Pigeons love foods that are considered to be grain. These include corn and peas. Most often, they prefer grains to be raw. So, raw corn that has become “unfit” for human consumption is a good idea - just make sure it’s not rotting or gross before leaving it out for the pigeons. If out in a park and have to choose between regular old bird seed and some bread, they’ll go for the birdseed. This isn't too surprising, since the birdseed has a lot of grains inside. Bread is made of grains, yes, but they’re not raw. The birdseed is raw grain.
However, they’ll also go for things like popcorn kernels and small rocks. This is to help them digest their food easier.