Bird collision with power lines: estimating carcass persistence and detection associated with ground search surveys

Abstract

The quantification of bird mortality due to collision with power lines is complicated by the heterogeneity of survey methods used and the bias related to searching for carcasses on the ground (e.g., carcass persistence and imperfect detection by observers). To estimate the bias associated with ground search surveys, we conducted three 30‐d trials to test carcass persistence by placing and monitoring carcasses of red‐legged partridges (Alectoris rufa) and common pheasants (Phasianus colchicus) below power lines at 14 sites. We also conducted two detection experiments, testing the ability of 19 observers to detect bird carcasses. We used survival analysis and generalized linear mixed‐effects models to investigate the effects of site, habitat, carcass size, and survey period on both carcass persistence and detection. We also investigated the effect of carcass age on carcass persistence and the effect of the observer on carcass detection. Our findings show significant variations in carcass persistence between sites and survey periods, as well as significant interaction between these variables. The daily carcass persistence probability was highly variable between sites, with an up to eightfold variation. Carcass detection increased with increasing carcass size and was significantly affected by the microhabitat surrounding the carcass; it also varied between observers. These findings suggest that both carcass persistence and detection vary strongly and unpredictably at a small scale. As a result, conservation managers should be encouraged to conduct carcass persistence and detection experiments on sites where they aim to produce unbiased estimates of bird mortality below power lines, and these trials should be carried out in conditions similar to the mortality survey. A large‐scale, unbiased, and accurate estimate of bird collision mortality due to power lines may require substantial field effort, with a survey frequency of more than once a week.

Response of vertebrate scavengers to power line and road rights-of-way and its implications for bird fatality estimates

“…This pattern of rapid scavenging has previously been observed in road verges and power lines that bisect agricultural landscapes (e.g. [10][11][12] ), as well as for other anthropogenic sources of bird mortality, like wind farms, fences and pesticides (e.g. 21,25,57 ).…”

“…Hill et al 31 , which performed a similar carcass persistence experiment during winter but in forested habitats, also observed almost no differences in scavenger species identity, as well as in the proportion of carcasses scavenged within power line clearings, compared to control areas. A possible explanation, hypothesized by Borner et al 11 , is that scavenger behaviour may be less predictable under power lines because mortality due to collision with wires may be less frequent than mortality on roads.…”

“…Our experiments were carried out during winter, but seasonal changes in scavenging efficiency are likely to occur (e.g. 11,17,66 ). For example, carnivores' ability to locate carrion is expected to increase with higher temperatures during spring and summer, when olfactory cues are more marked due to decomposers' activity 26,63 .…”

“…Several studies provide, however, evidence of a significant removal of small-bird carcasses in the first hours or days after fatality events, both in power lines (e.g. 10 , 11 ) and roads (e.g. 12 , 13 ), hence a large proportion of carcasses (or its remains) have low persistence rates and are missed by searchers.…”

Preliminary results on bird collision with overhead power lines in Hungary: a case study around Pusztaszer Landscape Protection Area

“…However, data collection was not done regularly and there were no exact notes on carcasses in many times, that is why we may have missed a huge amount of information on the number of individuals and species as well. Results are also influenced by detectability: a large number of individuals would be never detected, if they fall into water or reedbed, and small species are also difficult to find (Borner et al 2017). Furthermore, many injured or dead birds could be collected by predators, in which case carcasses can never be found and never reported (Molnár & Andrési 2019).…”