Much like how common sense guides us to huddle for a private conversation, animals use innate strategies to adapt their vocal displays to fit the social situation at hand.
By developing a new method to detect the volume and direction of sounds, which have been traditionally difficult to measure in the field, UC Davis researcher Gail Patricelli discovered that social context can shape how particular species of birds project their calls during communication.
Sound waves radiating from a source are considered directional if they are much louder in certain directions than others. We might attempt to adjust our vocal directionality by cupping our hands around our mouths while whispering in someone’s ear or yelling to someone in the distance.
The main strategy that birds use to adjust the directionality of their calls is by controlling the pitch, said Patricelli, an assistant professor of evolution and ecology.
Along with Marc Dantzker, a curator at Cornell Laboratory of Ornithology, Patricelli constructed a video and audio recording system to determine how male red-winged blackbirds use vocal directionality to send different messages to females in their territory.
The researchers surrounded birds in the field with microphones to record and analyze sounds emitted in all directions simultaneously by the vocalizing animals.
“With this system, we can really make a map for an individual vocal display,” Dantzker said.
When a predator was nearby, males beamed alarm calls in all directions to warn surrounding females.
“But when they’re trying to communicate with a particular female and convince her to mate, then they use much more directional vocalizations,” Patricelli said. “I think the reason for that is if anybody else overhears the male and female pair talk about mating, then the other males will often fly in and try to disrupt and harass them.“
An ongoing study in Patricelli’s lab suggests that different bird species likewise narrow or widen the directionality of their alarm calls depending on whether the warning signal is intended for a predator or members of their own group.
Patricelli also adapted the sound recording system to study the distinctive and elaborate courtship behavior of sage grouse.
Unlike other types of birds, sage grouse communicate using vocal sacs on the sides of their chests rather than their beaks, meaning that sound radiates much more strongly from the sides compared to the front of the bird.
“Now that we know about the directionality of their courtship sounds, it looks like they are trying to beam the loudest part of their call towards females by not directly facing them,” said Alan Krakauer, a postdoctoral researcher in Patricelli’s lab.
By adding a robotic female decoy equipped with video camera and microphone to a system of microphones set up around the sage grouse breeding grounds, the researchers can measure how well males aim their calls at the decoy, providing a way to gauge whether males with better aim are more successful at mating.
This is an interesting case of how the mechanism behind signal production affects all the other behaviors that are involved with courtship, Patricelli said.
“It’s also a neat example of how males need to not just be able to produce the biggest, loudest, flashiest signal but also use it appropriately during courtship,” she said.
Patricelli next plans to use the methods she developed to determine how sound radiation patterns in local songbird species influence courtship behavior and mate selection. Another goal will be to understand how noise pollution from nearby human development might interfere with communication between sage grouse during the breeding season.
The system can also be adapted to study insects, frogs and mammals, she said.
Patricelli presented an overview of her work in February during the annual meeting of the American Association for the Advancement of Science in Chicago.
ELAINE HSIA can be reached at firstname.lastname@example.org.