Analysis of the ecological impact of building a major new road scheme near landscape made famous by poet Seamus Heaney is not fit for purpose, the Court of Appeal heard today.
Ornithologist Chris Murphy claimed the environmental assessment for part of the £160m A6 Belfast to Derry was out of date, and that the project would destroy the tranquility which draws migratory birds back every year.
But lawyers for the Department for Infrastructure insisted surveys of whooper swan patterns on Lough Neagh and Lough Beg close to the disputed Toome to Castledawson stretch have been maintained annually.
Mr Murphy is seeking to overturn a ruling that the proposals do not breach a habitats directive on specially protected areas.
Reserving judgment on his appeal, Lord Chief Justice Sir Declan Morgan pledged to give a decision as soon as possible.
The environmentalist's continuing legal battle centres on plans to construct part of the road near Moosbawn, Co Derry - the childhood home of the former Nobel laureate poet.
The route was identified following a public inquiry nearly a decade ago.
Former Infrastructure Minister Chris Hazzard gave the green light to the scheme last year in a bid to significantly improve a major transport corridor and ease rush-hour gridlock.
Proceedings centred on ecological checks made to potential disturbance to the wintering swans.
Earlier this year a High Court judge held that the Department's decision was lawful and rational.
Representing himself once again in an appeal against that verdict, Mr Murphy claimed the project was based on environmental information from nearly a decade ago.
He insisted the authorities were legally required to carry out a new appropriate assessment of the ecological impact under Article 6 of the Habitats Directive.
Instead, he claimed, the analysis carried out in 2016 was "not fit for purpose".
According to Mr Murphy the area represents the most important wintering site for whooper swans in Ireland - but their numbers are now in decline.
Building a dual carriageway there will destroy the seclusion and tranquility which brings the birds back, he contended.
"This whole complex provides a mosaic of fields and provides familiar territory when these swans come in to land," he said.
However, Paul McLaughlin, for the Department stressed how the Royal Society for the Protection of Birds (RSPB) was consulted and concluded that the scheme will not have an adverse impact.
He argued that data has been kept up to date with annual surveys on the swans carried out to ensure complete monitoring of the entire complex.
"This was not just someone going out with a pair of binoculars and counting the number of swans in a field," Mr McLaughlin added.
Following submissions Sir Declan, sitting with Lord Justice Gillen, confirmed a decision on the appeal will be given at a later date.
He said: "We are going to reserve our judgment and give it as soon as we can."