Human-made noise is very common in bird environments. It is known to have the potential to disrupt habitats, and it can also disrupt the perception and communication of bird species. The potential impacts on the physiological and fitness levels in wildlife, however, are largely unknown. Given the constant presence of human-made noise and a predicted global rise in noise-producing infrastructure, clarifying these potential impacts is timely and needed. 

This study set out to test the hypothesis that exposure to noise causes glucocorticoid signaling dysfunction and decreases fitness in a community of secondary cavity-nesting birds. To do this, the researchers performed a natural experiment in northern New Mexico’s San Juan Basin during the late spring and early summers of 2011 to 2014. A network of 240 cedar latched-roof nest boxes were installed and monitored during breeding season each year on a bi-weekly basis. A distinction was made between treatment and control sites by placing large compressor engines close to the nest boxes. These engines produced high-amplitude, low-frequency noise on the treatment sites.

The researchers measured noise levels, corticosterone (cort) levels and fitness-relevant variables, e.g. hatching success and nestling body condition, in three species of cavity-nesting birds: the western and mountain bluebirds (Sialia mexicana and Sialia currucoides, respectively), and ash-throated flycatchers (Myiarchus cinerascens). To acquire data on these studied bird species, capture methods were used. Nestlings were hand-captured from the box. Adults were hand-captured with the use of a sliding-door trap manually triggered by a researcher.

The study found strong support for the negative influence of noise on physiological traits such as stress-levels, as well as several fitness consequences for the development of the studied bird species. The compressor noise had a negative effect on baseline corticosterone levels in adult females across species. Nestlings also showed reduced levels of baseline corticosterone with increased noise levels. The findings strongly suggest that human-made noise induces stress and hypercorticism in birds.

Meanwhile, the results confirmed model-based predictions in that hatchling success was negatively affected by noise in the western bluebird, a species previously thought to be noise tolerant. Egg-hatching rates in the ash-throated flycatchers showed a weak, positive relationship with noise and there was no clear effect of noise on hatching rates in mountain bluebirds. Overall, the varying results may be best explained by different nesting patterns between the studied species.

Noise had a positive impact on feather growth until 70dB(F), after which feather development was strongly reduced. Body size showed a similar response. The results supported the hypothesis that noise decreases fitness. However the reduced development at high noise levels was not predicted, demonstrating that noise disturbance can create complex responses in these bird species. Moderate levels may increase pressure to develop faster and fledge earlier, while high and low noise levels may reduce provisioning possibilities and perceived (and real) detectability of predators or signaling about predatory dangers.

These effects of noise on development, chronic stress and hypercorticism are best explained by the masking of acoustic cues, which then weakens risk perception, which can lead to decreased efforts to find food. Regular noise exposure therefore creates a condition where birds constantly perceive a high threat situation, leading to increased vigilance or distraction.

The results from this study are consistent with recent experimental laboratory studies, showing that human-made noise is a persistent and unavoidable source of stress for wildlife. Noises from human environments likely impair the risk perception of bird species that rely heavy on environmental acoustic cues, masking biologically relevant cues and causing animals in these living areas to fail to receive important information about their habitats. This leads to a continual state of perceived unpredictability, reduced security, and ultimately impacts the fitness of these species.

In this day and age, large-scale human-driven environmental change has grown to an unprecedented level. It is more important than ever to preserve and recover the natural acoustic conditions that are needed for successful conservation of natural environments and protected species.