A correct control of the balance between heat production and heat loss is needed for an optimal functioning of birds. As any warm blooded animal, birds will try to keep their body temperature at a certain optimum. If the heat production is not in balance with the heat loss, the bird will try to compensate one or both of them, to maintain its body temperature at the optimum level.
An important factor for the regulation of the heat loss is the effective temperature that the bird experiences. This effective temperature is a combination of the actual temperature of the air and the air velocity.
The temperature difference (so-called delta T) between the bird (or the egg) and the environmental temperature, creates a heat flow from warm to cold. For birds, that will be a heat flow from bird to air. For eggs in an incubator, it can be both ways as during the start of the incubation process, eggs are cooler than the air.
The higher the difference in temperature, the more heat will be moved within the same time period. If there is no temperature difference, so air and object have the same temperature, no heat will be transported. Isolation, for instance in the form of feathers, will slow down the process, because it will decrease the temperature difference at the surface.
Air velocity dramatically increases speed of heat transfer at a given temperature difference. This is because the boundary of air around the object that is warmed up, is immediately replaced by a new air, restoring the delta T. This combination of air temperature and air velocity is expressed as effective temperature.
For birds, often a maximum air velocity of 3 m/s is used (when birds are old enough) and then it is assumed that the effective temperature is 10-12oC lower than the actual temperature.
However, it is important to realize that the effective temperature will only be reduced when there is a significant delta T. If the object and the air have the same temperature, no heat will be transferred, hence, air velocity will have no influence. This means that the actual difference between air temperature and effective temperature depends on the delta T between the object and the air. Therefore at higher delta T level, air velocity will be more effective in reducing the effective temperature.