I was trying to clear out my backlog of owl research papers to discuss, but I followed a citation to this short paper. Some very interesting stuff on how noise we generate impacts animals in ways that are probably not obvious to us.
Since most owls rely on hearing very quiet sounds to find food, it does not take much to interfere with that process. This study examined various noise levels and its effects on hunting.
Some key takeaways:
impacts of traffic noise on owls’ ability to detect prey has the potential to reach >120 m from a road. In other words, owls’ ability to detect prey was impacted even at the lowest level of TN (40 dB[A]) and was approximately 17% lower than detections in ambient sound conditions.
40db is equated to a quiet library or the hum of a refrigerator. 120 meters is about 400 feet for us in America. This is over twice the distance a prior study had shown background noise affecting bats, though that was a lab study while this owl study was done outdoors.
Distraction, in which owls attend to traffic noise rather than rustling sounds, could also explain declines in prey detectability and could operate along side masking. However, it is also possible that distraction or compromised attention could decrease with habituation to traffic noise over time. Distinguishing among these potential mechanisms must be a next step.
Good details to consider. I'm always ready to read more owl studies!
High frequency components of TN attenuate faster with distance from roads than lower frequency components, suggesting overestimation of the masking effects of TN playbacks at amplitudes reflective of 55, 105, 155, 205 m from the road. However, because APRS playbacks were louder than natural prey rustling sounds and APRS might be easier for owls to detect than actual prey rustling sounds with broadband energy, effects of TN on owls’ prey detection may extend well-beyond our 120 m estimate.
To answer your first questions: TN is traffic noise, APRS is artificial prey rustling noise.
Besides the omnidirectional broadcasting of the APRS not replicating the correct sound vectors of real prey, it sounds like they tried to compensate the effects of constant traffic noise generated against the simulated prey sounds since their recorded traffic noise didn't ebb and flow like actual traffic.
Nevertheless, given our playback is representative of traffic noise propagating from other roadways, it is likely that impairment of foraging at similar distances is generalizable to other roadways. Moreover, a recently published captive study showed that experimental playback of compressor noise, which has similar power spectrum with traffic noise, negatively impacts hunting behavior of northern saw-whet owls (Aegolius acadius) at sound levels as low as 46 dB(A), which corresponds to approximately 800 m from compressor stations.
That seems crazy at first. 800 meters is about 2600 feet or 0.5 miles! But considering how quiet a rodent rustling through leaves would be (35 dB max, by this study) that makes a lot of sense that it needn't take much to mask that level of sound.
These potentially sizable footprints from energy-sector and traffic noise highlight the pervasive impacts of noise on acoustic predators because many sources of noise, including road densities, are high and increasing. For example, 83% of the continental US is within 1061 m of a road and globally, >25 million kilometers of new roads are anticipated by 2050.
This has a staggering potential impact to the owls! In a world where food is life, a 17% minimum impact over so much land mass sounds severely consequential! 😮
Moreover, it is critical to understand how common prey species respond to roadways and traffic and determine whether the cumulative effects are additive, synergistic or even antagonistic, as some nocturnal small mammals appear to increase in noise exposed areas and along roadways. Regardless of the shape of these interactions, it is likely that wild owls and other acoustically-oriented predators will continue to be impacted by noise.
This is why I decided to do this article first, despite it adding to my backlog. The original study I was going to share today was a study about how owls adapt to crossing roads, so I felt it made sense to cover the impact first.
Full article here if you want to read it all.
