"If you take a drive through some parts of my country, very high tension power-line pylons with stork nests are an increasingly common sight," said Francisco Moreira of the Universidade de Porto and Universidade de Lisboa in Portugal. "Power lines are usually associated with negative impacts on bird populations, through collision and electrocution. However, white storks seemed to be taking advantage of these structures, so we sought to analyze the pros and cons of this behavioural change."

The researchers estimate that 3282 white storks – around 11% of the population during the breeding season – were killed by power lines owned by power company REN (Redes Energéticas Nacionais) in 2004.

The storks’ choice of nest site didn’t affect the number of chicks they successfully raised, Moreira and colleagues found. The birds’ average productivity was 1.88 fledglings per nest and there was no difference between nests on electricity pylons and nests on other structures, although buildings did show lower values. That said, the technique the team used would only have registered negative impacts larger than an average decrease of 0.5–1 fledglings per nest.

Storks build nests on trees, buildings, cliffs and poles like pylons, telephone poles and poles kitted out with artificial nest platforms. From 1958 to 2004, trees were their number-one choice. Since then, the birds have nested on more poles, making them the most popular nest site in 2014. Storks seem particularly keen on the pylons of very high tension power lines, which are large and can hold as many as 30 nests.

"This progressive ‘wiring’ of white stork has consequences for the bird population and for the power-line companies," said Moreira. "For the storks, a preliminary estimate of the contribution of the very high tension power-line grid to mortality and productivity (number of chicks produced in pylon nests) suggests that, through time, the number of individuals entering the population has been progressively compensating the increased mortality due to this infrastructure."

Moreira and colleagues believe this is because the length of the REN power-line grid in stork habitat, which increased by a factor of 1.8, has risen more slowly than the number of nests in power-line pylons, which increased 12.1 times.

"From the power-line managers’ perspective, an increased use of pylons by storks implies an increased risk of power outages and a need for impact mitigation measures," said Moreira. "The current policy includes translocating nests from risk areas to dedicated nesting platforms in safe areas of the pylons, followed by the installation of anti-nesting devices to avoid storks coming back to previous nest sites."

Portugal has a very high voltage (150–400 kV) transmission grid managed by the REN (Redes Energéticas Nacionais) company, and high, medium and low voltage distribution grids managed by the EDP (Energias de Portugal) company. The length of the very high voltage power-line grid increased from 1958 to 2014, boosting the number of high-voltage pylons from 1256 to 11,002.

In 2015, REN installed 8,233 anti-nesting devices and 3143 nesting platforms. Between 1999 and 2012 the company translocated 1347 nests. As a result, its number of outages per 1000 nests fell from 739 in 1993 to 13 in 2014.

"Currently, this progressive ‘wiring’ of an entire bird population seems to be a win–win situation, with companies showing a biodiversity-friendly attitude, promoting their image to the public and being able to keep negative impacts and mitigation measures at acceptable costs, with no apparent impacts at the stork population level," said Moreira.

It’s likely that the stork population has soared in Portugal because of improved climate and feeding conditions in the birds’ wintering range in the West African Sahel and in Iberia. Iberia now sees milder winters and more food from a higher area of rice fields, open garbage dumps and the introduction of the red-swamp crayfish from the US. In 2014, storks were living in around 70% of Portugal’s area.

According to Moreira, if the white stork population continues to increase and exploit the expanding power-line grid in Portugal and elsewhere in Europe, the costs of impact mitigation measures will probably rise, "eventually representing a significant investment in financial and human resources." And the extra stork deaths due to collision with and electrocution by additional power lines could start to affect the stork population trend.

The next steps for research include understanding why storks are using pylons by investigating the importance of pylon technical features, the surrounding landscape and distance to stork feeding grounds; and evaluating the consequences of pylon use for population dynamics, taking into account other sources of mortality and productivity.

Moreira and colleagues reported their findings in Environmental Research Letters (ERL).

• This article was updated on 21.6.17 to correct the figure for anti-nesting devices.

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