Foraging plasticity and physiological adaptations enable hummingbirds to subsist on dilute nectars

Hummingbirds frequently feed on small volumes (< 30 µl) of sucrose-rich nectars. Climate change is expected to affect both the abundance and the concentrations of accumulated nectar. Using experimental feeding trials and field-collected measures of daily energy expenditure and floral nectar contents, we test the nectar dilutions at which hummingbirds can no longer meet their daily energy needs. We found across 14 hummingbird species that they are limited in how much they can increase their feeding rate (compensatory feeding) with decreasing nectar concentrations: nectar consumption increased until dilutions of 4-6 % sucrose, where birds fed at a maximum feeding rate (0.70 gNECTAR gBODYMASS -1 hr-1), after which nectar consumption (and corresponding sucrose intake) decreased. Integrating lab-tested maximum feeding rate with daily energy expenditure estimated from free-living hummingbirds, we found most species could meet daily energy needs while feeding on dilute nectars, but there is an average dilution threshold (~7 % sucrose). Hummingbirds may cope with energy deficits caused by variability in resources and competition with other nectarivores by adjusting their time spent foraging. Our energy budget models show that to meet average daily energetic needs, hummingbirds could feed ~2 hours on concentrated nectars, or an unattainable 19 hours feeding on dilute nectars. We show that maximum feeding rates pose a physiological cap on hummingbirds’ ability to use dilute nectars, which has implications for hummingbird survival and ultimately pollination. Thus, nectars provided by trochilophilous plants should be more concentrated than this dilution threshold. This mechanistic understanding may inform future work on the impacts of changes in flower phenology and nectar quality on hummingbirds.