Birdsong can fill many functions, including communicating patterns that scientists call 'memes'. Photo: Supplied
Why do no two bird songs sound the same? Evolution, Dianne Brunton, Auckland professor of biological sciences and birdsong expert says.
Brunton has made a study of honeyeaters in New Zealand, and told RNZ's Nights that all birds have different "morphological traits", which informs their various songs and calls.
Songs, unlike calls, are learned, she said.
"That adds another layer of ability to diversify of the species. By learning and innovating, the songs can change quite rapidly."
Her study of bellbirds and saddleback in relatively isolated populations shows songs evolve, she said.
"Through cultural evolution, which happens in the same kind of way as genetic evolution, except with what we call memes or units of transmission, then you can get mistakes happening and you can get innovations.
Shower Thoughts: Why do birds all sound different?
"And these things become, using very human language here - popular."
A bird with a novel song is going to grab attention, she said.
"A bird might be a bird other birds would like to be, has the best territory, has the best plumage and sings a particular song.
"And so, you get the development very rapidly, of unique syllables, unique song. And that can change and you see this, especially with our honeyeaters, tui and bellbird, we get lots of this very localised innovation in song."
So why do birds sing?
The reasons birds sing varies, she said.
"They sing for a number of different reasons. So, it's not always the same thing. It might be to defend a resource, it might be to defend the territory, which is a resource, but a bit more specific.
"It might be to attract a mate."
But whatever the reason may be, it is certainly not for fun, she said.
"Why would you sing for fun if there are so many predators around listening?
"It's not particularly energetically expensive to sing, although the way bellbirds sing it is, but why expend energy on something when it's cold, and you need to conserve your energy?"
They may not sing for fun like we humans, but some birds have better pipes than others, she said.
"There's studies that have shown that birds that are able to sing and have bright vibrant colours actually are in healthier condition. And so females can use that as a proxy for 'this is a good potential mate'."
In common with humans, birds also have regional variations, she said.
Research on translocated birds shows them singing different songs to ancestral songs for their species, she said.
"We've done playback experiments in the past where we've played back on an island the local song and then ancestral song and then songs from another population somewhere in between, and the birds respond most strongly to the local song."