MCWHORTER, T.J.*; HARTMAN BAKKEN, B.; TSAHAR, E.; MARTINEZ DEL RIO, C.; University of Wisconsin, Madison; University of Wyoming; University of Wyoming; University of Wyoming: Renal function in nectar-feeding birds: hummingbirds turn off the faucet at night

Nectar-feeding birds must often deal with exceptionally high water intake rates in order to meet their mass-specific energy demands. Hummingbirds face the dual osmoregulatory challenge of eliminating large water loads when feeding and preventing dehydration during the night when they do not feed. We investigated diurnal variation in the renal function of broad-tailed hummingbirds (Selasphorus platycercus). Because hummingbird kidneys are poorly suited for urine concentration we predicted that they would greatly reduce glomerular filtration rate (GFR, L-glucose clearance) during the scotophase. GFR measured in the evening (2234 � 520 �L h-1, mean � SD) was approximately 72% of the value predicted based on body mass. There was a significant effect of diet sugar concentration (and hence water intake rate) on GFR. At night GFR was significantly lower than during the evening and was not significantly different from zero (-41 � 165 �L h-1). In the morning L-glucose washout rate, which is directly proportional to GFR, was significantly lower than in the evening (calculating GFR requires a marker distribution space estimate, which was not obtained in the morning), presumably because birds were dehydrated from overnight evaporative water losses. Renal fractional water reabsorption (FWR) measured in the evening ranged from 0.66 to 0.89 (0.79 � 0.07) and was significantly correlated with water intake rate. Our results suggest that hummingbirds may not only conserve water and electrolytes by reducing their GFR at night, but may also reduce energy expenditures from retaining filtered glucose and metabolites.