These Tropical Birds Adjust Their Songs Because Of Urban Noise

How does urban noise affect bird songs in the city? In Brazil, tropical bananaquits are changing their song quality, a study has found.

Previous work had already shown that great tits in Leiden communicate differently in response to noisier and quieter areas, the University of Leiden noted in a news release. According to the researchers of a study, published in the Frontiers in Ecology and Evolution, there has been evidence that female great tits "respond less" to the males' low-frequency songs in noisy conditions.

"Anthropogenic noise has increased in natural and human-altered habitats, where it might negatively affect vocally communicating animals," the researchers wrote. "The elevated noise levels can cause acoustic interference by masking the functional variation in frequency and amplitude of vocal signals, for example in frogs, birds, and mammals."

For their study, the researchers looked at the effect of urban noise on the songs of bananaquit songbirds in Brazil. These birds are an "excellent model system," the researchers explained, as they have complex vocalization and are abundant in both rural and urban areas.

To do their study, the researchers recorded the bananaquit songs in noisy and quiet areas in the city of Salvador in Brazil.

"And those were scattered within the city," study co-author, Hans Slabbekoorn of the University of Leiden, said in the news release. "For example, on one side of the building, there was a bustling traffic point, while on the other side there was a peaceful park."

The researchers found that bananaquit songs were actually "significantly" different in noisy areas. Specifically, they sang in higher frequencies to be more audible in contrast to the low frequencies of the city noise, but this change also "diminished" the quality of their songs, the university noted.

"Songs in more noisy territories were also shorter, more repetitive and less diverse in terms of the number of different syllable types, which may reduce signal value for mate attraction and territorial defense," the researchers wrote.

According to the researchers, their findings suggest that urban birds have had to "compromise between audibility and signal quality" in that the songs in higher frequencies are more audible but the songs being less elaborate possibly "weaken the message of sender quality."

"That means that the bananaquits would have to spend more time defending their territory and spend more energy to attract a partner," Slabbekoorn said, adding: "Consequently, less time and energy is left to look for food."

"We believe our findings indicate that anthropogenic noise may not only alter the song features birds sing in urban areas, but that these changes can also affect fundamental processes of sexual selection that may undermine individual fitness and the fate of populations," the researchers concluded.