Chickens are easy to keep practically anywhere provided they have access to several essentials such as fresh air, food, water, space, and protection from harsh weather and predators. Current mobile chicken houses are often designed for either 5-10 birds or several hundred. Small chicken tractors are easily moved by hand while larger mobile chicken coops may require a tractor or other motorized vehicle to move them. There are several advantages and disadvantages when building a mobile chicken house. The positive points of mobile chicken houses are that they lead to a reduction in vegetation damage and allow a more even distribution of manure compared to permanent housing systems, allow chickens to scavenge for extra food, which cuts down on food costs, chickens consume pests which are present in the grass, give the farmer very easy access to the eggs and are usually cheaper than permanent houses. The major weak point with the mobile chicken houses is the need to move them regularly, which can be especially unpleasant in inclement weather. Since it is movable, the poultry house must be light and easy to move and therefore the number of chickens one can keep here are limited. For farmers, wildlife can pose a huge threat to the survival of the chickens in mobile houses. The best ways to protect a flock from predators are to keep birds confined during the night time, to install a fence to keep out land predators and to cover outside run with mesh wire or netting to prevent problems with predator birds.