The long range global forecast is for greater numbers of chickens being reared in extreme heat conditions; thus, genetic stocks will need to be selected for performance in warmer production environments. Eggs from three genetic lines were either incubated by conventional “normal” or thermal conditioning (elevated) and the hatched chickens were reared in either normal temperature or heat-stressed environments. Biological and genetic data were collected to identify biomarkers that could be used for genetic selection. The differences observed among lines indicate that a portion of heat tolerance is related to genetics. This study also demonstrates that elevated embryonic incubation alters the chickens’ response to heat stress. Furthermore, blood parameters may be used as biomarkers for selection.