Bi-parental care is necessary in a wide range of avian species to successfully raise offspring. I investigated a range of topics relating to sex-specific breeding ecology in common terns Sterna hirundo, a monogamous seabird with negligible sexual size dimorphism. Subtle size differences can be utilised to identify the sex of terns, enhanced by within-pair comparisons which increased the accuracy and simplified computational procedures. Under natural conditions, parental contributions were found to be flexible with respect to adult quality and body condition. Males were also found to provision more efficiently and to deliver more energy to offspring than females. Therefore there was no evidence for females investing more than males during a breeding attempt. Parental favouritism with respect to offspring sex was found, although why this should have occurred is uncertain. Experimentally increased egg production highlighted adult quality as an important factor in determining clutch size. Experimentally increasing male body mass did not result in lower provisioning rates or chick condition, suggesting that this species has a greater buffering capacity than previously thought. Environmental sensitivity of male and female offspring was examined under natural conditions. Mothers produced more female offspring at the end of the laying sequence, and male chicks from these eggs had higher mortality than females. This suggests that gender influences environmental sensitivity, even without sexual size differences.