Birds in cities often sound different from the same species in nearby countryside. The differences are systematic and trace back to the acoustic properties of urban noise.
Traffic noise occupies the low frequencies
Engines, tyres and machinery produce sound concentrated in the lower part of the audible range, and that band is broadly continuous rather than intermittent.
Many bird songs contain substantial low-frequency content, which is precisely where the masking is strongest, so those elements become inaudible at short range.
The practical effect is a reduced communication distance. A song that carried across a territory in quiet conditions may not reach its edge in a noisy one.
Birds shift pitch, volume and timing
Urban populations of several species sing at higher minimum frequencies than rural populations, moving the song out of the masked band.
Birds also sing more loudly in noise, an adjustment shared with many vocal animals, and some sing earlier in the morning before traffic builds.
These are not separate strategies so much as a set of adjustments, and which one dominates varies by species and by the noise pattern at a given site.
The adjustments carry costs
Song is a signal of quality as well as an announcement of territory, and lower-frequency song is harder to produce, particularly for larger-bodied birds.
Shifting upward may therefore make a male sound smaller or less able than he is, which affects both territorial disputes and mate choice.
Singing earlier moves activity into colder, darker conditions and can desynchronise a bird from the rest of its local population.
Some species cope and others leave
Species with naturally higher-pitched songs and flexible singing behaviour adjust readily, which is part of why the same few species dominate city bird communities.
Species with low-pitched or structurally rigid songs have fewer options, and they tend to be absent from noisy areas even where suitable habitat remains.
Noise therefore acts as a filter on community composition rather than as a uniform pressure, sorting which species can hold territory in a city.
Learning spreads the change
Songbirds learn their songs from adults nearby, so an adjustment adopted by established males is copied by the next generation.
Over time this produces recognisable urban song variants that persist as local traditions, distinct from those of populations a short distance outside the city.
Because the mechanism is cultural rather than genetic, changes can appear and spread within a few years rather than over evolutionary timescales.