Counterintuitively, After Extreme Droughts, Wading Birds Flourish

The white ibis and other birds spend their entire lives around water, foraging in it for fish and nesting in the grasses above it for protection against predators. To Frederick, a wading-bird expert at the University of Florida Institute of Food and Agricultural Sciences, three years of bone-dry conditions would surely force the birds to fly to wetter places. Instead, he was shocked to note a surge in breeding pairs of white ibis, wood storks, snowy egrets and tricolor herons.

“It was a classic case of scientists being caught with their pants down,” Frederick said. “We thought there would be nothing, and it was the biggest year in 25 years.”

Frederick, a UF associate professor of wildlife ecology and conservation, now believes what happened in 1992 is only the latest instance of wading-bird surges that occur in the aftermath of severe droughts. In a paper in the December issue of the journal Wetlands, he and a colleague argue that these proliferations occur not only in the Everglades, but also worldwide among diverse wading bird populations.

The cause remains a mystery, but Frederick and co-author John Ogden, a biologist at the South Florida Water Management District, speculate it relates to how droughts affect the population dynamics of another water-dependent animal: fish.

Biologists have long been puzzled by large fluctuations in nesting wading birds from year to year, with one year experiencing thousands or tens of thousands more birds than the next, Frederick said. After the 1992 upsurge, he and Ogden decided to probe the mystery using Everglades climate and nesting records dating back decades.

They identified two spans of years, 1931-1946 and 1974-1998, when there were good records of consecutive years of nesting data. Within these periods, they identified eight severe droughts, when there was little or no water during the January-May nesting season, based on surface water recordings.

Their research revealed that “supranormal breeding events” – years that had statistically many more breeding birds than the average – occurred in the two years following seven of the eight severe droughts. Many years had 10,000 or fewer nesting birds. But during surge years, there were as many as 150,000 birds, the Wetlands paper says.

“You would have these years when, all of a sudden, you’d have birds coming out of your ears – they’d be everywhere,” Frederick said.

Biologists had long believed that droughts benefit wading birds because as rivers and ponds dry up, small prey fish become concentrated and easier to hunt. But Frederick’s and Ogden’s findings showed birds proliferated not during but after droughts.

Frederick and Ogden hypothesize this is because of the drought’s impact on fish populations. In major droughts, there is so little water available that most big fish and the small fish they eat die. But the small fish typically have rapid life cycles – mosquito fish, for example, can reach maturity and breed in two months. By contrast, predator fish, such as largemouth bass, require at least 18 months to reproduce.

This means that, in the year or so following major droughts, there are no major predators going after small fish Frederick said. As a result, they proliferate, creating lots of food for the ibis and other wading birds that eat them.

The theory is bolstered by other environmental changes during major droughts, Frederick said. For example, the Everglades has very low nutrient levels, restricting the number of fish and animals up the food chain. But with the droughts come fires, followed by ash, with a large influx of nutrients into the system. This influx also would tend to encourage fish and, by extension, bird populations, Frederick said.

Although it may have many causes besides drought, the pattern of “pulsed productivity” of wading birds occurs elsewhere in the United States and the world, according to Frederick and Ogden’s research. Unfortunately, the droughts or other environmental disturbances that spur the pulses also tend to be events that people have the most trouble living with, he said. The practical message may be that people should try to avoid mitigating these disturbances.

“If the birds depend on these pulsed conditions, there are likely to be other wetland plants and animals that are similarly dependent on some aspect of drought-flood cycles. Flattening out these cycles would be a mistake,” Frederick said.