Water Economy of Granivorous Birds: California House Finches

Abstract

A water economy model for granivorous birds has been proposed that assumes equality between exogenous water intake and excretory (fecal + renal) water loss, with the variables of metabolic water production (MWP) and evaporative water loss (EWL) representing the determinants of states of water balance. The model further states that for each species employing seeds as the primary foodstuff, some ambient temperature (T a ) exists at and below which MWP ≥ EWL, and positive water balance is achieved. We tested this model with California coastal and desert populations of House Finches (Carpodacus mexicanus), both of which conform to the model, but the desert forms invariably are more economical in water regulation than are the coastal forms. We also compared both populations while fully hydrated and while subsisting on minimal water rations (dehydrated), and during both daytime and night-time. Under these treatments the gradient of water economy from most to least economical is night-time/dehydrated > night-time/hydrated > daytime/ dehydrated > daytime/hydrated. Ecologically, our studies confirm that House Finches under most circumstances are dependent upon exogenous water supplies; although more economical, the desert forms approach water independence only during the night, when the T a at MWP = EWL in hydrated birds is 4.9°C and that for dehydrated birds is 11.5°C. These temperatures are frequently encountered in their desert habitat during winter nights.