If the fitness benefits gained from producing male and female offspring differ due to parental or environmental conditions, parents should adjust their level of investment accordingly. We studied sex allocation and reproductive investment in a population of common guillemots (Uria aalge) in 2 breeding seasons. The common guillemot is a single-egg species and the male is only slightly larger than the female implying small, if any differential costs of raising male and female offspring. We use 4 variables to characterize reproductive allocation: 1) sex of the chick at hatching, 2) adult male and female body condition, 3) baseline corticosterone (CORT) level of adults early and late in the chick-rearing period, and 4) body mass of the chicks just prior to fledging. Females that produced female offspring were in better body condition during early chick-rearing than those producing males and both parents raising a female offspring lost more body mass during the chick-rearing period. Female offspring were heavier than male offspring at the end of the chick-rearing period. Whereas hatching sex ratio was at unity in one of the years, it was strongly skewed toward females (72.5%) in the other year, and this pattern was consistent for a subset of pairs studied in both years. Early baseline CORT levels of adults were lower in the year when the sex ratio was skewed toward female offspring. We discuss this unexpected pattern of sex allocation in relation to variation in feeding conditions and the role of females in competing for good nesting sites.