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Defence of offspring against predators is an important form of parental investment in many species. We derive a model for the optimal level of parental defence during a predator attack. A higher level of defence increases offspring security, but it also exposes the parent to a higher risk. Other conditions being equal, the model predicts that the optimal level of defence increases with offspring age. This is because the relative difference between parent and offspring in expected future survival decreases with increasing offspring age. Compared with the parent itself, the relative importance of the offspring for parental inclusive fitness therefore increases. The risk that the parent should take in defending offspring therefore increases with its age. The model is applied to fieldfare (Turdus pilaris) nest defence. As predicted, parent fieldfares increase their defence throughout the nest period. The model also predicts the …