Fig. 1. Semi-logarithmic plot of data from one representative broad-tailed hummingbird illustrating (1) our protocol for estimating evening, night and morning glomerular filtration rates (GFR) and evening renal fractional water reabsorption and (2) that ¹⁴C-labeled L-glucose appearance in excreta with time follows single-compartment, first-order kinetics. This particular humming bird had an evening and morning GFR of 1.9 and 1.1 ml h^–1, respectively (determined using equation 2); night GFR for this bird was 0.0 ml h^–1 (determined using equation 4). Although data of [¹⁴C] of excreta are log_e-transformed here for clarity, our analyses were performed on non-transformed data (Motulsky and Ransnas, 1987). We injected this particular hummingbird at 17:43 h and collected excreta samples until 19:43 h. Ureteral urine and plasma samples were taken from this hummingbird at 19:45 and 19:47 h, respectively. Our morning excreta collections suggested that whole-kidney GFR was interrupted overnight: there were no differences between the [¹⁴C] of excreta in the first morning and last evening samples.