Suprise Finding: Wading Birds Thrive After Droughts

Peter Frederick knows better than most of us that sometimes the facts just defy common sense.

It took years for him to accept his own findings, because the evidence he collected as he groped his way through the Everglades suggested something that, at first blush, doesn't make any sense at all. Astonishingly, the evidence indicates the prolonged droughts that periodically threaten the wildlife in that remarkable part of Florida actually cause the population of wading birds to soar.

To Frederick, a wading bird expert at the University of Florida's Institute of Food and Agricultural Sciences, that seemed very unlikely.

The white ibis, wood storks, snowy egrets and tricolor herons depend on the marshy wetlands for the small fish they eat and the tall grasses that hide them from predators. In a dry year, much of that disappears, so it only seems logical that the birds would go elsewhere to breed, or not breed at all.

Surprise Appearances

So no wonder he had trouble believing his own findings when he began his current research more than 13 years ago. A severe drought had gripped the Everglades, and it lasted until 1992.

"It was the most severe drought ever recorded there," says Frederick. "Over 90 percent of the gator holes had dried out," so there was no water in the depressions left in the soft soil by sleeping alligators. And there was little water elsewhere in the area, depriving the birds of many of the resources they need for survival.

So of course Frederick didn't expect to find many nesting birds when the drought finally came to an end.

"We had predicted no nesting, except maybe for a few foolish birds, and we were pretty confident of that," says Frederick, who teamed up with John Ogden, a biologist at the South Florida Water Management District.

But he admits today he and Ogden were caught with their "pants down."

There were birds everywhere. Where they had expected to find no members of some species, they found up to 30,000 nesting pairs.

"It was the biggest year in 25 years," Frederick says.

It didn't make a lot of sense for birds to nest like mad just after a three year severe drought, and for several years the scientists didn't know what to make of it. Could it be that droughts actually improve the survival rates for nesting wading birds?

"We decided to see if there was a real pattern here," Frederick says, although he admits he was "very, very skeptical."


Fortunately, numerous studies had been done over the years of climate and nesting rates for birds in the Everglades. That gave the scientists the raw data they needed to see if the bird explosion they witnessed a decade ago was a fluke, or part of a natural pattern.

The records were particularly complete for two periods, 1931-1946, and 1974-1998. During those periods, there were a total of eight severe droughts. And when they coupled the droughts with nesting rates at the end of each drought, they were blown away.

Although normal years saw 10,000 or fewer nesting birds, there were as many as 150,000 in the year after seven of the eight droughts ended, clearly revealing what the scientists call "supranormal breeding events." The eighth drought ended with a high nesting rate, but not as spectacular as the others.

"It was a nearly perfect pattern," Frederick says. Further research turned up similar patterns in wetlands around the world.

The record was clear, but the mystery remained. Why should wading birds, which depend so much on wetlands, breed with such gusto just as a drought ends? The research, published by Frederick and Ogden in a recent issue of the journal Wetlands, offers a clue.

Getting Rid of the Competition

Wading birds depend on a steady supply of small fish for their diet. But they have to compete with larger fish that also dine on the smaller fish.

During major droughts, there is so little water that nearly all of the bigger fish are wiped out, as well as many of the smaller fish. But most smaller fish, like the mosquito fish, have very rapid reproduction rates. Some can reach maturity and breed within a couple of months. The larger fish take longer, and thus their recovery rate is much slower.

So after a major drought, there are plenty of smaller fish to nourish the birds, and few larger fish to compete with them for that resource. That makes ideal nesting conditions for the birds.

Some scientists think predator fish have little to do with it. Joel Trexler, a biologist with Florida International University, argues that if there's enough water to support the fish after the drought, then the nesting birds will find all they need to survive.

Frederick thinks it's probably both the water level and the absence of predator fish.

But whatever the reason, the research suggests that maybe we ought to think twice about messing with nature. We humans tend to want to "eliminate Biblical disasters," like severe droughts, Frederick says.

"But that may be exactly what the wetlands need," he adds.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.