Red-shouldered Hawk (Buteo lineatus) eggs were examined to determine whether mass, eggshell thickness, and a thickness index corrected for egg shape and blowhole size changed as embryos developed. Four different stages of embryonic development (fresh, slight, large, and advanced) were described by the collector (C.F. Stone), or inferred from blowhole diameter measurements, for 286 Red-shouldered Hawk eggs collected in the late 1800s and early 1900s in central New York State. We examined changes in eggshell mass, thickness, and thickness index with linear mixed models with year of collection and clutch size specified as random variables. We also evaluated annual differences in eggshell characteristics with linear mixed models. There was an 8.8% decrease in eggshell mass between eggs with fresh and advanced embryos at the time of collection (F_3,242 = 3.4, P = 0.02). Eggshell thickness tended to decrease by 7.5% between eggs with fresh and advanced embryos, but the difference was not significant (F_3,119 = 2.06, P = 0.11). The thickness index also did not change due to stage of embryonic development. The year in which eggs were collected influenced eggshell mass (F_12,187 = 2.35, P = 0.008) and the thickness index (F_12,188 = 2.68, P = 0.002), but not thickness. This is the first study of eggs collected from wild raptors that demonstrates the magnitude of the effect embryonic development has on eggshell characteristics.