Red or ‘blood’ swallow's nests. These bird's nests are build in the second and third nesting season, when they are affected by the high humidity of the rainy season, which spurs rust from the caves' walls to infiltrate into the nests, turning them red.

The quality is hence less pure than that of white bird's nests, i.e. the first nests, which are of premium quality (fig.) and which were built during the Thai cold season, around February-March, and consequently are not affected by humidity or rust.