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

Abstract

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.

Microbial communities in hummingbird feeders are distinct from floral nectar and influenced by bird visitation

“…Novel taxa, including Zymobacter and another ASV from the family Halomonadaceae, were detected in nearly all faecal samples. The related species Zymobacter palmae is facultatively anaerobic and ethanol-fermenting, suggesting the potential for this taxon to participate in the hummingbird's unique physiology [26,27]. Previous studies suggest that gut-associated microbial communities may participate in nitrogen recycling through uric acid degradation [25]; our study could not detect if microbes contain genes relating to urease activity, but shotgun metagenomic approaches could examine this potential.…”

“…Avian microbial associates are just beginning to be studied in depth [18,20,21], and little is known about microbiome assembly in free-ranging birds or its association with avian health. In hummingbirds, some microorganisms have been linked with disease and mortality [22][23][24], while other microbes may be beneficial [21,25], particularly considering the birds' high-sugar diet [26,27].…”

Nectar intake and foraging efficiency: the responses of sunbirds to flower morphology

“…Observations on how flower morphology affects the feeding performance of hummingbirds may not apply to the main pollinators of African ornithophilous flowers, sunbirds (Cronk and Ojeda 2008;Nicolson and Fleming 2014;Skead 1967). Sunbirds (Nectariniidae) are larger than hummingbirds (Trochilidae), have lower mass-specific metabolic rates and typically perch rather than hover when they feed (Westerkamp 1990;Fleming and Muchhala 2008), although hovering during foraging has been reported (Geerts and Pauw 2009b;Padysakova and Jacenek 2016).…”

Detoxification and elimination of nicotine by nectar-feeding birds

“…This compensatory feeding means that toxic secondary metabolites could potentially be consumed in large quantities if the nectar is very dilute (Lerch-Henning and Nicolson 2013). On the other hand, the high water turnover experienced by these birds (Nicolson and Fleming 2014) may also be advantageous in facilitating the excretion of secondary metabolites.…”