However, respiratory evaporative heat loss is the principal pathway of heat
loss in the birds; however, reduced feed intake is also considered an adaptive mechanism (Blem, 2000). Moreover, Van kampen (1981) suggested a limited scope for increasing heat loss by vasodilatation in birds. It was noted that eggshell formation is under the effect of the acid-base balance in blood. Laying hens will try to conquer thermal stress by panting. However, this causes a reduction in the availability of carbon dioxide (CO2) in the hens’ blood, a condition recognized as respiratory alkalosis. As egg shells are composed mainly from calcium carbonate (CaCO3), this reduction in blood CO2 levels, consolidated with an increase in blood pH and a subsequent decrease in Ca ions utilized by the shell gland, leads to thinner or softer shelled eggs (Arima etal.,1976).