Give them a plug of sod from an untreated lawn (about the size of a pie plate). Put it upside down. Hopefully there will be some insects and worms in it. When you start to introduce different foods, be sure they have grit. This can be the little bits in that sod, or a bit of gravel from your driveway. Put some marks on the side of their brooder with a sharpie, high enough so they have to jump to catch them. Give them multi height areas to play on. be sure their brooder is big enough. By the time my chicks are 2 - 3 weeks old, I want them to have 2 s.f./bird.
x1000 Best thing I ever did was follow the sod advice (love ya, LG) and toss one in for my first brooder raised chicks. Now it's as automatic as filling the feeders!
I'm not impressed you can get sod in march ... my snow is 8 feet deep in some places..
Mail me some sod please ....