RICHMOND, Va. (AP) — Developers of the proposed Atlantic Coast Pipeline have asked federal regulators for more time to cut down trees along the project’s route, saying they likely can’t finish the work under an initial deadline designed to protect birds and bats. The request, which drew criticism from pipeline opponents, was made in a letter to the Federal Energy Regulatory Commission posted online Friday. “Despite their best efforts, it now appears that Atlantic and (Dominion Energy Transmission Inc.) will be unable to complete the scheduled tree felling” in West Virginia, Virginia and North Carolina on time, the letter said. It seeks approval to continue cutting trees to clear the way for construction of the 600-mile (970-kilometer) natural gas pipeline until May 15, with certain exceptions. The proposed modification would still provide “equal or greater” environmental protection, the letter said.

Developers initially agreed to the tree-felling restrictions to protect migratory birds, and threatened and endangered species — two types of bats, in this case. The time restrictions vary from state to state but generally prohibit tree cutting between mid-March or early April through mid-September or mid-November. The earliest restriction to kick in was Virginia’s migratory bird window, which started Thursday. Pipeline spokesman Aaron Ruby said in an email that about 40 percent of the work in Virginia has been completed. Completing the tree felling within one season would limit “the amount and frequency of disturbance in the general area, benefiting landowners. Finally, failure to complete the tree felling will affect the 2018 construction season,” the letter said.

A presentation Dominion recently gave state environmental officials, obtained through a public records request and provided to The Associated Press, provides insight on the business impacts project delays would have. The presentation said that if “full mechanized construction” can’t start until spring, there would be a $150 million to $200 million impact. If only partial construction is started by summer, there would be a $250 million to $350 million total impact, the presentation said. Finally, it said that if “we cannot start in time to ensure a full and efficient construction season and have to delay service by one year, the impact would be $1 billion.”

According to the letter, the proposed modification will be at least as environmentally protective as the initially agreed-to limits. “Biological monitors will survey all work sites before we begin any tree felling,” Ruby said. “If we find any bird nests, we’ll place a protective buffer around them. All of the work will be closely monitored by state and federal inspectors.” But opponents said the request should be denied. “It would be unconscionable for FERC to allow Dominion to slide around an important protection merely for the company’s convenience,” said David Sligh, conservation director for Wild Virginia, which is fighting the pipeline. “Too much destruction has already been caused. It must not be allowed to continue,” he said.

Separately Friday, the Virginia Department of Environmental Quality issued a violation notice to the pipeline for conducting tree felling work in buffer areas intended to protect stream and wetlands crossings, in violation of state law. The notice covers self-reported violations on 15 separate sites resulting in an estimated 0.84 acres (0.34 hectares) in impact to wetlands and streams, according to a statement from the department. Ruby said immediate steps were taken to prevent such a violation from happening again, including halting all work on the project for three days “to determine the root cause and reinforce environmental compliance” with work crews. “We will have zero tolerance for regulatory non-compliance, and we have reinforced this policy with all employees and contractors associated with the project,” he said in an email.