Bluebonnet Swamp: A center for warbler research

05.16.2019

A charismatic yellow bird worth saving
By Sara Bongiorni

GeoDad was more or less standard issue as Prothonotary Warblers go.

Bright-eyed, blunt-tailed and just five inches long, the songbird with a yellow head and a plump body was a confirmed father by his second breeding season at BREC’s Bluebonnet Swamp Nature Center—all standard stuff for a species also called the Golden Swamp Warbler.

But what happened in GeoDad’s second year wasn’t customary.

Researchers and volunteers trapped the bird, affixed a tiny tracking device to his back and released him back into the urban forest that bumps up against attorneys’ offices and surgery centers on busy Bluebonnet Boulevard.

Their hopes? That GeoDad would return to the BREC park to breed the following year. That the 0.5-gram light-sensitive device on his back would not fall off or malfunction. That capturing the path of his winter migration could help save a species whose declining population has made it a major focus of U.S. conservation efforts.

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GeoDad did not disappoint. The bird returned to the BREC park eight months later with the geolocator on his back. The device showed he had flown 4,800 miles since leaving Baton Rouge the previous summer.

The migration data wasn’t just a big deal for Louisiana researchers. The journey that began at Bluebonnet Swamp spurred similar Prothonotary-tracking projects across multiple states. That work is shedding light on how to save a bird whose numbers have fallen by 50% over 40 years.

Erik Johnson, director of bird conservation for Audubon Louisiana, describes the warbler as charismatic. It would be hard to argue the point. The bird in 1948 made an appearance of sorts before the House Un-American Activities Committee when Whittaker Chambers testified that birdwatcher and accused spy Alger Hiss once boasted he had seen one.

Hiss, who had denied knowing Chambers, later mentioned the bird sighting himself, sealing his traitor’s place in Cold War history.

The warbler feeds mostly on swamp insects, spiders and snails. It is one of just two North America cavity-nesting warblers. Its beak is ill-suited for boring holes in tree trunks, so it typically nests in holes vacated by woodpeckers. It will use manmade nesting boxes, however, like the 26 that are attached to stakes above the water and out of predators’ reach at Bluebonnet Swamp. Mating pairs can raise three clutches of chicks in one season.

The warbler is also pretty. Johnson compares the bird to a “swamp candle” in reference to the brilliant yellow head that contrasts with a blue-olive body. It likes stagnant water. Its call is a series of loud zweets.

“It’s an easy bird to like,” Johnson says.

The warbler has ample Louisiana connections. Between 15% and 20% of its global breeding population is found here, including 5% in the Atchafalaya Basin. The Prothonotary appears on Plate No. 3 of John J. Audubon’s Birds of America. Bluebonnet Swamp is a microcosm of the still, watery habit the bird likes best.

Volunteers and researchers have banded birds at Bluebonnet Swamp for years, not just Prothonotaries but other species, too, to measure survivor rates in urban-forest habitat.

A volunteer in 2013 recognized a male Prothonotary that had been banded a year before at Bluebonnet. The bird reappeared with its U.S. government-issued aluminum band on one leg and two orange plastic bands on the other, a color combination unique to that bird.

“Foundation Fact: In 1989, the Baton Rouge Area Foundation made the first pledge – $25,000 – and assisted the Nature Conservancy in raising more funding to purchase 165 acres that became BREC’s Bluebonnet Swamp Nature Center. The Foundation has also made grants to support Audubon Louisiana.”
The warbler’s return to Bluebonnet meant researchers had a suitable candidate for winter-migration tracking. “We had birds coming back to the swamp to breed,” Johnson says.

Simultaneous development in technology gave the research team a way to capitalize on the bird’s reappearance at the BREC park.

Geolocators had been used for 20 years to track migration of large birds like albatross and raptors. By 2013, the devices with three components—a battery, a data logger and a light sensor—were small enough to attach to birds like the Prothonotary Warbler, which weighs about half an ounce. “That was a door opening for us,” Johnson says.

The team went to work. The bird with the orange leg bands was lured into a finely threaded “mist” net strung between posts near a nesting box at Bluebonnet. The male warbler—around this time acquiring the name GeoDad—was quickly removed from the net once caught, then fitted with the tiny pouch containing the geolocator before being released.

Birds like GeoDad can be caught and released again in about 5 minutes, explained Audubon researcher Katie Percy, who works alongside fellow scientists and volunteers at conservation sites across south Louisiana.

Audubon Louisiana Avian Biologist Katie Percy shows off a warbler to visitors at Bluebonnet Swamp Nature Center.
Audubon Louisiana Avian Biologist Katie Percy shows off a warbler to visitors at Bluebonnet Swamp Nature Center.

GeoDad left Baton Rouge in mid-summer 2013. Audubon’s team settled in to wait. A question they hoped the bird would answer: Exactly where did it go once it flew south for the winter? A still bigger question: Could the path of its migration help unlock reasons for the species’ continuing decline?

Pinpointing where GeoDad went for the winter was of special focus because the warbler’s numbers have fallen faster than its U.S. breeding habitat had dwindled. Johnson and other researchers wondered if something taking place during the bird’s yearly migration outside the U.S. was impacting its survival.

Johnson got a call from a volunteer checking nesting boxes at Bluebonnet in spring 2014. The man had spotted the male Prothonotary with the orange leg bands. GeoDad was back. Johnson dropped everything and rushed to capture the bird and retrieve the locator.

The geolocator showed it had taken GeoDad 3.5 months to get to his farthest point south, a river valley in northern Colombia. It had taken the bird a day to cross the 500-mile-wide stretch of the Gulf of Mexico after jumping off from Louisiana’s coast a few days after leaving Baton Rouge. He had flown across seven countries since leaving Baton Rouge.

The bird’s return almost immediately prompted similar use of geotrackers to trace winter migrations across the bird’s multistate breeding territory, mostly in southeastern states. The work that began at Bluebonnet Swamp was the first time a geotracker had been successfully used for this purpose.

Findings from additional deployments of the devices in recent years showed something remarkable. Nearly all the tracked warblers, regardless of where they began their winter migration, spent time in northern Colombia’s Magdalena River Valley, where much of its natural habitat has been converted to cattle ranching or farmland.

Elsewhere across the region, hotel construction and other development has destroyed mangrove forests, including on Colombia’s Caribbean coast—another critical winter habitat for Prothonotaries.

Researchers found something else, too. The rate of decline in the warbler population is much closer to the rate of habitat loss in Central and South America than in the U.S., something of key importance in shaping conservation efforts.

Those findings will be published later this year in The Condor, one of the nation’s most important scientific journals on bird research.

“We wanted to see where the birds go to understand what might be happening,” Johnson says of his work and that of fellow researchers.

Adds Percy, “It’s very exciting to see this.” •