Blood in mouth, nose, and eye tissues of birds cools by evaporation, then flows to a cephalic vascular heat exchanger, the ophthalmic rete. There, acting as a heat sink, blood from the evaporative surfaces cools arterial blood flowing counter-current to it toward the brain. The brain thus remains cooler than the body core. Data for unanesthetized domestic pigeons (Columba livia) suggest that in addition to losing heat, blood perfusing the evaporative surfaces also exchanges oxygen and carbon dioxide with air. In the heat exchanger, this blood apparently gives up oxygen to, and gains carbon dioxide from, arterial blood. The consequent increase in oxygen and decrease in carbon dioxide in the brain's arterial blood enhance diffusion of these gases in, and oxygen supply to, the brain. Such events may help birds maintain the brain's oxygen supply during the high systemic demand of exercise and at the reduced oxygen availability of high altitude.