Birds are homeothermic – they produce and dissipate heat to maintain a relatively constant body temperature. The internal body temperature of birds shows more variability than mammals, and therefore there is no absolute body temperature. In the adult chicken the variability is between 105°F and 107°F (40.6° and 41.7°C). The body temperature of a newly hatched chick is about 103.5°F (39.7°C), and increases daily until it reaches a stable level at about three weeks of age. Smaller chicken breeds have a higher body temperature than larger breeds. Male chickens have a slightly higher body temperature than females, probably the result of a higher metabolic rate and larger muscle mass. Activity increases body temperature. For example, the body temperature of chickens on the floor is higher than that of chickens kept in cages.

Birds have feathers that help them regulate their body temperature. Their relatively high body temperature makes it easier for them to lose heat into the air around them. Their air sacs allow inhaled air (usually cooler than body temperature) to reach deep into the abdominal capacity so when the bird exhales heat is removed from the body. Birds do not have sweat glands. Broilers use a panting mechanism (referred to as gular flutter) during hot weather to evaporate water from its throat, thus reducing body temperature. Panting is extremely effective in cooling birds. Feathers are great insulation in cold weather but inhibit heat loss in hot weather.

As previously stated chickens are homeothermic and have the ability to maintain a rather uniform internal body temperature (homeostasis). However, the mechanism for accomplishing this is efficient only when the ambient temperature is within certain limits; chickens are not able to adjust well to extremes. It is important, therefore, that broilers be housed and cared for so as to provide an environment that will enable them to maintain their thermal balance. This is known as the thermoneutral zone (see Figure 7.1 below) which is a range of temperatures at which an animal does not have to actively regulate body temperature. There is considerable margin in cold weather, a chicken’s body temperature can drop to as low as 73°F before death occurs. However, there is much less flexibility on the high side. The upper lethal limit on body temperature is 113-117°F.

Figure 7.1 - Environmental temperatures and thermal zones

The poultry thermal comfort zone, or thermoneutrality, depends on species and age, with younger birds responding better to warmer temperatures. Broiler feed conversion deteriorates when temperatures are outside the recommended comfort zone. Bird responses are predominantly affected by the dry-bulb temperature of the air space.

Broilers produce heat that must be lost to the environment to maintain constant body temperatures. Broiler heat loss is comprised of two components; latent heat loss and sensible heat. Latent heat loss is usually expressed as the amount of water evaporated from the broiler, referred to as moisture production. Evaporation uses broiler heat to change water state from liquid to vapor. The evaporation takes place inside the broiler as water passes over the wet surfaces of its respiratory system. Sensible heat lossrefers to heat dissipated through heat transfer from the broiler to the surrounding air. If the air is cooler than the broiler’s surface temperature, heat flows from the broiler to the surroundings. If the air is warmer than the broiler’s surface temperature, broilers will not be able to dissipate heat and heat stress will occur.

Air temperatures that cause heat stress and mortality are considerably below broiler body temperature. Broiler surface temperatures typically range from 95-100°F, with skin temperatures warmer than feathers. Air temperatures in this range can virtually stop heat loss from the broiler and accelerate heat prostration. For this reason, an important goal for hot weather ventilation systems is to keep air temperatures below 95°F.

During cold weather, the optimal temperature may depend on feed prices. When feed price is high, temperatures at the high end of the comfort zone may be more economical since higher temperatures improve feed conversion. When feed prices are low (or fuel costs are high), lower temperatures would increase feed consumption but save on supplemental heating costs. The right management strategy needs to be determined for each situation.

Broiler mortality is influenced by their thermal history. Once acclimated to heat stress, broilers can tolerate higher temperatures that would have been lethal to a large portion of the flock during the first exposure. Consequently, some producers gradually raise the temperature set point for cooling systems before arrival of a heat wave in an effort to prepare broilers to combat heat stress. However, extreme caution must be exercised when employing new control or management strategies that attempt to improve profitability but might also affect mortality rates.

Heat stress in poultry is a serious problem for the poultry industry. Mortality during extremely hot weather can be significant, especially when combined with high humidity. However, probably even more costly is the routine loss of weight and feed conversion efficiency during less severe periods of heat stress. Under normal conditions, chickens do a good job of cooling themselves with physiological and behavioral mechanisms. One of the keys to minimizing production losses during hot weather is proper ventilation system design.

Although air temperature represents the major component of the thermal environment, the term ‘effective temperature’ describes the combined effects of air temperature, air velocity, relative humidity, and radiation. The concept of effective temperature recognizes that the broiler regulates heat dissipation and thus maintains homeostasis by integrating all the environmental factors. Effective temperature is particularly useful when the air temperature is below or above the thermal comfort zone.

Over the last decade, there have been tremendous changes in broiler strains. As broiler nutrition improves and daily gain increases, the pattern of broiler heat loss has changed, and older data on heat loss have become obsolete. Heat and moisture production data for broilers that required ten weeks to reach a 4 lb body weight are very different than that for broilers that will reach the same weight in six weeks. Caution is needed when applying historical data.

Daily fluctuations in temperatures may result in temperatures outside the thermal comfort zone. As long as the daily mean temperatureremains in the comfort zone, mature birds can tolerate a temperature cycle of ± 15-20°F without adverse effect on performance. The cycle range of ± 15-20°F should be applied with caution as it will vary with species, age, nutrition, and other stress factors. For instance, young chicks or poults that have just been set in the brooder house will benefit from a ‘draft-free,’ constant-temperature environment while fully-feathered birds may actually benefit from temperature fluctuations. In general, temperature variations should be minimized until the broilers are fully feathered.

The inside surfaces of the walls and ceiling radiate energy based on their temperature. During warm periods, radiant heat loads from these surfaces and sunlight coming through open sidewalls or curtains will contribute to heat stress on the birds.

By contrast, in cooler weather, the relatively warm broiler body will lose radiant heat to its colder walls and ceiling. A primary function of insulation is to keep the interior surface of the wall or ceiling closer to the interior temperature to minimize radiant heat loss from the birds.

Radiant heaters direct heat toward the floor and broilers to provide localized heating while allowing lower room temperatures. This reduces building heat losses and saves fuel during brooding periods when young broilers need high temperatures. The radiant heat effect diminishes with distance from the heater.