Biologists from UMass Amherst are the first to comprehensively link bird songs' pitch and volume, providing insight into bird evolution
AMHERST, Mass. — Biologists at the University of Massachusetts Amherst recently announced that they have released the first broad scale, comparative, fine-grained analysis linking the amplitude, or volume, of a birds' song to its vocal frequency, or pitch. Though biologists have long wondered whether birds are able to control their pitch as they get louder or if their vocal abilities are mechanically limited, until now there has been no wide-ranging data with which to probe this aspect of the evolutionary history of birdsong. The study, published recently in Proceedings B, is a large step forward in the goal of understanding how communication and evolution have influenced each other.
If you've ever been at an outdoor concert and had to yell across a few rows to your friends, you've probably noticed that as your voice gets louder, increasing in amplitude, it also increases in frequency by getting higher pitched.
However, you've also probably noticed that you can hear the low bass frequencies from a concert at a far greater distance than you can hear the higher, trebly voices and instruments.
It would make sense, then, that if you wanted to be heard at a greater distance, your voice would drop in pitch while rising in volume—but it doesn't. The physiology of our own vocal apparatus cannot do both low and loud. We're mechanically constrained, and that's why screams are ear-piercingly high and not rumblingly low.
"The difference is between what you can do and what you want to do," says João C. T. Menezes , graduate student in the Organismic and Evolutionary Biology Program at UMass Amherst and lead author of the new paper.
But what about birds?
"There have been two main arguments over the decades," says Jeffrey Podos , professor of biology at UMass Amherst and the paper's senior author. "One camp holds that if birds sing loudly, their pitch should increase because they are mechanically constrained." This is the textbook prediction, Podos says, but there's another camp, which makes the opposite argument and points to evolution. "Since lower sounds carry farther and therefore should increase a singer's chance of being heard, it would make sense that birds should be both loud and deep at the same time."
So, which camp is right?
Until now, the two main camps have remained speculative largely because it has been challenging to measure with any precision the volume of a wild bird's song in its natural setting. "It's only been in the last 10 years or so that devices capable of measuring amplitude precisely enough, in the millisecond bursts that birds sing and with the durability and portability to take out into the field, have been available," says Podos.
But now that the technology exists, Menezes and Podos intensively sampled the amplitude and frequency of hundreds of songs and calls from 53 species of birds, ranging from the Canada goose to the elusive black-and-gold cotinga from Brazil.
What they discovered was a mix.
"There's considerable variation across species," says Menezes. Of the 53 species the team sampled, 27 species' calls got higher pitched as they got louder, 12 species' calls got lower and louder, and 14 species showed no consistent pattern in the linkage between volume and pitch at all. What this means is that we can't simply point our finger at either physiology or evolution as an explanation for how birds' calls work.
But the team did find that songbirds, in particular, tended to narrow the range of frequencies they used as their volume increased. Some songbirds were able to focus their loudest calls on the higher frequencies, some at the lower. But nearly all of them were able to narrow their range.
"One of the specializations that songbirds have developed," says Podos, "is an ability to control the tension of their vocal apparatus as they ramp up and down in volume. Most birds don't have this. It means that songbirds could, in theory, have more evolutionary freedom in evolving their songs."
Or as Menezes puts it, "this might be a hint that songbirds are like opera singers and can actively control their frequencies to the ones that are most amplified by their bodies for the best projection."
Whatever the ultimate answer to the argument may be, we now know much more about how birdsong pitch and volume relate—next time you're listening to the birds, see if you can hear the dynamic relationship between pitch and volume.
