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Osmoregulation/Ionoregulation
The Rufous-collared sparrow relies entirely on its kidneys for osmoregulation and ionoregulation. It is able to tolerate a wide range of salt intake despite lacking a salt gland, however the metabolic cost in energy is too great to maintain the necessary osmoregulatory processes for an extended period of time. As a result, the Rufous-collared sparrow tends not to inhabit marine environments such as salt marshes. Under conditions of higher salt intake, the mass of the kidney and heart can increase up to 20%. This response in organ size causes an increase in basal metabolic rate (BMR) by up to 30%. Kidney size is also affected by the amount of water available in the environment. In arid environments, the urine is more highly concentrated, and the kidneys tend to be smaller than in wetter environments.

Thermoregulation
In association with its non-migratory behavior, and its tendency to be found at a wide range of elevations, the Rufous-collared sparrow experiences significant fluctuations in temperature throughout its range each year. Strategies used to acclimate to changing seasonal temperatures include limiting the amount of evaporative water loss (EWL) and increasing metabolic rate. Total evaporative water loss (TEWL) increases during summer months, which may help prevent overheating, and remains lower during winter months. In response to cold temperatures, both basal metabolic rate (BMR), and maximum metabolic rate (MMR) will increase.