Male Broad-tailed Hummingbirds perform dramatic aerial courtship dives to impress females. In a new study, scientists have shown that diving males closely time key events to produce a burst of signals for the viewer. They synchronize maximal horizontal speed, loud noises generated with their tail feathers, and a display of their iridescent throat-patch (gorget), performed in a mere 300 milliseconds, roughly the duration of a human blink.
When it comes to flirting, animals know how to put on a show. In the bird world, males often go to great lengths to attract female attention, like peacocks shaking their tail feathers and manakins performing complex dance moves. These behaviors often stimulate multiple senses, making them hard for biologists to quantify.
Hummingbirds are no exception when it comes to snazzy performances, as males of many species perform spectacular courtship dives. Broad-tailed hummingbirds (Selasphorus platycercus) fly up to 100 feet in the air before sweeping down toward a perched female, then climb back up for a subsequent dive in the opposite direction. At the Rocky Mountain Biological Laboratory in Gothic, Colorado, home to a population of breeding broad-tailed hummingbirds, researchers from Princeton University have been investigating how hummingbirds combine speed, sound and color in their displays.
The team created video and audio recordings of 48 dives performed by wild male broad-tailed hummingbirds. They then used image-tracking software to estimate each male's trajectory and speed throughout the dive. Combining these estimates with the audio data, the researchers measured the precise time at which the males produce a mechanical "buzz" with their tail feathers.
To incorporate information about iridescent plumage color, which is difficult to extract from the video recordings, the team headed to the American Museum of Natural History in New York City. Using a multi-angle imaging technique and an ultraviolet-sensitive camera, they photographed broad-tailed hummingbird specimens. Hummingbirds are tetrachromatic, their eyes have four color cones, one of which is sensitive to ultraviolet wavelengths, so by combining the photographs with a model of hummingbird color vision and details of the U-shaped flight path, the researchers were able to estimate a female "bird's-eye view" of the male's iridescent throat feathers.
First, the male starts the tail-generated buzz. Then his bright red throat feathers become visible to the female and quickly appear to change to black, due to his speed and orientation.
During this time, the male reaches top horizontal speed. Because of his high speed, the researchers estimate that a female will perceive an upward and then downward shift in pitch as he approaches and departs.
This is due to the Doppler effect, the same phenomenon responsible for the perceived change in pitch as a car with its horn blaring drives past you. All of these key events occur in a 300-millisecond window, roughly the duration of a human blink.