The graceful and majestic egrets of Hunter Wetlands Centre at Shortland have gone missing for only the second time in 40 years.
The centre's general manager James Wilson said "we don't know where they are".
The birds may have learned from the death of 100 egret chicks at the site in 2018, when hot weather dried out the ecosystem.
Records show the first time the egrets didn't nest at the centre in its 40-year history was 16 years ago.
The birds usually come in their hundreds and build lots of nests at the wetlands centre.
"When they come in numbers, it's an incredible sight," Mr Wilson said.
"Last year, the count was 150 nests. The year before it was 300. It ranges from 100 to 400 nests each year."
Mr Wilson said it was not known why the egrets weren't nesting there this year but "we have theories".
"The strongest theory is the dry weather," he said.
It was estimated that about a quarter of chicks nesting at the site died in the dry summer of 2018.
Mr Wilson wondered "whether the birds remembered that".
More than 100 egrets did arrive at the site in November last year, but left after a few days.
"Whether they came and realised it was too dry and didn't want to go through that and left - we're not too sure," he said.
Records from the first time the egrets didn't nest there - in the summer of 2007-08 - show it was dry then, too.
"The ground is really dry now. All our ponds are dry, except for one," he said.
"We had a dry winter and then the heat came and the water evaporated. Any water that has come, it's soaked straight in."
He said it was "somewhat concerning" that the birds may be in "a slight decline".
The centre would consider "how to build that back up".
Four egret species nest at the wetlands centre - mainly cattle egrets, with great, intermediate and little egrets in smaller numbers.
Hunter bird expert Rob Kyte said the site was "normally a hive of activity this time of year".
"The birds have usually bred and there's a cacophony of noise. It's a fantastic place," Mr Kyte said.
"It's just so dry that the birds are having to move on, which is quite sad.
"They're not going to breed if there's no food and water around. Even at Hexham Swamp, it's very dry at the moment."
Mr Wilson said the egrets were also missing from other nesting areas around the Hunter.
