The taphonomic effects of heat and or fire on bones, conodonts, and plant tissues have received much attention from paleontologists and geologists, but the effect of high temperatures on amniote eggshell has been unknown. We compared the effects of heat on extant avian eggshell, which is constructed primarily of CaCO₃ deposited within a protein matrix. Ostrich (Struthio camelus) and glaucous-winged gull (Larus glaucescens) eggshell fragments were heated to temperatures up to 800 °C for varying lengths of time. A series of dramatic color changes, similar to those reported for conodonts, began to occur at about 200 °C, especially in ostrich eggshell. Reverse curling of eggshell was observed at temperatures above 600 °C. Thermogravimetric analysis showed a negligible decrease in mass below 200 °C, which was attributable to water loss. For both eggshell types, sharp decreases in mass occurred at about 710 °C and the remaining CaO residue represented 55% of the original mass. Heat should not be overlooked as a factor contributing to the variability of fossil eggshell, including color differences and reverse curling. Existence of these features, in turn, may provide important clues for understanding environmental conditions and events preceding or accompanying eggshell fossilization.