The hypothesis that parent birds regulate nest humidity and consequently egg dehydration by varying ventilation of the nest was tested in the house finch (Carpodacus mexicanus) and the phainopepla (Phainopepla nitens). Injection into the nest cup of dry air or air saturated with water vapor varied nest humidity two- to threefold. This treatment substantially modified egg dehydration, but the frequency at which the adult ventilated the nest was unchanged. These data indicate that these species do not regulate nest humidity.