Respiratory gas concentrations and thermal gradients were measured in specially designed artificial nestboxes occupied by American kestrels, Falco sparverius. During 1988 samples were collected from five occupied nest boxes. Means and standard error of respiratory gas within occupied boxes at the top sampling ports were 0.14+0.01% C02 (n=55) and 19.84+0.12% 02 (n=55). Means from the bottom samplings ports were 0.15+0.01% C02 (n=51) and 19.86+0.10% 02 (n=51). During the 1989 field season samples were collected from both occupied and unoccupied nestboxes. Means for 1989 were similar to those of 1988. Scheffe’s test indicated no statistically distinguishable difference between occupied and unoccupied nestboxes (alpha=0.05). Review of minute thermal differences within occupied nestboxes indicates air flow. This bulk flow of air is likely to cause sufficient air turnover in the cavities so there is no buildup of C02 or depletion of 02. Thermal extremes were measured in the nestboxes, with temperatures as high as 39.1 C recorded.