Use a feed with higher protein. I use Purina Flock Raiser at 20% whereas most layer blends hover around 16 or 17% protein. Also, bread isn't exactly nutritious for chickens; at least a regular meal of it. You'd be better off getting whole red wheat and/or race horse oats and making a mixture for some scratch. Red winter wheat is about 14% protein, so I've heard.
The chickens will naturally gravitate toward protein foods like cat food, hamburger, scrambled eggs, etc. They're omnivores with a taste for meat so that's what they want.
In the winter, I would feed mine a small bit of natural ground beef to give them a bit more protein through the winter. Now that summer's here and they don't need to keep so warm, they're off the protein but still get Flock Raiser.
Free ranging doesn't mean they get a lot of bugs to eat. Mine have pretty much worked through the yard.
Also, if you free feed, they'll regulate their intake to match their nutritional needs. Flock Raiser is more expensive than Layer food but I think that they eat less because it's more nutritionally dense.