Nope, they are obligate omnivores. Grains, leafy greens, fruits and veggies for sure, but they'll also take insects and any animal small enough to tackle (yes, even snakes). In fact, they are also cannibals. Even as day old chicks, if one in a group starts bleeding, there's a more than good chance the others will peck it to death. The sight of blood throws them into a frenzy. To top it off, break one of their own eggs in front of them and they'll consume the entire thing, shell and all. I even on occasion give them carcasses from roast chicken and turkey, they completely pick the bones clean.
In short, chickens ain't the cutesy little things you see in media where they just nibble on grass and pellets, they're surprisingly tenacious creatures.
...Although this time it didn't go according to plan. It was already sundown by the time I threw the mouse in their pen, so it refroze overnight to the point where it didn't garner much interest from them. Ah well.