There is massive variation in how loud bird calls are, and scientists are trying to understand why birds alter their volume
Loud signals can be heard from further away, so a male bird hoping to attract a flurry of females might find more success by belting at the top of his voice. A loud song is also easier to pick out from background noise, so a loud bird can make sure it is heard. What’s the point of calling out if no one can hear you? The thing is, the volume or amplitude of birdsong changes and researchers are piecing together what information the volume of a signal is communicating.
They think the volume itself is an encoded signal, and listening birds can get information from how loud the song is. They are yet to pinpoint exactly what information this is, however.
It is difficult to accurately measure the volume of a bird’s song. Wild birds are highly mobile which makes it difficult to calculate the exact distance between the bird and the microphone. It’s also hard to judge whether a bird is calling at maximum volume in a lab setting as there’s a big difference between singing from a wind-battered treetop and an air conditioned room. This is something researchers have had to find their way around before even beginning their work.
The volume of a call could be a bird’s way of telling those in earshot about its quality as a mate. Females of some bird species like zebra finches and red-winged blackbirds have been shown to prefer males with loud songs. This is yet to be proven for all species, but scientists have ruled out several underlying factors. They know that larger birds don’t produce louder songs, so the volume doesn’t communicate information about the bird’s size. It also doesn’t reflect their hormones, with male birds with high testosterone singing more frequently rather than louder.
Some birds have been shown to sing louder when food availability is high. This could be an honest signal about the bird’s overall body condition, with the healthiest males calling at top volume. This begs the question; why would a male in poor condition be honest about his shortcomings?
A bird with DNA that codes for honest signalling will have offspring with that same trait. If only the strongest birds are selected by females, then this honesty is passed down to the next generation. It seems that these honest calls are the way forward for the entire species, and offspring from birds at the top of their game are likely to survive.
What’s the cost?
Animals constantly need to manage their energy. It’s not an endless commodity and needs to be budgeted to make sure it doesn’t run out. Emitting loud sounds counts an an energy cost, albeit a small one. However, it is difficult to know exactly how much energy it takes for a bird to make a loud sound. It may be easier for a bird to reach maximum volume in a silent, windless lab than in outdoors in gale-force winds.
Advertising their presence may make birds more vulnerable to predation, so that is likely to affect how loud a bird signals. It has also been demonstrated that male-to-male aggression increases when birds sing louder. If a male bird wants to avoid being attacked by a neighbour, he’s likely to want to keep the volume down.
Image from flickr.com/photos/kwl