Nasal respiratory turbinate function in birds.

Nasal respiratory turbinates are complex, epithelially lined structures in nearly all birds and mammals that act as intermittent countercurrent heat exchangers during routine lung ventilation. This study examined avian respiratory turbinate function in five large bird species (115-1,900 g) inhabiting mesic temperate climates. Evaporative water loss and oxygen consumption rates of birds breathing normally (nasopharyngeal breathing) and with nasal turbinates experimentally bypassed (oropharyngeal breathing) were measured. Water and heat loss rates were calculated from lung tidal volumes and nasal and oropharyngeal exhaled air temperatures (T(ex)). Resulting data indicate that respiratory turbinates are equally adaptive across a range of avian orders, regardless of environment, by conserving significant fractions of the daily water and heat budget. Nasal T(ex) of birds was compared to that of lizards, which lack respiratory turbinates. The comparatively high nasal T(ex) of the lizards in similar ambient conditions suggests that their relatively low metabolic rates and correspondingly reduced lung ventilation rates may have constrained selection on similar respiratory adaptations.