If you want to give them an excellent probiotic, much better than yogurt, get some buttermilk and mix it with plain boiled white rice. Make sure the rice has cooled. Then give it to your chickens kind of runny with the buttermilk and watch them scoff it up.
Buttermilk mixed with rice helps control diarrhea as well.
That sounds like a hearty breakfast. They have been a bit runny today so I'll see what buttermilk I can find. Thanks!