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When checking the chick body temperature at hatch time, it should be taken into account that there are three physiological systems of chicks that are not fully developed: digestive, immune and body temperature regulation.
Upon hatching, the birds are poikilothermic animals: their body temperature will rise or fall depending on the outside temperature, requiring a metabolic effort by the birds. It will take around 4 days for chicks from adult flocks and 5 to 6 days for birds from young flocks for the thermoregulatory system to behave homeothermically – constant body temperature.
It is postulated that birds from younger flocks present a different composition of lipids in their yolk, this being the factor that makes them need more time to go completely from poikilothermic to homeothermic.
Comfort zone of chick body temperature
There is a range of chick body temperature known as the comfort zone, where the birds are comfortable with the temperature that surrounds them:
The best way to “find” the comfort zone is to measure the birds’ cloacal temperature.
If the cloacal temperature is above or below this range, adjustments should be made to the ambient temperature.
Where to measure the cloacal temperature?
This information will help us adjust the temperature and ventilation of the hatcher according to the flock of birds that are currently inside the machine.
What happens with chick body temperature after processing or service?
During sexing, vaccination or in beak or spur treatments, the temperature of the birds will drop to ranges of 38.3 – 38.9 ºC (101-102 ° F).
Particular care should be taken with birds after being exposed to spray vaccines, as their temperature can drop greatly if the drop is not correct. At the end of these operations, in the waiting room to be loaded in the truck, the birds must recover their body temperature between 39.5 – 40.5 ºC (103-105 ° F). 
During transportation
Upon arrival at your farm’s destination, as the birds are being lowered from the truck, the temperature of the birds inside the boxes should be monitored.
Preferably, the temperature of birds from different parts of the truck should be monitored, particularly when it is a large truck.
In the farm
In addition to the correct air temperature of the house, it is important that the floor is acclimatized before the birds arrive. 
Note: not only the low temperature but also the high temperature will make the birds not look for food and water. They will survive two or three days because of the nutrients they have in the yolk sac, but they will eventually die.
- Emaciation and dehydration, kidney inflammation
- Ureters distended with urates
- Increased gallbladder size and distention with bile
- Darkened and dehydrated muscles, especially the pecs which lose their fleshy appearance
The energy expenditure associated with different metabolic adjustments will be reflected in low weight gain, poor dietary conversion and unevenness in uniformity. If the heat stress situation is strong and prolonged, the birds will stop drinking and eating to a point that death will occur.