Effects of air humidity during incubation and age after hatch on heat tolerance of neonatal male and female chicks.

Effects of incubation 45 versus 55% relative humidity (RH) and early versus late hatching time on heat tolerance of neonatal male and female chicks were studied. Chicks were exposed for 48 h to temperatures of 35 (Experiment 1), 37 (Experiment 2), or 39 C (Experiment 3). Chicks that hatched from eggs incubated at 45% RH were lighter at hatch than chicks that hatched from eggs incubated at 55% RH. Chicks that hatched from eggs incubated at 55% RH lost more body weight and water during heat exposure than those that hatched from eggs incubated at 45% RH. Body weight and water loss during heat exposure of chicks that hatched early and late was similar. However, chicks that hatched late maintained their initial heat production and respiratory quotient better during heat exposure than chicks that hatched early. Body weight and water loss of male and female chicks was similar. At 37 and 39 C, heat production of chicks fell to lower values during the 2nd day of exposure compared with the 1st day. It was concluded that chicks that hatched late, i.e., with a short holding period in the hatcher, and coming from eggs incubated at 45% RH had increased heat tolerance in comparison with the other chicks.