Which traits best allow hummingbirds to turn on a dime, in midair, at fast speeds? The most agile hummingbirds owe their nimbleness to muscle strength and power, as well as wing shape and size, according to a new study in the February 9 issue of Science.
Fiery-throated hummingbird (Panterpe insignis) | Paolo Segre
Teasing apart the physical traits that allow birds to be agile during flight can be very challenging.
"There are many different traits that could affect maneuverability," explained Roslyn Dakin of the University of British Columbia (UBC). "For example, in birds these could include body mass and shape, wing size and shape, and muscles, to name a few."
To date, many studies aiming to pinpoint the ideal traits for agility have focused on just one species. But Dakin notes the importance of studying flight abilities across many species, since species vary more from one another than individuals of the same species.
To learn more about interspecies flight differences, Dakin's UBC colleague Paolo Segre caught more than 200 hummingbirds across 25 species. The wild birds were captured at four different field sites in Ecuador, Peru and Costa Rica, and were released after the study.
The researchers studied each bird for half an hour, as it flew in an enclosed space and was allowed to do as many maneuvers as it wanted. A computer tracking system recorded these movements, which included thousands of accelerations, rotations and turns.
Remarkably, the flight differences between species were sufficiently distinct that the authors could correctly classify species in 34% of cases based solely on their maneuvering patterns, indicating that differences among species in maneuvering style are significant.
At first, it appeared that bigger body size endowed hummingbirds with greater agility. "A heavier hummingbird should be slower to accelerate and not as good at making tight turns," Dakin said. "So we were surprised to find that the larger hummingbird species were actually more maneuverable on those metrics and a few others."
Gould’s jewelfront (Heliodoxa aurescens) | Paolo Segre
However, it was not just body size that was helping birds maneuver with great agility. Curiously, bigger birds were slower than their peers of the same species when attempting certain maneuvers.
This disparity prompted the researchers to conduct another analysis, this time accounting for a broader range of traits. In their more comprehensive analysis, Dakin and her colleagues found that muscle capacity — strength and power — was associated with better deceleration, accelerating swiftly and upward rotations. Lower wing loading, which is the ratio of the wing area compared to body mass, was associated with better rotational movement and turning capabilities.
"Our results also explain why larger hummingbird species are more maneuverable: they have proportionately greater muscle capacity and/or wing size, which allows them to achieve greater performance," explained Dakin.
Moving forward, Dakin said she would like to explore which maneuvers provide the best advantage during dynamic interactions with predators, competitors, potential mates and prey. She would also like to know if there a trade-off between acceleration abilities driven by large muscles and rotational performance driven by large wings.
"It's exciting that we now have the technology and computational framework to quantify maneuverability, to begin to address these other questions," she said.
[Credit for associated image: Paolo Segre]