Temperature regulation in poultry: The shell is made up of three thermal insulators: 1) boundary layer, 2) feathers and 3) skin, including comb, wattles and feet.

The first thermal insulator is the boundary layer, which is a very thin, still layer of air that surrounds an animal (immediately adjacent to the feathers on a bird). Air is a very good insulator, so the boundary layer decreases the transfer of heat between an animal and the environment, which is beneficial when for cold animals and detrimental for hot animals.

The second thermal insulator in the shell is cover insulation (feathers in birds). Cover insulation functions by trapping air, increasing the insulating value of the shell. When cover insulation is wet, dead air space is removed and heat loss increases.

The third thermal insulator is tissue insulation, which refers to the outermost layer of skin and fat. Tissue is a good insulator because it contains a lot of water, which can absorb a lot of heat energy and decrease heat transfer between the core and the environment.