HawkWatch International conducted a study, funded by the Bureau of Land Management (BLM), to assess the ability of perch deterrents to limit raptor and raven perch use on a new power line constructed in southwestern Wyoming. The results are published in the July 2010 edition of the Journal of Wildlife Management.
In open habitats, power line poles and other support structures provide attractive perches for raptors and ravens where few natural perches exist. The study area is home to many species of management concern, including the Greater Sage Grouse, Sage Thrasher, and Pygmy Rabbit, due to recent declines in their populations and threats to their persistence.
Were perch deterrents effective in excluding raptors and ravens that may be attracted to and concentrate around the new power lines perches?
Three types of surveys were conducted along both the “deterrent” line and a control line (no deterrent device) between September 2006 and August 2007 in order to monitor raptor and raven activity.
1. Driving surveys were conducted along roads parallel to the power lines at a predefined speed during which the observer simply scanned the power poles for any perched birds; when seen, they stopped and recorded information, including species, location, behavior, etc.
2. Behavioral observation surveys were conducted for 1 hour periods where the observer sat at a pre-selected point and watched a segment of the powerline for any activity. Their survey “window” was the pole directly in front of them and three on either side, seven poles in total.
3. Prey-remains surveys consisted of searches under three pre-selected poles for any pellets or prey remains within a 10 meter radius of the poles.
Each of the three survey types showed that raptor and raven activity was significantly lower on the deterrent line, but perching was not prohibited entirely.
During 192 driving surveys along each line, only 42 raptor and raven sightings were noted on the deterrent line, compared to 551 sightings on the control line.
The behavioral studies recorded 31 deterrent line behaviors and 124 control line behaviors during 192 hours of observation. The behaviors were mostly general perching, flights to and from perches, and flights through the observation area.
Results from 576 prey-remains surveys near each line noted 17 single prey items and 65 grouped items near the deterrent line compared to 398 single prey items and 493 grouped items near the control line
THE BOTTOM LINE:
Overall, our results suggest that perch-deterrent devices can reduce raptor and raven activity
on power-line structures, but to determine their utility on entire power-line segments, we suggest managers consider 1) what level of reduction in perch activity is worth the cost, and 2) the availability of alternate perches in the surrounding landscape.