When we purchased this property, it was unimproved pasture land. There were horses on it at one point, but the organic matter was virtually non-existent and the grasses were very poor quality.
A benefit of pasturing our birds across the field is that they get to eat the grass, clover, bugs, and insects. Another massive benefit is what the birds do for the land itself.
As part of their daily activities and natural behaviors, chickens will scratch at the ground and dig for insects. This helps to aerate the soil and remove thatching of dead grass. They will also eat and trample down the grasses and after eating, well, they poop.
Chicken manure is a nitrogen rich fertilizer that does wonders for pasture land. The small area they are confined to means that their manure is spread evenly over the entire area they occupy. After just one day they are moved to the next patch of fresh grass where they repeat the cycle.
The immediate aftermath of the chickens is an eaten-down patch of grass that is covered with manure. The grass will die off for a bit before growing back even stronger, greener, and more plush.
