Here's how traffic noise affects bird physiology and reproductive health

Birds may undergo a change in physiology and reproductive health when living in environments with constant vehicular traffic noise, compared to when they are breeding in a quiet habitat, according to a study which throws light on how disturbances in urban landscapes affects birds.

The study, published in the journal Conservation Biology, noted that the chicks of noise-exposed birds were smaller than the young ones from quiet nests.

The researchers, including those from The Max Planck Society for the Advancement of Science in Germany (MPG), studied the effect of traffic noise on stress hormone levels, health, and reproductive success in breeding zebra finches -- birds native to the arid areas of Central Australia.

They observed a total of 88 birds split into two groups bred in both noise and no-noise conditions. The noise groups, the researchers said, were exposed to traffic noise recorded at several busy intersections in and around the city of Munich in Germany during the whole breeding period. The study noted that the traffic noise used by the researchers varied throughout the day, with the sounds of heavier traffic during the day, and lighter traffic during the night.

After the first breeding period, the researchers said that the noise conditions changed for both groups, and the same bird pairs bred again.

The research team recorded the level of stress hormones before, during, and after the breeding period. They also took measures of the functioning of the birds' immune systems, and reproductive success, as well as the growth rates of their chicks.

The findings of the study revealed that the birds in constant traffic noise had lower levels of the stress hormone corticosterone in their blood compared to when they were breeding in a quiet environment. The researchers said that this was surprising since stress often resulted in higher levels of the hormone.

"In the birds breeding in quiet environments, their baseline corticosterone remained low throughout the breeding season," said Sue Anne Zollinger, lead author of the study from MPG.

According to the researchers, the low level of the hormone found in the birds may have been a natural defence mechanism to ensure that chronically increased corticosterone levels did not affect their immune systems.

"This suggests that the birds didn't habituate to, or get used to the noise, since their hormone levels did not track the normal ups and downs that occur during the normal breeding cycle in non-noise exposed birds. Instead the suppression of corticosterone levels may be a way to protect from the negative consequences of chronically elevated stress on the immune system," said Zollinger. The researchers also found that chicks whose parents were exposed to traffic noise were smaller in size than those from parents that bred in quiet aviaries. However, the study noted that once the chicks left the nest and could feed on their own, away from the noisy environment, they managed to catch up in size with the chicks from quiet nests.

To exclude the effects of other factors associated with traffic noise, such as chemical pollution, light pollution, and other differences found in urban areas, the researchers conducted the study in aviaries. "Our data shows that traffic noise alone, without all the other disturbances of an urban environment, changes the physiology of birds and has consequences on their growth," said Henrik Brumm, co-author of the study from the Max Planck Institute for Ornithology in Germany.