Parental defence was studied during the breeding season with individually marked tawny owls (Strix aluco). Attacking tawny owls pose a serious threat to an intruder because of their dangerous talons. As predicted by theory, parental defence was positively correlated with offspring age and brood size. Pairs with equal numbers of offspring but with different initial clutch sizes showed equal defence levels. The level of defence is therefore determined more by expected gain in number of offspring produced than by cumulative investment. Pairs breeding early in the season defended more strongly than those breeding later on. This was partly explained by a seasonal decrease in brood size, but the trend also persisted within each brood size. One possible explanation is that the probability of breeding failure increase as the breeding season progresses, making a risky defence less worthwhile for late-breeding pairs. The level of defence was positively correlated with the female's body mass. Defence behaviour did not change significantly with female age. Females in pairs that defended their offspring had a lower probability of breeding the following season than did females in non-defending pairs. Individual females changed their defence behaviour between years in relation to their seasonal start of breeding, suggesting that parental defence is a plastic behaviour.