Nasal Salt Secretion in Falconiform Birds

Abstract

Many birds of prey exude a clear fluid from their nares while eating. In an examination of 16 spp. of Acclpitridae and 8 spp. of Falconidae, these secretions were found to contain high concentrations of sodium and chloride, which were always hypertonic to known values for blood. Potassium ions were excreted mainly in the urine. The gustatory process seems to provide some nonosmotic stimuli that promote secretion. Presumably the secretion comes from the nasal glands. Except for the aegypine vultures, the nasal glands of falconiforms are much smaller than the functional salt-secreting glands of comparable-sized marine birds, but there is no a priori reason why a small gland should not be functional. Size of the gland may be associated only with the volume of secretion and not with concentrating ability. The morphology of falconlform nasal glands shows considerable differences among spp. groupings and might serve as a useful tool for taxonomic considerations. The accipitrid species typically void the secretion by shaking their heads from side to side, and they secrete larger volumes than the falcons, which sneeze out the fluid. The main adaptive value of a functional salt gland for terrestrial, carnivorous birds seems to be associated with the fact that they acquire a sodium load from their food that is not easily excreted by their typically hypotonic urine. Evaporation results in a further concentration of the ingested electrolytes, but the salt gland may function effectively enough in some spp. to permit them to remain in water balance solely on the preformed and oxidative water associated with their food, even in the face of relatively high rates of evaporative loss. Some quantitative aspects of this problem were examined.