The federal government had successfully sued developers under the Migratory Bird Treaty Act (MBTA), resulting in multi-million dollar fines.
But a memorandum published by the DoI on 22 December confirmed the Act, established to protect migratory birds, "does not prohibit incidental take", meaning deaths "that results from an activity, but is not the purpose of that activity".
"Incidental take", which is not to be prohibited by the Act, would include activities such as wind farm or solar park development, the DoI affirmed.
Daniel Jorjani, the principal deputy solicitor of the DoI, concluded that the Migratory Bird Treaty Act only applies to "direct and affirmative purposeful actions" that kill migratory birds, or harm their eggs or nests — hunting, for example.
It should not be interpreted to include "incidental" killing of birds, for example, by colliding with wind turbines, Jorjani wrote.
According to the latest US Fish and Wildlife Service (FWS) figures for "human-caused threats to birds", which were last updated in May 2016, collisions with wind turbines account for approximately 174,000 bird deaths every year.
By comparison, the estimated figures show 25 million birds are killed every year after colliding with electrical lines; 750,000 die in oil pits; while cats kill an estimated 2.4 billion birds per year, making feline predators the greatest threat to US birds.
Jorjani described the MBTA as the "epitome of vague law", arguing that developers could be prosecuted even if they fully complied with everything requested of them by the FWS when building a wind farm.
Cases prosecuted under the Act had resulted in courts extending the MBTA to include "incidental" deaths, while others had limited the law to exclude "incidental" deaths, he wrote. The latter interpretation raised serious concerns, Jorjani added.
He concluded that courts and agencies should "avoid interpreting ambiguous laws in ways that raise grave constitutional doubts if alternative interpretations are available".
Jorjani added: "Interpreting the MBTA to criminalise incidental takings raises serious due process concerns and is contrary to the fundamental principle that ambiguity in criminal statutes must be resolved in favour of defendants."
Accordingly, the US federal government should not prosecute individuals or companies over the incidental killing of migratory birds, he wrote.
In November 2013, Duke Energy became the first wind power company to be successfully prosecuted under the Act and paid $1,000,000 in fines after pleading guilty to violating the MBTA.
The prosecution followed 163 migratory bird deaths, including 14 golden eagles, at Duke Energy’s 200MW Top of the World and 99MW Campbell Hill projects in Wyoming between 2009 and 2013.
Utility PacifiCorp Energy pleaded guilty to violating the MBTA in 2014, after 374 migratory birds were killed at its 118.5MW Seven Mile Hill project and 99MW Rolling Hill wind farm in Wyoming. It was fined $2.5 million.
Both companies were also ordered to adopt a migratory bird compliance plan at all of their projects in Wyoming.