The poops are similar to foo, maybe firmer probably due to diet. He poops like all the time however!
There is an excellent site online talkingstarling, that gives a lot of information on starlings and their diet. You can also find a few studies done by zoo's and such on their diet. Originally people were trying to feed them low iron soft bill food-which was still causing iron storage and other issues, or they would feed mostly insects-then they were getting bad feathers, leg bumps ect. Though they eat mostly insects in the wild, in captivity we are not able to provide the same variation in bugs which causes nutritional issues.
For an adult starling their diet would be a quality dog/cat food with either poultry mash or softbill food. You would grind 1 c dog food to 1/4 cup bird food that can be left dry to free feed. Daily extra egg and fruit. Treats-fruit, veggies, flowers, meats, insects ect.
Seeds are not really recommended at all unless sprouted.