Birds’ affinity for sound has its upsides and downsides, it seems.
On the one hand, scientists have found that zebra finches have a surprisingly good memory for dozens of different bird songs. Birds that live in forests, on the other hand, can be disrupted by human-made sounds – as well as light pollution.
In the first study, published in the journal Science Advances, researchers investigated how many individual bird calls zebra finches could learn to distinguish.
Songbird vocalisations convey different messages – sounding an alarm or recognising intruders, for instance – as well as unique information about the identity of the vocaliser, like voice recognition in humans.
To make sense of these and identify individuals to form social bonds, it’s necessary to remember the different sounds. Humans can do that rapidly and retain the auditory memories for long periods of time – a process known as fast mapping.
A group from the University of California, US, found that zebra finches can do it as well.
Previously they showed that every call type in the songbirds’ repertoire is distinctly unique, and that other finches can recognise different individuals. Because the birds are highly social, the team was curious to know how many vocalisers they could tell apart.
“We believe that this is an important question because it requires auditory memories for not just particular sounds but for features of sounds that carry information,” says senior author Frederic Theunissen, from the University of California.
“This is the type of memory that animals and humans need to make for voice recognition and in humans to map sounds to word meanings.”
Because they couldn’t just ask the birds to identify callers, the team set up an experiment.
They trained 19 adult zebra finches to differentiate calls from unfamiliar vocalisers that were rewarded with food from calls from unrewarded vocalisers. In each trial a different call rendition from each vocaliser was used to make sure the birds could really discriminate between individuals rather than just the different sounds.
Starting with one vocaliser versus another, the team tested them until they used their entire repertoire of 56 vocalisers, finding that the birds could still tell individuals apart – 42 on average. The finches were able to make the representations very quickly, after hearing less than 10 call renditions from a new individual, and retain the memories for up to a month.
“We found that the memory capacity for vocaliser identity is very high – we did not expect this result,” says Theunissen. “In retrospect, it makes ecological sense because the zebra finch is a gregarious species with strong social bonds with partners and other birds in their group.
“Birds are really phenomenal in terms of their skills for vocal communication – this study is just one from a long list of studies that have shown these skills.”
There is a downside to birds’ reliance on sounds, though.
Thanks to citizen scientists and the NestWatch program, another team collated a massive amount of data covering 142 bird species from nearly 60,000 nests across North America to clarify how the world’s accelerating noise and light pollution affects them.
After controlling for the impact of other human activities and natural environmental variation, they found that noise and light “can profoundly alter reproduction of birds”, according to senior author Clinton Francis from California Polytechnic State University.
“Anthropogenic noise impairs the perception of auditory signals, altering communication, orientation, foraging and vigilance behaviours,” the team writes in the journal Nature.
Similarly, they say, night lighting changes bird activities, circadian rhythms and interactions that rely on vision.
The study showed complex effects depending on where the birds live and how they interact with the environment.
Forest-dwelling birds were most responsive to both stimuli. Noise exposure was associated with fewer eggs and declining nest success and light exposure with earlier nesting and tendency to lay more eggs.
Nesting delays due to noise were strongest in birds with low frequency vocalisations, such as the white-breasted nuthatch songbird, which had lower reproductive success. Human noise has a similar frequency, likely making it harder for them to hear each other’s breeding cues.
Birds that responded most strongly to light pollution were those that can see well in dim light. They showed advanced breeding with light exposure, and, unexpectedly says Francis says, had greater nest success. The light might have helped them align with plants and insects that emerge earlier as a result of warming temperatures.
The study adds an important perspective to current understanding of climate change impacts on birds, Francis says, which hasn’t factored in noise and light pollution and may therefore be inaccurate.
It has strong implications for proposed developments and conservation efforts, he adds.
“We should do as much as we can to restore natural sound levels and lighting at night. Unnecessary noise and light should be eliminated or minimised.”
Initiatives could include smarter lighting technologies, for instance, to turn lights on only when they’re needed. Noise pollution could be drastically lowered with quiet road surfaces, electric vehicles and increased vegetation near roads.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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