Since all life depends on water, the movement of fluids is profoundly important in both natural and engineered systems. The physics of flow in large volumes of fluids is fairly well understood, but we know less about how to move very small volumes of liquid efficiently. Rico-Guevara and Rubega recently showed that hummingbirds use a previously unknown method to move tiny volumes of fluids very rapidly while feeding. Hummingbirds eat more than twice their body weight in nectar daily. Thus, hummingbirds are a premier natural example to mimic in engineering effective fluid-moving devices for biomedical and other purposes. Rico-Guevara and Rubega's previous work generated predictions about how much fluid can be collected and transported in a second. This project will use high-speed camera traps to test those predictions by measuring actual rates of nectar extraction (in microliters/second) for hummingbirds feeding at wild flowers. The results will have two benefits: 1) They will confirm a model of the physics of a method of fluid transport that has potential for the engineering of microfluidic devices; and 2) will provide insight into how the limits on how fast birds can feed on nectar have shaped the behavior, distribution, ecology and evolution of hummingbirds. The project will contribute to the professional development of four students (Hispanics) from one of the most underrepresented groups in science. The project is already engaging members of the public through a database of bird-plant interactions called "Hummingbirds and flowers" on iNaturalist.org (see www.inaturalist.org/projects/hummingbirds-and-flowers), making behavioral and ecological data publicly available.
Hummingbirds are fascinating, charismatic creatures that fuel their high-speed lifestyles with flower nectar. However, flowers only provide tiny amounts of nectar at a time. So, hummingbirds need to visit hundreds of flowers a day, and get the nectar out of each as fast as possible. Although scientists have been studying hummingbirds for the last hundred years, no one had yet obtained direct evidence that showed exactly how hummingbirds used their tongues to remove tiny amounts of nectar from flowers. We used high speed video cameras to film hummingbirds drinking nectar at feeders to reveal how the tongue picks up the nectar, how the nectar fills the tongue, how hummingbirds get the nectar off the tongue into their mouths, and how they move nectar up their long bills so that they can swallow it. In order to be sure that what we saw when hummingbirds were at feeders is what happens when wild birds are feeding at actual flowers, in this project we studied the mechanics, and measured the speed and efficiency of feeding of hummingbirds feeding at wild flowers. Hummingbirds have very long, forked tongues that are rolled into two tubes that run most of the length of the tongue. Historically, scientists thought that hummingbirds collected nectar from flowers by dipping the tips of their tongue into nectar, and that the nectar filled the tubes in the same way that liquid rises up a capillary tube. Our research has shown that this is not true. Our high speed videos show that a hummingbird’s tongue unrolls and spreads open at the tips when it touches nectar. When the bird pulls its tongue back in, the edges of the tongue roll back up, which keeps the nectar on the tongue, a process we call fluid trapping. The tubes on the parts of the tongue that remain outside the nectar are squeezed flat by the bird while it was pushing the tongue out through its bill tips. These tubes are then filled with nectar by forces created by the re-expansion of the tubes, which recover their cylindrical shape when they contact the nectar. We call this process expansive filling. Since capillarity is not the reason nectar moves onto hummingbird tongues, we needed to recalculate how much nectar hummingbirds could pick up, and how fast. Our new understanding of fluid trapping and expansive filling means that we can predict how hummingbirds should behave and feed in the wild, if we understand correctly how much energy they need. By using high speed video to film hummingbirds in the wild, we were able to test how, and how fast, hummingbirds actually extract nectar from real flowers. Hummingbirds are so fast while licking nectar and so quick when visiting a flower, that studying their feeding behavior in the wild with the naked eye is impractical. Therefore, we developed a system of high-speed camera traps that remotely videotaped every hummingbird visit. We also devised macro, backlit-filming techniques to visualize and measure the amount of liquid inside the nectar chamber of the flowers, and even to track the bill and tongue inside the flower. Through this combination of macro, high-speed, and backlit videography, we were able to observe what was previously unobservable – live, wild hummingbirds depleting the nectar inside the flowers. We filmed 11 pecies of hummingbirds in the Colombian Andes, and collected data on hundreds of visits. We found that about half of the time only a very thin layer of nectar was available on any given visit to a flower. The realization that hummingbirds are collecting nectar in such thin layers supports our hypothesis that hummingbirds employ fluid trapping rather than capillarity, since capillarity would not work in very thin fluid layers. Instead of placing the tip of the tongue in the nectar and waiting for capillarity to draw the nectar up the tongue, we observed the hummingbirds mopping the inside of the flowers with their tongue tips. The other half of the time the nectar pool is deep enough for the hummingbirds to dip their tongues in the nectar. We calculated licking and nectar uptake rates and found that the speed at which the birds extracted the nectar was too fast to be explained by capillarity, and instead matched the predictions of expansive filling. Our results offer potential engineering applications (e.g. microfluidics, or movements of tiny amounts of fluid), and the development and testing of field-ready devices to study wild animals (i.e. low-cost and versatile "high-speed camera traps"). We have made this research available to the public through talking with the media (e.g. interviews and documentaries) and directly via public presentations and websites. Finally, we have incorporated citizen science through a project called "Hummingbirds and flowers" on the iNaturalist.org platform, where interested citizens are entering geographic records of their observations of hummingbirds at flowers.
