Hummingbirds are aeronautical wonders. They hover, zoom, dive and halt with arresting speed and accuracy. Most of what we know of their flight capabilities comes from studies in the lab – wind tunnel experiments that test maximum flight speeds and force. However, Katherine Sholtis, of the University of North Carolina at Chapel Hill, USA, and her collaborators wanted to understand how the birds' fantastic maneuverability in the lab related to what they were doing in more real-world circumstances. That is, how the patterns that were recorded in the lab match what we see out of our windows or in the wild.
The authors used multiple high-speed camera videography to film wild male and female Ruby-throated hummingbirds (Archilochus colubris) as the birds fed at a hummingbird feeder adjacent to a wooded area. They recorded the flying behavior of hummingbirds as they fed alone and as they interacted with another competing bird at the feeder. They also recorded differences between when the birds were approaching the feeder from a nearby perch and when they were flying in from elsewhere. The team was interested in how flying speed, acceleration, force and energy use varied under these different circumstances.
They found that, not surprisingly, birds that were being chased flew faster than birds that were alone. But interestingly, birds that were chased away from the feeder and the unrestricted birds both used about the same amount of energy when they left the feeders. The birds that were chased sped away faster, but because they chose a less energetically expensive route, forward or down, compared with the unrestricted birds’ choice to fly upward, the energy use of both birds balanced out in the end. So, while being chased away might be inopportune, it was not more energetically expensive.
The authors also found that the birds that were flying in to defend the feeders from perches flew in at greater speeds and velocities than birds flying in from elsewhere. They found that defending birds turned with the greatest force and that, based on calculations from other studies, the birds in this study appeared to use most of their available maximum flight force when departing from the feeder. While these results provide many real-world insights into hummingbird flight maneuvers, because most of the defending and unrestricted birds were male and most of the birds being chased were female, these differences could also reflect sex-specific differences in performance or behavior.
Previous lab studies with these birds have measured maximum level flying speeds as a standard of performance. But Sholtis and her colleagues found that the birds they were studying never reached those maximum level flying speeds. Birds defending the feeder flew the fastest, but their speed was still below the maximum recorded in wind tunnels in the lab, and the wild birds flew downward rather than along a level trajectory.
The authors argue that because only maximum flight force and power – not maximum level flying speed – were observed under natural conditions, these are the performance measures that should be studied in hummingbirds. The authors also caution that these natural competitive interactions in Ruby-throated hummingbirds may not be the best model if the goal is to quantify maximum forward flight performance. And although this study shows why their natural exertions may not be as straightforward to study as those of animals in the lab, the interactions are still fun to watch.
