For years, two European birds underpinned the scientific evidence that city life changes birdsong. One had repeatedly been recorded singing at a higher pitch in Amsterdam than in nearby forests.
The other had been observed doing much the same in urban suburbs and parks.
Together, they helped establish an appealing idea: when cities become noisy, birds learn to sing above the din.
A new study has tested that idea on a continental scale. Researchers brought together recordings of 65 species and searched for the same rise in pitch.
Most species showed no such pattern – but what they did reveal was arguably more intriguing.
Birdsong across Europe
Researchers at Hungary’s HUN-REN Centre for Ecological Research examined the urban birdsong hypothesis across an entire continent.
The study was led by evolutionary biologist Mónika Jablonszky and her colleague László Zsolt Garamszegi.
Their approach was straightforward but ambitious: assemble field recordings of 65 European passerine species – songbirds, in everyday terms – and see whether birds in built-up areas sang differently from those in fields and forests.
These birds use song to defend territories, attract mates and warn neighbours. If urban noise drowns out those signals, they must either find another way to be heard or face a real cost.
Previous studies had suggested that some city birds raise the pitch of their songs to cut through traffic noise.
Searching a sound archive
The recordings came from xeno-canto, an open library to which birdwatchers and researchers around the world contribute field audio.
The team selected clips linked to precise map locations, then matched each site with satellite data showing the extent of surrounding urban cover.
For each recording, they extracted the dominant frequency – the pitch that carries the greatest sound energy in a song.
They tracked the average pitch used by each species at each site, as well as how much it varied between notes.
Their statistical analysis accounted for shared ancestry, preventing similar songs in closely related species from being treated as independent evidence.
Social behaviour, migration and vocal complexity were taken into account too.
A broader range of notes
The strongest pattern was not the one the researchers had anticipated. Across the 65 species, birds in urbanised areas used a broader range of dominant frequencies than members of the same species in quieter habitats.
The central pitch remained much the same, while the spread became wider.
Cities are not merely louder – their noise also covers a more complicated mixture of pitches. A broader spread could help birds find a frequency gap not occupied by traffic, although the recordings alone cannot establish whether that is the cause.
The researchers do not claim that this is a proven adaptation.
The recording library does not directly capture noise levels for each clip, and the broader range might partly result from singing strategies that have nothing to do with traffic.
Even so, the consistency across species is striking.
Birds that altered their pitch
Looking within individual species made the picture more interesting. Only a small number clearly raised or lowered the central pitch of their songs as urban cover increased. Three familiar birds stood out.
They were the European robin, the European serin – a small yellow-green finch – and the hooded crow, a grey-and-black corvid common across central and eastern Europe.
Each changed in its own direction. There was no single neat pattern shared by all three.
The finding shows that some city birds do exhibit a genuine frequency change. The original idea about avoiding noise is therefore not wrong, but incomplete.
Other research on the European robin in Scotland reached similar conclusions.
Familiar birds showed no change
The greater surprise lay in the absence of change. Two species central to the story of urban birdsong – the great tit and the European blackbird – showed no change in dominant frequency linked to urban cover.
Both had appeared in widely cited papers as classic examples of birds raising their pitch in cities.
Jablonszky’s team is not challenging those earlier results. But across Europe as a whole, a local pattern identified in one city does not hold for the entire species.
A great tit in one neighbourhood may sing at a higher pitch than its rural neighbours. When the data are averaged across the continent, that difference disappears.
Changes in urban birdsong appear to depend on particular cities, habitats or populations, rather than on a quality inherent in the species.
No single rule emerges
The team also asked whether any obvious species trait could predict which birds changed their songs and which did not.
They examined sociality, migration habits, song complexity and how frequently each species occurs in cities. None proved a reliable predictor.
Highly social species were no more flexible than solitary birds. Migrants behaved no differently from year-round residents. Birds with complex repertoires adjusted no more than those with simple songs.
Researchers had long suspected that characteristics such as social complexity or vocal flexibility would help a species adapt to city life.
The continent-wide data suggest otherwise. Whatever drives changes in urban birdsong appears to be more subtle than these surface-level predictors.
Rethinking city birds
There is no single formula for how birdsong responds to city life. Across species, frequency ranges become broader. Within species, only a few birds raise or lower their pitch.
That distinction has practical implications for urban planning. Efforts to create greener pockets in cities cannot rely on the assumption that every songbird will adjust.
Some will and some will not, while even those that do may not overcome noise entirely.
Until this study, the well-known urban adjusters were thought to do much of that work everywhere.
Those birds are now an open question. Future research is likely to focus less on whether birds adapt and more on which cities prompt them to do so.
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