. I build and sell steel swingers and have a set that is 7 years old that has not yet wore out.