Alejandro Rico-Guevara
Hummingbird tongues pick up a liquid, calorie-dense food that cannot be grasped; a physical challenge that has long inspired the study of nectar-transport mechanics. Existing biophysical models predict optimal hummingbird foraging on the basis of equations that assume that fluid rises through the tongue in the same way as through capillary tubes.
Nectar feeding biomechanics
We demonstrate that the hummingbird tongue does not function like a pair of tiny, static tubes passively drawing up floral nectar via capillarity. Instead we show the tongue tip as a dynamic liquid-trapping device that changes configuration and shape dramatically as it moves in and out of fluids.
Proximate and ultimate mechanisms of nectarivory in birds
Footage taken with a high-speed camera trap, Fusa, Colombia
Ecological implications of hummingbird feeding mechanisms
Opportunistic nectar consumption is widespread in nature probably because this highly energetic resource is relatively easy to find. Simply obtaining nectar is one thing, but getting it efficiently is a different problem entirely. Some groups of birds have achieved nectar-feeding specializations that allow them to thrive in mutualistic systems of animal-plant associations. Some studies have described repetitive evolution of some well-known morphological traits (e.g. gross bill shape-size and bifurcated, fringed tongues). However different historical contingencies on each group (i.e. frugivory or insectivory as 
ancestral conditions) have imposed different constraints on nectar-feeding adaptations. Consequently, the same function (nectar gathering) is achieved by similar but not identical structural changes. The focus of this project is to describe convergent traits and mechanisms for nectar feeding, as well as alternative solutions to attain efficiency given the different morphological constraints of each group.
Rainbow lorikeet in Sydney, Australia, © Alejandro Rico
I was fortunate to spend 3 months in Australia, working with Maude Baldwin (Harvard University) and Dr. William Buttemer (Deakin University).
Fowlers Gap Research Station (Arid Zone, NSW, Australia, aka "The Outback").
Slow capillarity
The hummingbird tongue bifurcates distally, ending in two parallel incomplete tubes or grooves, formed by rolling of the thin tongue margin. The first step during nectar consumption is loading the tongue, which is protruded out of the bill into the nectar pool up to 20 times a second. We explain why capillarity is too slow to load the tongue at the licking rates observed in nature. The slow speed of capillary filling means that a hummingbird loading its tongue by using it as a capillary tube would need to slow down its licking rate. Low licking frequencies translate directly into lower nectar extraction rates and diminished energy intake efficiency.
Currently, we are advancing on modeling the entire feeding process to inform predictions on energy intake efficiency, optimal foraging, and coevolution.
Excerpt from Flight: the genius of birds, © Illustra Media. Based on Rico-Guevara and Rubega 2011.
Golden-Bellied Starfrontlet (Coeligena bonapartei) feeding on Cavendishia nitida. Alban, Colombia.
© Alejandro Rico-Guevara.
Using data collected in the wild to test conflicting hypotheses about how nectar is actually extracted from real flowers allows us to validate the hypotheses in a realistic behavioral and ecological context. In collaboration with Dr. Kristiina Hurme, our results to date demonstrate that capillarity equations are unsuitable to calculate energy intake rate (the building unit of foraging theories); therefore we need a new elucidation of hummingbird foraging ecology. We have generated falsifiable predictions on the basis of the feeding biomechanics of hummingbirds. We translated these predictions into testable nectar extraction rates for hummingbirds feeding at wild flowers, and propose how this should impact their foraging behavior.
Intrasexually Selected Weapons (ISW) in hummingbirds
We have found that many species of hummingbirds, in several clades, independently evolved secondary sexual traits in their bill tips, and I propose that they are functional intrasexually selected weapons. We are currently reviewing this kind of weaponry across the tree of life!
Four movies showing different interaction sequences of Long-billed Hermits © Alejandro Rico-Guevara.
With Dr. Marcelo Araya-Salas, we described (in Behavioral Ecology) a previously undocumented trait in a species of hummingbird with a lek mating system, the Long-billed Hermit (Phaethornis longirostris). The trait under consideration is a dagger-like structure at the bill tip, which is a secondary sexual trait that we hypothesize is a sexually dimorphic weapon. We tested our hypothesis by examining five leks during 4 consecutive years, and by employing morphological analyses, performance experiments, and behavioral
Close-up of serrations and hook of the obviously called “Tooth-billed Hummingbird” Androdon aequatorialis, © Alejandro Rico
observations. We found that 1) adult male bill tips were longer and pointier than their counterparts in females and juvenile males; 2) juvenile males acquired dagger-like tips during their transition to adulthood; 3) variation in bill tip morphology reflected puncture capability; and 4) males with larger and bill tips were more successful in achieving lek territory tenure. Our study provides the first evidence of ISW in bird bill tips (i.e. function as daggers).
In collaboration with Dr. Kristiina Hurme, we are exploring the potential trade-off between possessing ISWs and feeding efficiently. The elegant fit between a hummingbird's bill and the flowers they pollinate has long been an example of co-evolution. When males and females have distinct bill shapes, these differences have traditionally been ascribed to differences in floral foraging between the sexes (termed ‘ecological causation’). In contrast, we propose that when males have different bills than females, it is the result of the struggle for territorial dominance through male-male combat, rather than the result of feeding on different flowers. Bill modifications should lower feeding efficiency, and seem to contradict the selective pressures towards feeding on nectar. By examining the nectar extraction efficiency of males with weapons, we can tease apart the roles of natural and sexual selection in hummingbirds.
One way in which secondary sexual traits can influence differential reproductive success is by playing a key role in the outcome of direct physical contests for mates.
Excerpt from the documentary film "Hummingbirds: Jewelled Messengers"
© Terra Mater Factual Studios.
Alejandro Rico-Guevara and Kristiina Hurme served as Scientific and Filming Advisors for the footage of this sequence.
With Dr. James Mickley, we designed customized “high-speed camera traps” that solved many of the logistical challenges of filming hummingbirds feeding on wild flowers. We published our results in Ecology and Evolution, to make our system widely accessible.