Respiratory and cutaneous evaporative water loss at high environmental temperatures in a small bird

We measured rates of respiratory and cutaneous evaporative water loss as a function of air temperature in a small desert bird, the verdin Auriparus flaviceps. Birds were placed in a two-compartment metabolic chamber that separately collected water evaporated from the bird's head and body. Cutaneous and respiratory evaporative water loss, as well as CO2 production, were measured in resting birds at 2 °C intervals between 30 and 50 °C. Metabolic rate was lowest at 38 °C (19 mW g-1) and increased to 28 mW g-1 at 50 °C. At the lowest air temperature, 30 °C, resting metabolic rate was 34 mW g-1. As air temperature increased from 30 to 50 °C, cutaneous water loss increased from 3.3 to 10.3 mg g-1 h-1 and respiratory water loss increased from 2.1-64.1 mg g-1 h-1. At moderate air temperatures (30-36 °C), water loss was divided almost evenly between respiratory and cutaneous components. As air temperature increased, however, verdins became heavily dependent on respiratory evaporation for heat dissipation. Evaporative water loss data for other species at high air temperatures suggest that partitioning of water loss may follow two different patterns. Evaporative heat dissipation may depend primarily on either cutaneous or respiratory modes of evaporative heat transfer. The physiological mechanisms and functional significance of these contrasting patterns of evaporative heat loss remain unknown.