We compared eggshell strength in a group of falcon taxa including the peregrine falcon (Falco peregrinus peregrinus), the red shaheen falcon (F. peregrinus babylonicus), the saker falcon (F. cherrug), the gyr falcon (F. rusticolus) and some interspecific and intraspecific hybrids. Our results showed that smaller falcons (<1,000 g) of the peregrine group have eggshells that are significantly softer (equation image=13.3 N) and thinner (equation image=0.26 mm) (n=107 eggs) than larger falcons (>1,000 g) of the gyr-saker group (equation image=20.8 N and 0.39 mm, respectively, n=81 eggs). We found a significant positive correlation between egg hardness and eggshell thickness. Linear mixed models showed that clutches from heavier females consisted of larger and harder eggs with thicker shells and thicker egg membranes. Eggs produced by older females and eggs laid later in the laying sequence were relatively smaller and softer and had relatively thin egg membranes and eggshells. Individual females, irrespective of their age, contributed significantly to the observed variation in egg strength. Egg size and hardness of hybrid eggs were similar to that of the pure species suggesting that hybridization does not affect eggshell hardness or thickness. Our study provides quantitative evidence of several factors, other than levels of contamination, which may affect eggshell thickness and hardness in falcons. J. Exp. Zool. 311A:303–311, 2009. © 2009 Wiley-Liss, Inc.