Acoustic differences persist in urban <i>Parus major</i> (Great Tit) over two decades of declining anthropogenic noise in Paris, France

Abstract Anthropogenic noise pollution induces changes in the behavior of wild animals. Birds exhibit a well-characterized response to noise pollution: by vocalizing at higher frequencies, birds can escape the masking effects of low-frequency anthropogenic sounds. Although Paris ranks among the world’s largest and most densely populated cities, in recent years Parisians have piloted a multi-faceted effort to reduce anthropogenic noise. Are the city’s noise reduction efforts sufficient to affect the frequency characteristics of city-dwelling birds? We used directional microphones to collect field recordings of Parus major (Great Tit) in noisy urban sites within Paris and quiet forest sites outside of Paris in Fontainebleau. We recorded birds at both sites during two time periods, separated by two decades, in 2003 and 2023. We collected field measurements of ambient noise using a calibrated sound level meter and we evaluated long-term noise monitoring data collected by the French government. We predicted that birds would sing higher frequency songs in noisy urban Paris compared to quiet natural areas, and we predicted that the magnitude of these differences would diminish in 2023 compared to 2003 given the city’s recent noise reduction efforts. We show that urban birds sing at higher frequencies when noise levels increase, matching previous findings from across Europe. Long term data reveal a decline in anthropogenic noise in Paris over the past decades, with average noise levels in 2023 reaching half of their previous intensity. Despite noise reduction efforts, urban birds continue to sing with higher frequencies compared to non-urban birds, and those frequencies have not declined over two decades. We conclude that the noise reduction efforts in Paris have decreased anthropogenic noise levels, but that birds maintain songs with higher minimum frequencies in spite of this change.