Little is known about early indicators of heat stress in bird erythrocytes. We investigated the effects of elevated temperatures on the morphology and cellular responses of avian erythrocytes. Hen red blood cells were subjected to 22–45 °C temperatures for 1 h and 4 h, then stained and examined by light microscopy to assess morphological alterations. Cell viability, cytotoxicity and caspases 3 and 7 activity also were investigated. We found that short-term exposure of hen blood to 43–45 °C caused morphological alterations and increased the activity of pro-apoptotic caspases 3 and 7; hemolytic cells also were found. Reduction of erythrocytes may be a consequence of direct disruption of the cell membrane, although apoptotic disintegration also may occur. Because changes in erythrocyte morphology were rapid, they may be useful indicators of thermal stress in birds.