At 5.51pm on Sunday (May 31), firefighters, alongside specialist Land Rovers were called to a fire in the open in the Combe Valley Countryside Park.
The blaze stretched across three hectares of reed beds, according to East Sussex Fire.
Firefighters used beaters and flexipacs to tackle the flames.
A spokesman for East Sussex Fire, on Monday (June 1), said the fire is believed to be accidental but Sussex Police said it is now investigating the cause.
Firefighters spent hours tackling the fire before the incident was left with the park rangers.
Hastings Police said: "Sadly we received a report of a fire at Combe Valley Countryside Park yesterday evening (May 31), it is estimated three hectares of reed beds, located in a SSSI have been lost.
"This is such a sad loss to the environment, the fire will likely have killed wildlife and destroyed birds’ nests in the area. The cause of the fire is unexplained, and there is no current evidence that it was deliberately started, anyone who saw anything suspicious or may have any information is asked to contact us by e-mail or phone 101, quoting serial 0270 of 01/06."
PCSO Daryl Holter said: “The area is an important breeding ground for birds. They have now lost their nesting areas. Wild birds and wildlife are protected under the Wildlife and Countryside Act, it is an offence to intentionally take, injure or kill or to take, damage or destroy a nest, eggs or young."
Following the fire, Hastings Police warned people to properly dispose of their cigarettes and barbecues as the recent warm weather has increased the likelihood of fires starting in long, dry grass.
Jay Brewerton, independent councillor for Bexhill Pebsham and St Michaels, said the 'major fire' was the fourth in the last few days that volunteers have been sent to.
She added: "We have had a major fire in the Combe Valley Park started late afternoon early evening, it unfortunately consumed a very large area of the reed beds, with flames up to ten foot high.
"It's truly sad as we have many nesting birds in the area and other wildlife too. Hastings Fire Service were in attendance and it took several firefighters to extinguish it.
"It's the fourth fire in the last few days that the volunteers have been out to. Most have started accidentally with portable BBQs in tin foil trays. Can I please remind people, if using these, do not leave them unattended or burning out."