Starlings are a highly intelligent breed that has been maligned and charged with the offenses of other birds (mainly grackles). They eat a mainly insectivorous diet (yes, that means they eat all the nasty catepillars and beetles that plague farmer and gardener alike). So, if you see a flock desending on a field of grain or some such, be thankful... they're not after the grain, they're after the bugs eating the grain!!
To answer your question about the diet more fully, it would be easier if I knew if you were keeping one as a pet or feeding the wild ones. For the wild ones, anything with a high insect, high fat content would be great .... look at the same things being fed to woodpeckers. (There's a suet cake with mealworms #GS-16607 and a seed/nut/fruit woodpecker diet #GS-16446 and any of the dried bugs.) They will eat some seed, but they don't prefer it. Also, cat or dog food that has chicken or turkey as its first ingredient would also be appreciated by them [either wet it down or break it up]. I use Innova Prime cat food personally because it's sky high protein/fat content allows me to put lots of "goodies" in the mix without sacrificing the bird's needs.
The cat/dog food is usually the basis for a home kept starling as well. Add bugs, especially wax worms [#GS-79065] the canned wax worms are a bit better since they don't wander off. The Quiko Classic Egg Food [#CC-18169] is another great addition. There's a softbill diet typically for mynahs but great for starlings as well [#CC-416815], that's usually a winner as well. Beyond all that remember that they need a high protein, high fat diet. I aim for 30% protein and 15-20% fat. They are usually fruit lovers too, especially small, dark colored fruits like cherries, blue berries, etc.
Hope I managed to answer your question.