The nest is unusually deep for the size of the bird, a tight cylinder of grass and reed flowers woven directly around three or more standing stems. It has no support underneath. Because the reeds bend in wind, a conventional shallow open cup would spill its contents; the depth and the firm attachment together solve that.
Reed warblers are also the best studied host of the common cuckoo, and the relationship has shaped their behaviour. They mob adult cuckoos that approach the nest, and they inspect their own eggs closely, rejecting eggs that fail to match. This selection pressure is what maintains the close egg mimicry of the cuckoo's reed warbler race.
The rejection is not perfect, and that is the interesting part. A warbler that has just seen a cuckoo near its nest becomes more likely to reject an odd egg, meaning the decision depends on recent evidence rather than on egg appearance alone.