MCWHORTER, T.J.*; MARTINEZ DEL RIO, C.; PINSHOW, B.: Osmoregulation in avian nectarivores – an integrative approach

Nectar-feeding birds must often deal with large ingested water loads in order to meet their energetic demands. We investigated the importance of the integration of digestive and renal function for maintaining water homeostasis in these animals. We compared the fractional absorption of water from the gastrointestinal tracts of Broad-tailed Hummingbirds (Selasphorus platycercus) and Palestine Sunbirds (Nectarinia osea). Hummingbirds absorbed the majority of dietary water (~ 80%) regardless of the amount ingested, while sunbirds appeared to be able to modulate absorption of water across the intestine. Fractional water absorption by sunbirds decreased from 100% to 40% as water intake increased, allowing them to dispose of significant amounts of ingested water at the supply side when feeding on dilute nectars. To our knowledge this is the first documentation of adaptive regulation of water flux from the gastrointestinal tract to the body. Glomerular filtration rate in Palestine Sunbirds was 42% of that predicted based on body mass (1.82 � 0.89 mL h-1, mean � SD) and was not significantly correlated with water intake. The apparent ability of sunbirds to modulate the absorption of ingested water suggests that renal processing of water and recovery of filtered glucose may not limit energy assimilation because GFR, and thus glucose filtered load, remains relatively low even when birds are faced with large dietary water loads. Our results suggest that although hummingbirds and sunbirds are convergent in the ecological niche they exploit, they may deal with nectar diets in very different ways on a physiological level.