Dietary intake effects on arginine vasotocin and aldosterone in cloacal fluid of whitebellied sunbirds (Nectarinia talatala)

Detoxification and elimination of nicotine by nectar-feeding birds

Many dilute nectars consumed by bird pollinators contain secondary metabolites, potentially toxic chemicals produced by plants as defences against herbivores. Consequently, nectar-feeding birds are challenged not only by frequent water excess, but also by the toxin content of their diet. High water turnover, however, could be advantageous to nectar consumers by enabling them to excrete secondary metabolites or their transformation products more easily. We investigated how the alkaloid nicotine, naturally present in nectar of Nicotiana species, influences osmoregulation in white-bellied sunbirds Cinnyris talatala and Cape white-eyes Zosterops virens. We also examined the metabolic fate of nicotine in these two species to shed more light on the post-ingestive mechanisms that allow nectar-feeding birds to tolerate nectar nicotine. A high concentration of nicotine (50 µM) decreased cloacal fluid output and increased its osmolality in both species, due to reduced food intake that led to dehydration. White-eyes excreted a higher proportion of the ingested nicotine-containing diet than sunbirds. However, sugar concentration did not affect nicotine detoxification and elimination. Both species metabolised nicotine, excreting very little unchanged nicotine. Cape white-eyes mainly metabolised nicotine through the cotinine metabolic pathway, with norcotinine being the most abundant metabolite in the excreta, while white-bellied sunbirds excreted mainly nornicotine. Both species also utilized phase II conjugation reactions to detoxify nicotine, with Cape white-eyes depending more on the mercapturic acid pathway to detoxify nicotine than white-bellied sunbirds. We found that sunbirds and white-eyes, despite having a similar nicotine tolerance, responded differently and used different nicotine-derived metabolites to excrete nicotine.

Drinking problems on a 'simple' diet: physiological convergence in nectar-feeding birds

Regulation of energy and water are by necessity closely linked in avian nectarivores, because the easily available sugars in nectar are accompanied by an excess of water but few electrolytes. In general, there is convergence in morphology and physiology between three main lineages of avian nectarivores that have evolved on different continents - the hummingbirds, sunbirds and honeyeaters. These birds show similar dependence of sugar preferences on nectar concentration, high intestinal sucrase activity and rapid absorption of hexoses via mediated and paracellular routes. There are differences, however, in how these lineages deal with energy challenges, as well as processing the large volumes of preformed water ingested in nectar. While hummingbirds rely on varying renal water reabsorption, the passerine nectarivores modulate intestinal water absorption during water loading, thus reducing the impact on the kidneys. Hummingbirds do not generally cope with salt loading, and have renal morphology consistent with their ability to produce copious dilute urine; by contrast, as well as being able to deal with dilute diets, honeyeaters and sunbirds are more than capable of dealing with moderately high levels of added electrolytes. And finally, in response to energy challenge, hummingbirds readily resort to torpor, while the passerines show renal and digestive responses that allow them to deal with short-term fasts and rapidly restore energy balance without using torpor. In conclusion, sunbirds and honeyeaters demonstrate a degree of physiological plasticity in dealing with digestive and renal challenges of their nectar diet, while hummingbirds appear to be more constrained by this diet.

Osmoregulatory response to acute diet change in an avian nectarivore: rapid rehydration following water shortage

Nectarivores may be required to switch between water conservation and water excretion as rapidly as they change food plants in nature. We examined the rehydration response in Whitebellied Sunbirds (Nectarinia talatala) that had been fed a concentrated sucrose diet (2.5 mol/l) for 2 days and then were switched to a diet 10 x less concentrated (0.25 mol/l) on the morning of the third day. We measured water gain as well as cloacal fluid (CF) volume hourly over 12 h, and analysed CF osmolality and calculated osmotic excretion. CF was also assayed for the osmoregulatory hormone aldosterone (ALDO). As in most water-deprived birds, whitebellied sunbirds cope with water shortage when fed a concentrated sugar diet by almost completely ceasing to void CF. Although osmolality of CF is high, volumes are not sufficient to maintain a steady rate of excretion and the birds retain osmolytes. Immediately upon switching to dilute diet, sunbirds produce copious volumes of CF and osmotic excretion is elevated and maintained at high levels over the first 6 h of rehydration. This stabilises by the afternoon at levels expected for hydrated birds. Some 2-3 h after peak osmotic excretion, there is a peak in the discrepancy between water intake and output, mirrored in an increase in ALDO output. These data suggest that excretion of retained osmolytes is undertaken as soon as water is available, with changes in the body fluid composition occurring subsequently. This study vindicates the use of CF to obtain repeated measurements of changes in the osmoregulatory steroid ALDO in small birds.