Field testing an acoustic lighthouse : Combined acoustic and visual cues provide a multimodal solution that reduces avian collision risk with tall human-made structures

Billions of birds fatally collide with human-made structures each year. These mortalities have consequences for population viability and conservation of endangered species. This source of human-wildlife conflict also places constraints on various industries. Furthermore, with continued increases in urbanization, the incidence of collisions continues to increase. Efforts to reduce collisions have largely focused on making structures more visible to birds through visual stimuli but have shown limited success. We investigated the efficacy of a multimodal combination of acoustic signals with visual cues to reduce avian collisions with tall structures in open airspace. Previous work has demonstrated that a combination of acoustic and visual cues can decrease collision risk of birds in captive flight trials. Extending to field tests, we predicted that novel acoustic signals would combine with the visual cues of tall communication towers to reduce collision risk for birds. We broadcast two audible frequency ranges (4 to 6 and 6 to 8 kHz) in front of tall communication towers at locations in the Atlantic migratory flyway of Virginia during annual migration and observed birds’ flight trajectories around the towers. We recorded an overall 12–16% lower rate of general bird activity surrounding towers during sound treatment conditions, compared with control (no broadcast sound) conditions. Furthermore, in 145 tracked “at-risk” flights, birds reduced flight velocity and deflected flight trajectories to a greater extent when exposed to the acoustic stimuli near the towers. In particular, the 4 to 6 kHz stimulus produced the greater effect sizes, with birds altering flight direction earlier in their trajectories and at larger distances from the towers, perhaps indicating that frequency range is more clearly audible to flying birds. This “acoustic lighthouse” concept reduces the risk of collision for birds in the field and could be applied to reduce collision risk associated with many human-made structures, such as wind turbines and tall buildings

Reducing Avian Collisions With Human-Made Structures: A Sensory Ecology Approach To Open-Air Settings

Billions of birds fatally collide with human-made structures each year. These mortalities have impacts on species of conservation concern and potentially on avian populations as a whole. This source of human-wildlife conflict also places economic and operational constraints on various human industries. Furthermore, with continued increases in urbanization, the rate and extent of collisions continues to increase. Efforts to reduce collisions have largely centered on making structures more visible to birds but have been met with limited success. Currently, there is a call for solutions to be tailored to both the environmental context of hazardous structures and to the sensory ecology of at-risk birds. In Chapter 1, we review how and why sensory ecology will help reduce in-flight collision risk for birds. A growing understanding of the sensory systems of birds and of the interface between these systems and the environment will enable the design of appropriate warning and deterrent signals. In particular, we review avian auditory and visual sensory ecology to better understand the susceptibility of birds to collisions and to recommend effective signal design. We highlight the ubiquity and salience of multi-modal signals in avian ecology and evolution, particularly as warning signals, and propose the use of multi-modal signals in mitigating collisions. We encourage the use of animal behavior frameworks to assess collision risk and collision mitigation approaches. Behavioral analyses offer numerous advantages over traditional collision measures, such as mortality estimates. Behavioral data can be generated quickly, render large sample sizes, and allow more nuanced perspectives of the context-dependence of collisions. In Chapter 2, we investigate the use of acoustic signals to reduce avian collisions with structures in open airspace. Birds have largely evolved without tall human-made structures in their flight paths and, consequently, avian perception and behavior may not be suitably primed to detect these novel hazards. Our previous work in captive settings showed that acoustic signals aid in drawing the attention of flying birds to potential collision hazards, influencing flight behavior. The current work corroborates these findings in a field setting. We projected acoustic signals into open airspace surrounding communication towers and quantified movement patterns of birds, to indicate potential collision avoidance behavior. Our results show a ~15% reduction in overall bird activity surrounding towers during sound treatment conditions, compared with control trials. Furthermore, flight movement patterns during sound treatments were characterized by significantly greater distances from and greater displacement of travel direction relative to towers, compared with control trials. Flights during sound treatments also showed significantly slower velocities, compared with control trials. Lower frequency sound stimuli (4-6 kHz) produced larger effect sizes than higher frequency stimuli (6-8 kHz). Results also co-varied with tower location and data collection date, reinforcing an appreciation of the context-dependent nature of collision risk. Our findings will inform the field of avian sensory ecology and help to assess the use of acoustic signals in collision mitigation measures