The mechanisms and ecology of water balance in hummingbirds

Abstract

Hummingbirds are confronted with an osmoregulatory dilemma: when feeding, their high rates of water intake require them to eliminate ingested water quickly; yet, during fasts, their capacity to eliminate water makes them susceptible to dehydration. Indeed, although their food is mostly water, hummingbirds may even dehydrate when they are feeding on moderately concentrated nectars. What mechanisms then do hummingbirds use to maintain water balance? To avoid overhydration, hummingbirds rely on two renal processes, the most important of which is to reduce water reabsorption; however, glomerular filtration rate (GFR) appears to be responsive to water loading under some circumstances. To mitigate dehydration during fasts, hummingbirds reduce, even cease, GFR. Although this strategy conserves body water by reducing losses in urine, rates of evaporative water loss suggest that hummingbirds still dehydrate during natural, overnight fasts. At present, however, it is unclear if hummingbirds are able to regulate evaporative water losses in response to hydration status. Of the eight avian lineages that independently evolved specialized nectar-feeding, the mechanisms of water balance are best understood in hummingbirds. Although more work in other lineages is needed to understand the evolutionary patterns of water balance strategies, studies to date have revealed both convergence and diversity in the physiological processes that nectar-feeding birds use to meet the osmoregulatory challenges resulting from their diet.