What are "eggmobiles" in The Omnivore's Dilemma?

In "The Ominovore's Dilemma," author Michael Pollan discusses many farming techniques and practices which are utilized by one Joel Salatin of Polyface Farms in rural Virginia. Mr. Salatin believes in using practices that observe animals' activities in nature and seek to emulate them in a farm setting. Salatin's egg-laying chickens are housed in a mobile trailer-style structure known as an "eggmobile." The eggmobile is essentially a chicken coop that can be moved around in order to best mimic a chicken's natural movement. This rectangular contraption on wheels is capable of housing about 400 laying hens. It features nesting boxes lined up from the inside, which allows the farmer to retrieve eggs from the outside. Chicken wire is placed at a distance around it so as not to let the chickens escape, and they are able to come and go as they please. When they need to be fed, a trailer hitched to the eggmobile slowly pulls them over to the pasture where the cows were three to four days earlier. 
This is beneficial to both the land and the chicken, as it is unhealthy to both to remain in one area for long. Salatin moves the eggmobiles four days after the cattle graze and deposit manure in an area. This allows the chickens to feed on the fly pupae that inhabit the manure. In fact, the chickens naturally get 15% of their feed from this practice, just as they might in a  natural setting. So, not only are they getting essential food for a healthy diet by moving around in the eggmobile, but by scratching for pupae, they are also depositing cow manure across the pasture, which is essential for healthy grass and plants to grow.