Abstracts Biological Sciences

The water economy of the Sage Sparrow, Amphispiza belli nevadensis (Ridgway)

by Ralph Roy Moldenhauer

This study investigated several physiological aspects of the water economy of the Sage Sparrow, Amphispiza belli nevadensis,as they relate to the ability of this species to inhabit xeric desert environments. When maintained in captivity on a dry diet without drinking water, Sage Sparrows gradually lost weight and died. During the period of water deprivation, motor activity increased and gross energy intake declined. However, Sage Sparrows were able to maintain their body weight when provided with only succulent foods as a source of water. When birds were transferred from ad libitum drinking water to a restricted water intake of either tap water (ca 2.0 ml /day) or Tenebrio larvae (ca 2.5 ml H₂O/day), excretory water losses were diminished by about 65 percent through a reduction in both the moisture content of droppings and excrement dry weight. At the same time, there was an increase in the percentage of assimilated energy, as indicated by the utilization coefficient. The renal capacity of the Sage Sparrow for concentrating electrolytes was not exceptional, with a mean maximum urine-plasma ratio of 2.4 for chloride. Individuals lost weight on NaCl drinking solutions of 0.25 M and above. The thermoneutral zone of the Sage Sparrow ranged from 28 to 37°C with a standard metabolic rate of 7.24 kcal/day (mean body wt = 18.8 g; temperature = 35°C). The temperature coefficient was 5.6 percent. From 10 to 35°C the passive rate of evaporative water loss was relatively, constant at ca 6.5 mg/g/hr. Above 35°C, evaporative cooling was initiated, and at high ambient temperatures and low humidities, Sage Sparrows were able to dissipate all their metabolic heat production by, evaporative cooling. There was indirect evidence for a reduction in evaporative water loss with water deprivation. The physiological adaptations of Sage Sparrows to water and heat stress were not unusual for small passerine birds, and were intermediate in relation to various species of desert and non-desert birds already investigated. The successful existence of Sage Sparrows in desert environments is probably dependent upon the utilization of succulent foods to satisfy their demands for water and on temperature-dependent behavioral responses which may reduce heat stress, thereby minimizing the need for evaporative cooling.