Scientists have discovered a new behavior that Costa's hummingbirds employ during their mating ritual. Male Costa's hummingbirds perform a high-speed dive that's unlike any other hummingbird species' dive.
The sound created from the high-speed dive rises from 7 to 9 kHz and then drops to 6.5 kHz as it decreases speed.
Researchers from the University of California, Riverside published the study in Current Biology showing the Costa's hummingbird dives to the side during its courtship display. They believe that the Costa's hummingbird is doing this to manipulate the sound that is reaching the female.
During courtship display, males spread the feathers around their throats which makes them resemble a tiny octopus. They also make dives near the females where they start flying downward before pulling up toward her location. During the dive, at a critical speed, the male holds and spreads his tail feathers which makes the back edge of the outer tail feather to vibrate in the airflow.
By doing this, the male reduces the Doppler effect that affects the sound of the hummingbird's dive. Doppler effect works by making an approaching noise sound higher-pitched and making a retreating noise sound at a lower pitch. A classic example is an ambulance's siren makes as it drives down the street during an emergency situation.
Scientists think that the hummingbirds do this during the display to mask the Doppler effect affecting the sound of their dives. This confuses scientists, if the hummingbird dove with a trajectory that went straight down, it would make the highest pitched sound. However, the dive to the side draws out the whistle and hides their speed.
To capture the hummingbird's dive, researchers used an acoustic camera. They also used a wind tunnel to determine if the bird's speed and direction would affect the sound it makes.
There are two theories as to why the Costa's hummingbird may have developed this mating behavior. One involves hummingbirds being able to find the right species in a place where two different hummingbird species may run into each other. The other theory involves female hummingbird's preference for mating behaviors.
In the first case, it's not that the Costa's hummingbird is hiding the true speed of the dive, but that the dive serves as a way to tell it apart. Another species of hummingbird, Anna's hummingbird, lives in the same region and its dive produces a lower pitch.
Another theory suggests that this behavior may have been a random preference inherited through generations of Costa's hummingbirds. The displays by the Costa's hummingbird may not do anything except being pleasing to the female of the species. Males dive to the side because it is the preference of the females.
More study is required to figure out why the birds dive this way. Scientists would still need to work out what the female Costa's hummingbird hears during the display to determine the reason why the males dive to the side.