2009

Flight observations and carcass searches were conducted at six sites at San Luis National Wildlife Refuge Complex, Merced County, California, to evaluate the effectiveness of Bird‐Flight DivertersTM (BFD) and Swan‐Flight DivertersTM (SFD)1 at reducing avian collisions with power lines and identify conditions associated with collisions. In 791 hours of observation in three winter seasons from 2005 to 2008, few actual collisions were observed, but many reactions to power lines were documented, such as altitude changes or more sudden changes in flight direction, called flutter/flares. Birds exhibited reactions at greater distances from power lines after diverters were installed, especially on SFD lines.

Along with the presence or absence of diverters, wind conditions and site were the strongest predictors of a flutter/flare reaction. Based on 403 carcass recoveries, adjusted to account for searcher and scavenger bias, 550 total collisions were estimated in two winter seasons from 2006 to 2008. Estimated total collisions were significantly higher for control lines 2 than for BFD or SFD lines.

The difference between control and diverter lines was even greater when the analysis excluded American coot (Fulica americana), a species accounting for approximately half of all carcasses found. Although conclusions regarding diverter effects are limited by the lack of carcass searches before diverter installation, the consistent pattern of more estimated collisions at control lines, along with the increase in reaction distances after diverters were installed, indicate that both diverter types had a positive effect on reducing the number of avian collisions at San Luis National Wildlife RefugeComplex.

However, there were exceptions. Although the estimated number of collisions for BFD lines was considerably less than for control lines at all sites, the estimated number of collisions for SFD lines was relatively high at Merced East and North, indicating that SFD might have been ineffective at those sites. Also, both diverter types were apparently ineffective for American coot, which flies at night. To improve protection for coots and other species with nocturnal flight habits, diverters that illuminate at night or in low‐light conditions should be added, particularly at Merced West, where most of the coot collisions occurred, and Blue Goose, a second high‐risk site. Because a variety of factors, such as weather and location, can influence the risk of avian collisions, even with diverters on the line, managers should continue to monitor avian mortality after diverter installation.