Evolved differences in muscle power and wing size — along with a touch of skill — govern hummingbirds’ inflight agility, according to new research published today in Science. The findings by University of British Columbia biologists show that larger species of hummingbirds, despite their increased mass, are able to adapt to outmaneuver smaller species.
“Studies of bats, birds, and other animals show that increases in body mass can have a detrimental effect on many aspects of flight,” says Roslyn Dakin, co-lead author on the study. “But with hummingbirds, the correlated evolution of increased wing size and muscle mass helps larger species compensate for their greater body masses.”
Dakin, who is currently with the Smithsonian Migratory Bird Center, study co-lead Paolo Segre, and senior author Douglas Altshuler used sophisticated video capture and a novel geometrical framework to determine how maneuverability relates to differences in physiology — notoriously difficult relationships to quantify.
They found that acceleration is primarily driven by a bird’s muscle capacity, whereas maneuvers involving rotations are driven primarily by wing size. But skill also plays a role.
Wing loading, which is the ratio of the wing area compared to body mass, was associated with better rotational movement and turning capabilities. Since muscle capacity was found to be the primary species-level trait associated with accelerations, the authors suggest that changes in muscle capacity that hummingbirds evolved over time may be able to compensate for their relatively small wing size.
The research is featured as the cover article of the February 9 issue of Science.
“The hummingbirds tend to play to their strengths, especially with complex moves,” says Altshuler. “For example, species that have the ability to power through turns tend to use more arcing trajectories, and they shy away from performing turns in which they decelerate to turn on a dime.”
The researchers captured more than 200 individual hummingbirds from 25 Central and South American species. Computer vision technology developed by co-author Andrew Straw at the University of Freiburg in Germany enabled the researchers to record inflight maneuvers with precision. The birds flew randomly in an enclosed area while Straw’s tracking system recorded their movements. Remarkably, the differences between species was sufficiently distinct that the authors could correctly classify species in 34 percent of cases based solely on their maneuvering patterns, indicating that differences among species in maneuvering style are significant.
“We recorded over 330,000 maneuvers, including many repeated maneuvers for each bird,” says Segre, now a postdoctoral researcher at Stanford University. “Capturing that much data was a challenge. Our first field site was at a biological reserve deep in the Peruvian Amazon, an area with many species of hummingbirds, but only accessible by boat. We ran our computers and cameras using solar panels and generators in a thatched hut with strategically placed rain buckets.”
Hummingbirds vary greatly in body mass and wing shape, and many species have evolved to perform at high elevation, where the air density is low. That variety offers researchers a great opportunity to study how traits relate to agility in flight, says Dakin.
“There are a lot of questions we can look into now,” she says. “How do these differences impact their ability to survive and find mates? If maneuverability is an advantage to some species, how do others get by with less agility? What is it about their lifestyle that differs?”
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