In the Bluebonnet Swamp, a pair of biologists unfurled a net a bit larger than a volleyball setup and laid their bait — a wireless speaker and a yellow rubber ducky.

The "sweet-sweet-sweet" call of the prothonotary warbler piped out from behind the bath toy, and before long, a flummoxed male charged over to meet his would-be challenger, flying into the net and dropping into one of its pockets.

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Last year, Louisiana Audubon captured nearly two dozen warblers and attached geolocators to them. Once outfitted, the songbirds with heads and chests the color of sweet corn look like they're wearing backpacks. After wintering in Latin America, the warblers are back in the U.S. for the summer. Scientists hope the birds, which weigh about 5 cents in pennies, can teach them about habitat loss on two continents and provide insight into the spread of malaria.

Prothonotary warblers are predictable and a bit lazy, or at least opportunistic, which makes them ideal subjects, avian biologist Katie Percy said. They return to the same breeding grounds every year and dislike digging out their own tree cavities to build nests. On their own, they might take up residence in a discarded woodpecker's hole, and they're happy to settle down in a man-made birdhouse like the ones in the Bluebonnet Swamp.

That allows researchers like Percy and technician Lauren Solomon to deploy nets to catch females flying to and from their nests and males defending their territory. When the scientists snag a bird, they examine, age, weigh and measure it. If it has a geolocator, they'll remove it, though the specimen caught Monday morning wasn't equipped. Also, they did not draw its blood, since they had caught that particular bird — identified by a tag — just two weeks earlier.

Blood samples go to a lab at Virginia Commonwealth University, where avian ecologist Lesley Bulluck uses DNA to track prothonotary warblers as they migrate between the U.S. and their winter grounds in Central and South America, particularly Colombia.

The species' population has fallen about 40 percent in the past 50 years, said Erik Johnson, Louisiana Audubon's director of bird conservation. Warbler numbers are declining faster than the state's cypress and tupelo swamps are disappearing, leading observers to wonder if they're losing mangrove wetlands in their winter grounds.

However, while the population in the Bayou State falls about 2 percent each year, Virginia numbers climb 1 percent annually, Bulluck said. Scientists from multiple states and the Smithsonian Institute hope the new data from the warblers' DNA and geolocators will help them determine how to protect songbirds.

"We are really at the stage of 'we don't know what's going on at the moment,'" Bulluck said.

Prothonotary warbler populations aren't declining as fast as other species, but because they're relatively easy to catch, they can help scientists gauge the health of swamps and hardwood forests, she said

"We put the box out, and they come to us," she said.

Researchers also send the animals' blood to the University of Louisiana at Lafayette, where Scott Duke-Sylvester looks for evidence of avian malaria. Once they've been exposed to the disease, it stays in their system. Some birds die, and those that fight off the infection expend a lot of energy, wearing them down and presumably shortening their lifespans, Duke-Sylvester said.

Like humans, warblers contract the disease through mosquito bites, though it can't pass between birds and people, he said.

"We pretty much have a solid grip on human malaria. We know what makes it tick," he said. "This is really a wildlife issue of preserving threatened and endangered species."

The prothonotary warblers will remain in their nesting grounds through about August. They've already started to mate. On Monday a Bluebonnet nest had a single white and brown mottled egg smaller than a grape. Females will typically lay two or three broods per year, each with four to six eggs, Percy said.

Of the 22 Louisiana birds tagged with geolocators, four summer in the Bluebonnet swamp. Bird watchers have spotted two — a male and a female — though they haven't caught them yet. The geolocators can approximate a bird's latitude and longitude by recording sunrise and sunset measurements, but it doesn't transmit the data. Instead, it must be physically retrieved so the findings can be downloaded. Because the birds are so small, any tracking equipment must be ultra-lightweight, but scientists are hopeful that more advanced equipment can be deployed soon.

Monday, Solomon played the warbler call in the swamp while Percy looked on with binoculars, trying to determine if the bird in her sights was one of the ones with the geolocator equipment, but the bird didn't take the bait, preferring instead to munch on a caterpillar high in its tree, occasionally calling back through mouthfuls, "sweet, sweet sweet."

Follow Steve Hardy on Twitter, @SteveRHardy.