Recent noteworthy research is helping the Endangered Wildlife Trust (EWT) to better manage the interactions between birds and electrical infrastructure.
The EWT first identified a conservation need around wildlife and power line interactions in 1996. The most common problems are electrocution of perched birds, and birds colliding with power lines. Birds can also cause short circuits in the electricity supply, which costs both Eskom and the electricity user.
A species of particular concern to conservationists involved in wildlife and power line mitigation is the Ludwig's Bustard Neotis ludwigii. This bird, which is near-endemic to southern Africa, is extremely susceptible to collisions with power lines, and was uplisted to globally Endangered in 2010 based on the anticipated population decline stemming from this form of mortality. Surveys suggest that at least 11-15 percent of the population may be killed annually on high-voltage transmission lines, although actual mortality is probably much higher given biases in carcass detection and the fact that estimates exclude mortality on lower voltage distribution lines and telephone wires. Given a global population of about 56 000 - 81 000 birds in the late 1980s, this mortality seems unsustainable.
In light of this, research is being conducted to find out more about the impacts of collisions on populations of this long lived species, and to investigate ways to mitigate this through censuses, mortality surveys, marker experimentation and satellite tracking. Initial census counts indicate a population decline, and line surveys across the South African range reveal high variation in collision rates. Three satellite tags have already been deployed on male Ludwig's Bustards, and initial results indicate that their daily movements are relatively short. As such occasional large-scale movements may pose the greatest threat to these birds.
A 199 km stretch of power line was recently surveyed in the Overberg region, and 123 bird carcasses of at least 18 species were found. Collisions were more common than electrocutions, apparently killing 88 percent of the birds found on distribution lines.
Large terrestrial birds were the most numerous victims, with many Blue Cranes and Denham's Bustards Neotis denhami killed. 54 percent of all carcasses were Blue Cranes.
About 12 percent of the total Blue Crane population within the Overberg study area could be killed annually in power line collisions.
This represents a possibly unsustainable source of mortality, and highlights the urgent need for further research into risk factors.
The survey again highlights the hazard that power lines pose to birds, and the urgent need for further research into the population impacts of the high incidence of collisions.
Locating and mitigating collision 'hotspots' (short sections of line where multiple collision events have occurred) may be the most effective approach to reducing collision mortality in this region.
Bird mortality on power lines in South Africa is currently recorded on the Central Incident Register (CIR), which is a collation of incidentally reported cases, managed by the Endangered Wildlife Trust.
The Overberg study estimated that only 2.6 percent of power line mortalities in that area are reported, emphasising the importance of systematic surveys in quantifying mortality and directing mitigation.
An important factor in bird collisions with power lines that previously received little attention is birds' visual capacity. In a recent research project, visual fields were determined in three birds known to frequently collide with power lines: White Storks Ciconia ciconia, Blue Cranes Anthropoides paradiseus and Kori Bustards Ardeotis kori.
All three species live in open habitats and are visually guided ground feeders with large eyes. While all three have the narrow and vertically long binocular frontal visual fields typical of birds that use vision to guide the capture of prey in the bill, they differ markedly in the vertical extent of their binocular field.
As such, the extent of the blind spot ahead of the birds, above and below their binocular fields differs. When a bird is in flight, vertical movement of the head such as when birds scan the ground below for foraging or roost sites, can blind it in the direction of travel. In bustards and cranes head movements of only 25° and 35° respectively are enough to render the birds blind in the direction of travel, but storks require head movements of 55°.
As such, bustards and cranes are probably more vulnerable to power line collisions because of the small size of their binocular fields and their substantial blind spots.
These findings may also apply to other bird families that are prone to collisions, including some of the larger diurnal raptors that have small binocular fields similar to those of bustards and cranes.
Because visual fields vary among bird species, it is unlikely that we will find one solution to all collisions.
While line marking is probably effective for storks, birds such as bustards will need additional solutions, although we recommend that power lines continue to be marked.
Distracting these birds away from obstacles or encouraging them to land nearby, so making them more aware of their surroundings and of marked power lines, may help to prevent collisions.