They do most of the work... The N comes from Urea. When mixed with the soil and the rest of the bacteria-rich bird litter, it's exposed to various ureases to cr

They do most of the work... The N comes from Urea. When mixed with the soil and the rest of the bacteria-rich bird litter, it's exposed to various ureases to create NH3, which combines with water for NH4 which plants can use.