David M. Larson
Traffic noise may mute interspecific reactions to Tufted Titmouse alarm calls. Photograph by John Flannery (CC BY-SA 2.0).
I’ve been thinking a lot about noise lately. Early this summer I had to relocate one of my Breeding Bird Survey points to reduce the incessant noise from a propane station. The compressor made it nearly impossible for me to hear any bird sounds. Another survey point is too close to the traffic of Interstate 95. Those observations lead me to think about how birds react to anthropogenic noise. Recent studies have shown that noise affects animal distribution, behavior, and reproductive success (Francis and Barber 2013).
A study published recently (Grade and Sieving 2016) tested the effects of highway noise on interspecific communication in songbirds. Specifically, the researchers tested the extent to which highway noise affects the response of Northern Cardinals to playback of Tufted Titmouse alarm calls. They played the titmouse high-seet alarm calls at varying distances from two busy highways (3200–3600 vehicles per day) in north central Florida. Their hypothesis was that the typical anti-predator responses of the cardinals (freezing) would decrease with increased ambient noise levels. Indeed, they found that at background noise levels above 50 decibels (dB)—for comparison, 50 dB is as loud as a moderate rainfall, per the American Speech-Language-Hearing Association—none of the cardinals responded to playback of alarm calls, but at lower noise levels, almost 80 percent of the cardinals responded to the same alarm calls. Clearly, the response of the cardinals was blunted by higher ambient noise levels. What this project did not address was whether the noise masked the sound of the alarm calls so the cardinals could not hear them or whether the noise distracted the cardinals so that they did not respond normally.
And then there are birds that rely primarily on their hearing to hunt for food, such as nocturnally hunting owls. Mason et al., (2016) have conducted studies on the effects of noise on hunting success in Northern Saw-whet Owls. The authors, who all work in Idaho—a state in which energy extraction is an important industry—recorded the chronic broadband sounds generated by a natural gas compressor station and used this noise source to test hypotheses regarding effects on captive owls. They captured owls in mist nets and transferred them to a flight tent in the field. The tent was light-proof and the owls were allowed a day to acclimate to the tent with internal lights. Then the birds were tested for their ability to capture released mice, in total darkness, under noise levels replicating the sounds of compressor stations at distances from 800 down to 50 meters distance, corresponding to 29–73 dB. The researchers tested two hypotheses: if this anthropogenic noise reduced hunting success at all (threshold hypothesis); and if hunting success decreased with increasing noise (dose-response hypothesis). By monitoring the owls using infrared sensitive video recorders, the researchers also attempted to determine if any deficit in hunting was traceable to the problems in prey detection, attempted capture, or successful capture.
Testing 30 owls in 184 trials over two years, the research indicated that the dose-response hypothesis held for all three parts of the response to prey, and successful capture upon strike was equally modelled by both hypotheses. The odds of hunting success decreased by 8% for each decibel increase in noise. The odds of detection of the mouse decreased by 11% for each decibel increase in noise. Overall, capture of prey was degraded by noise up to 61 dB (equivalent to 200m from the original sound source). At higher volumes, there were no successful captures. Similar to the Grade and Sieving (2016) paper on Northern Cardinals, these authors could not distinguish between masking and distraction as causes.
These two papers present results consistent with the growing literature showing significant changes in bird behavior with increasing ambient noise from anthropogenic sources. In the first case, bird response to predator-avoidance cues were demonstrated and, in the second, degradation in auditory hunting by owls was shown. Much recent research has demonstrated that bird diversity and population density are lower near sources of high ambient noise. The researchers in Florida noted that Northern Cardinals were less abundant near those noisy highways in Florida than in woodlands far from the roads. Although Mason et al. (2016) did not address population levels, I suspect that Northern Saw-whet Owls would not choose to hunt near compressor stations where they cannot detect prey audibly.
The sound spectrum of noise can be a significant variable. Norman Smith, director of the Blue Hills Trailside Museum (Mass Audubon and Massachusetts Department of Conservation and Recreation) in Milton has noted that Snowy Owls at Logan Airport in Boston can detect and respond to mouselike squeaks against a backdrop of jet engine noise (personal communication). These observations suggest that Snowy Owls are able to pick out those high frequency sounds and are not overwhelmed by jet engine noise.
Anthropogenic noise is everywhere, from cities, highways, and many other sources. And, lest you think that gas compressing stations are limited to the west, all natural gas pipelines require compressing stations every 40–100 miles to keep the gas flowing; this topic has been controversial recently in Massachusetts, with plans for new pipelines. Noise mitigation efforts, designed to protect humans, may end up helping birds and other wildlife as well.
- Francis, C.D., and J.R. Barber. 2013. A framework for understanding noise impacts on wildlife: an urgent conservation priority. Frontiers in Ecology and the Environment 11: 305-13.
- Grade, A.M., and D.E. Sieving. 2016. When the birds go unheard: highway noise disrupts information transfer between bird species. Biology Letters 12: 20160113. Accessed on 8/8/2016.
- Mason, J.T., S.J.W. McClure, and J.R. Barber. 2016. Anthropogenic noise impairs owl hunting behavior. Biological Conservation 199: 29-32.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, Vice President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.