Oxidative stress has recently been suggested to play an important role in life-history evolution, but little is known about natural variation and heritability of this physiological trait. Here, we explore phenotypic variation in resistance to oxidative stress of cross-fostered yellow-legged gull (Larus cachinnans) chicks. Resistance to oxidative stress was not related to plasma antioxidants at hatching, which are mostly derived from maternal investment into eggs. Common environmental effects on phenotypic variation in resistance to oxidative stress were not significant. Heritability was relatively low and nonsignificant in hatchlings, but interestingly, the chicks of age 8 days showed high and significant heritability (h2 = 0.59). Our results suggest that resistance to oxidative stress is determined mainly by the genotype as chicks grow. Further work is required to explore the genetic role of oxidative stress in life-history evolution.