Combined effect of anthropogenic noise and artificial night lighting negatively affect Western Bluebird chick development

Sensory pollutants such as anthropogenic noise and night lighting now expose much of the world to evolutionarily novel sound and night lighting conditions. An emerging body of literature has reported a variety of deleterious effects caused by these stimuli, spanning behavioral, physiological, population, and community-level responses. However, the combined influence of noise and light has received almost no attention despite the co-occurrence of these stimuli in many landscapes. Here we evaluated the singular and combined effects of these stimuli on Western Bluebird (Sialia mexicana) reproductive success using a field-based manipulation. Nests exposed to noise and light together experienced less predation than control and light-exposed nests, and noise-exposed nests experienced less predation than control nests, yet overall nest success was only higher in noise-exposed nests compared to light-exposed nests. Although exposure to light decreased nestling body condition and evidence was mixed for the singular effects of noise or light on nestling size, those nestlings exposed to noise and light together were smaller across several metrics than nestlings in control nests. Our results support previous research on the singular effects of either stimuli, including potential benefits, such as reduced nest predation with noise exposure. However, our results also suggest that noise and light together can negatively affect some aspects of reproduction more strongly than either sensory pollutant alone. This finding is especially important given that these stimuli tend to covary and are projected to increase dramatically in the next several decades. LAY SUMMARY Noise and light pollution often occur together, but their combined influence on wildlife is poorly understood. We manipulated traffic noise and artificial night lighting exposure at Western Bluebird nests. Relative to control nests, those exposed to noise experienced lower nest predation and produced more fledglings whereas chicks in nests exposed to light took longer to fledge and had lower body condition. Nests exposed to noise and light together produced smaller nestlings than control nests and, for some body measurements, those exposed to either noise or light alone.