PLANTATION KEY -- Fewer roseate spoonbills built nests in Florida Bay this winter than in any nesting season since 1952-53, according to researchers at Audubon of Florida's Tavernier Science Center.

What caused the dramatic decline, from 223 nests last year to just 69 nests this year, for now remains a mystery, those researchers say.

"With this kind of work you want to see how they nest the next few years in a row, and then look back and see what is different," said Karen Dyer, Audubon's lead field worker for the spoonbill count.

Because they are especially dependant upon the normal summer wet/winter dry seasonal cycle of the Everglades, the spoonbill is considered a key indicator species on the health of Florida Bay.

Nest counts often go down in years when the dry season is not as dry as usual, a scenario that robs spoonbills of foraging grounds. That's what happened last year, when the nest count of 223 was the lowest since the late 1960s.

But what makes this year's exceptionally low count especially vexing is that climatic conditions have been ideal. A fairly typical wet season has been followed by a dry winter.

"All the data, from banding, from satellite tags, from nest monitoring over the years would indicate we would have had a good year this year. But it wasn't," Dyer said.

Still, the news might not be as bad as it sounds for South Florida's spoonbill population as a whole. Traditionally, Florida Bay is the primary nesting spot in the region for the beloved pink, red and white bird.

But this year the nest count in what is called Water Conservation Area 3 -- a swath of the Everglades watershed to the west of Fort Lauderdale -- was more than 100, up from just four last year.

"I think the birds breeding up there are very likely Florida Bay birds," Dyer said.

Spoonbills have also moved nests inland in Everglades National Park and to areas along the gulf coast, park service biologist Dave Hallac said.

"While we are preliminarily concerned about the Florida Bay numbers, it's difficult to understand or assess the situation without knowing the status of nesting across all possible nesting areas," Hallac wrote in an e-mail.

Dyer said the likelihood that Florida Bay spoonbills are nesting in places other than the bay brings up another line of inquiry.

The birds typically return to breed in the area where they were hatched. And there's no obvious reason relating to the bay's health that would cause them to break that pattern, she said.

Pollution and water management practices led to a precipitous decline in Florida Bay water quality in the 1980s and 1990s. But the bay has experienced no eventful deterioration in the past couple years. In fact, two major algae blooms that lingered in the bay between 2006 and 2008 have cleared.

And beginning in 2005, the South Florida Water Management District began considering the spoonbill's needs while deciding when and how much freshwater to release from inland Everglades canals toward the bay.

What happens next year is anybody's guess. But Dyer isn't optimistic.

"My opinion is we're going to see even fewer nests next year, but we really don't know," she said.

rsilk@keysnews.com