In busy cities, birds alter their songs, but not in the way that scientists had anticipated.

The scientific evidence for urban song change was anchored for many years by two European species. In Amsterdam, one was frequently heard singing at a higher pitch than in the surrounding woodlands. The other was observed doing the same in parks and city outskirts.

Together, the birds contributed to the development of a satisfactory theory: birds learn to sing above the noise of towns. A recent study tested the theory on a continental scale. In order to find the pitch-raising trend, researchers collected recordings from 65 different species. The majority of animals didn't exhibit it, but what they did may have been more intriguing.

Songs from all around Europe

The urban-song concept was tried on a continental scale by a team at the HUN-REN Centre for Ecological Research in Hungary. There, evolutionary biologist Mónika Jablonszky and colleague László Zsolt Garamszegi oversaw the project.

They had a straightforward yet ambitious idea. Compile field recordings of sixty-five European passerine species, or, to put it simply, songbirds, and determine whether the birds that sang in urban areas differed from those that sang in fields and forests.

These birds use songs to warn their neighbours, attract mates, and protect their territory. Birds either find a way around it or suffer a significant cost when those messages are muffled by urban noise. Some metropolitan birds may whistle at higher pitches to avoid vehicles, according to earlier research.

Mining an archive of sounds

The recordings were taken from xeno-canto, an open resource where scholars and birdwatchers from all around the world post field sounds. The scientists linked each place to satellite data on the surrounding urban cover after selecting clips associated with particular map points.

They extracted the dominant frequency—the pitch with the greatest sound energy in a song—from each clip. The average pitch that each species employed at each site, as well as the degree to which it changed over notes, were monitored by the researchers.

Similar songs among closely related species are not considered independent evidence because the analysis employed a statistical method intended to account for common ancestry. Vocal complexity, migration, and social behaviour were also taken into consideration.

A greater variety of notes

The most obvious pattern was not what was anticipated. The range of dominating frequencies employed by birds in urbanised areas was greater than that of the same species in more tranquil environments across 65 species. The spread increased while the central pitch remained unchanged.

Not only are cities louder, but they are also louder across a more chaotic range of pitches. Although the recordings by themselves cannot verify that this is the cause, a larger dispersion may allow birds to discover a frequency gap that traffic hasn't filled.

The researchers refrain from referring to this as a verified adaptation. The broader range may be partially due to singing techniques unrelated to traffic, since the audio collection does not directly record noise levels at each clip. Nonetheless, the uniformity across species is remarkable.

Pitch-changing birds

The picture becomes more intriguing within individual species. As the number of urban covers increased, only a few distinctly changed the center pitch of their tunes. Three well-known names jumped out.

They were the hooded crow, a gray-and-black corvid found throughout central and eastern Europe, the European robin, and the European serin, a tiny yellow-green bird. Each made its own adjustments. There isn't a single neat pattern among the three.

The discovery indicates that some city birds exhibit a true frequency shift, indicating that the initial noise-avoidance theory is not incorrect but rather inadequate. Similar findings were obtained from other research conducted in Scotland on the European robin.

No alterations were seen in familiar birds.

The negative aspect was the greater surprise. The great tit and the European blackbird, two species that have typified the urban song narrative, did not exhibit a shift in dominant frequency associated with urbanisation.

Both have been featured as traditional urban pitch-raisers in extensively cited publications. Those prior findings are not being contested by Jablonszky's team. However, a local trend observed in one place does not apply to the entire species over all of Europe.

In one neighbourhood, a huge tit might sing louder than its rural neighbours. When you take the average for the entire continent, the disparity vanishes. Instead of being ingrained in the species, urban song modification seems to be dependent on particular cities, environments, or populations.

There isn't just one rule.

Additionally, the scientists investigated whether any clear species characteristic could identify which birds modified their songs and which did not. They examined each species' sociality, travel patterns, song complexity, and frequency of appearance in urban areas. Nothing remained.

The flexibility of highly sociable species was equal to that of solitary ones. Year-round residents and migrants behaved in exactly the same ways. Compared to birds with basic songs, those with complex repertoires did not exhibit any greater adaptability.

For a long time, scientists believed that characteristics like vocal flexibility or social complexity should aid a species in adapting to city life. The statistics for the entire continent indicates otherwise. The factors influencing song change in urban areas are more nuanced than the obvious forecasters.

Rethinking birds of the city

There is no one formula for how a bird's song reacts to a metropolis. Frequency ranges expand among species. Only a small number of organisms change their pitch. The subtlety has practical significance in urban planning. It is not possible to design greener city enclaves by assuming that all songbirds will adapt.

Some will, some won't, and even the adjusters might not be able to totally eliminate noise. Prior to this study, it was believed that the well-known urban adjusters handled all of the heavy lifting. The question of those birds is now open. Future studies will probably focus more on which cities attract birds than if they can adapt.