Birds that nest in tree cavities, despite being relatively safe, are still threatened by predators, which not only can destroy broods but also can kill the parents. Therefore, using safe cavities is crucial for the survival of parents and nestlings. Safe cavities should simultaneously have the smallest entrance (yet large enough to admit the cavity occupant), walls made of hard wood, and depth sufficient for the nest to be placed beyond the reach of predators. I studied nests of Marsh Tits (Poecile palustris) that were attacked by predators and compared cavity features of nests that survived the attack with those that did not. Observations were gathered in pristine conditions (Bial̸owieża National Park, Poland) over 30 yr. There was no direct human interference, and cavities were superabundant. I observed 169 attacks on Marsh Tit broods by a diverse set of predators. Nests survived ∼30% of these attacks. Only 10% survived when a predator entered the cavity, but 29% survived if the predator attempted to pluck the nest from the cavity and 39% survived if the predator tried to remove the cavity wall. The probability of surviving an attack depended on predator identity and cavity attributes; the smallest predators passed through even the narrowest entrances, but small entrances combined with walls of sound wood were effective in stopping larger predators. Nests situated closer to the cavity entrance were less likely to survive attack than nests farther away, but nests were seldom placed beyond the predators' reach, which suggests the existence of some constraints. The probability of surviving an attack strongly increased with the nestlings' age; larger young survived attacks more often, demonstrating that, even in cavity-nesting birds, early development of locomotor capacity might be at premium. I discuss constraints on antipredator adaptations in cavity structure and trade-offs between opposing requirements.