Carcass removal by scavengers and search accuracy affect bird mortality estimates at power lines

Bird mortality as a result of collisions with power lines has been of increasing concern in recent decades, but the real impact on bird populations requires an experimental assessment of scavenger removal rates and searcher detection errors. Farmland and steppe birds, two of the most threatened avian groups, have been shown to be particularly vulnerable to collision with power lines, but few removal and detectability studies have been developed in cereal farmland habitats, and none in the Mediterranean region. We conducted five carcass disappearance trials in central Spain by placing 522 corpses of different sizes under power lines, and searching for remains four times during the following month. The influence of several factors was examined using multivariate approach. The accumulated number of carcasses removed by scavengers increased logarithmically, with 32% removed over the 2-day period after the initial placement, but only 1.5% removed on a daily basis by day 28. Small birds disappeared earlier and at a higher proportion than larger birds. Carcass removal rates were site-dependent, but were not influenced by carcass density or season. The detection rate increased with the observer's previous experience and carcass size. Carcass counts at power lines notably underestimate bird casualties. Our 4-week disappearance equations provide a full range of scavenging rates and observer efficiency correction factors for a wide range of bird weights. Fortnightly to monthly search frequencies may be adequate to detect medium- to large-sized corpses, but are insufficient for smaller birds. Finally, all personnel participating in carcass searches should be trained previously in this task.