Bonaire is a southern Caribbean island known for its stunning pink flamingos, world-class diving ... and salt. In fact, most of the southern part of the island is used for salt production. Bonaire's only export, the mineral has been harvested here for hundreds of years – but when locals noticed that their salty staple was interfering with the nesting habits of a small migratory bird, they knew they had to take action. And that's how Bonaire's least terns got an island all of their own.
Last spring, several birdwatchers noticed that least terns nesting along Bonaire's salt harvest roads were being disturbed by trucks belonging to Cargill Incorporated, the global food conglomerate that runs the solar salt works on the island. When it comes to human disturbance, terns have low tolerance levels: they'll abandon their nests and their young in the face of prolonged disruption. Concerned, the birdwatchers approached Bonaire's National Parks Foundation (STINAPA). The birds clearly needed a tern-friendly nesting site, one that was located away from all of the road activity.
With that mission in mind, STINAPA teamed up with experts from IMARES (the Institute for Marine Resources & Ecosystems Studies), as well as officials from the Cargill plant itself, including the plant's manager, Gary Rimmey. "We realised that we had to find a solution to this problem. We couldn't shut our operation down, but we needed to come up with an alternative. The initial objective was to get the terns off our harvest roads," Rimmey says.
“The island appeared to be an ideal alternative nesting site for the birds ... except for one thing: it was prone to flooding.”
The first step in this tern translocation was finding a suitable site. Paulo Bertuol, a STINAPA wildlife biologist, had noticed that some least terns frequented a small island in a Cargill salt pond. It appeared to be an ideal alternative nesting spot for the birds ... except for one thing: the low island was prone to flooding. Making it flood-proof would be a bit of an earth-moving project.
Already armed with the right equipment for the job, Cargill agreed to take that project on. "It took ten days to complete the island. We had to raise a half-acre area about one foot (30 centimeters) in order to prevent flooding. Once the fill material was delivered, we chopped a gap in the road that we had built to deliver the fill. That's when it became Tern Island."
Least terns usually begin nesting on Bonaire by May, and it takes about 6-8 weeks for the eggs to hatch and the chicks to fledge. Once the construction of the island was complete, STINAPA brought in wooden tern-like decoys to attract the birds, completing the task just in time. By that day's end, six birds had already settled on the island.
The resilient, diminutive birds are the smallest of the North American terns at about 22 centimeters in length and weighing in at approximately 45 grams. They reside along the southern United States coastline and migrate throughout the Caribbean, temporarily joining local birds before continuing to northern South America. On Bonaire, the last fledglings and adults typically leave before the end of August.
Since its construction, Bertuol has been regularly monitoring Tern Island twice a week. Each time a new nest is discovered, its location is tagged with a numbered marker. This allows Bertuol to track the number of eggs and chicks of each nest over time. To date, he's marked over 150 least tern nests. "I'm monitoring other least tern nests nearby so that in the future I can compare the success of Tern Island to those natural nesting sites," he explains.
While STINAPA's wooden tern decoys were effective in attracting the birds to the island, the terns did something extremely uncharacteristic once they got there: they grouped together. "Least terns don’t normally nest at a high concentration or density," explains avian biologist Adrian Del Nevo, who has monitored Bonaire's tern colonies over the past twelve years. "Each species has a different strategy for protecting themselves and their offspring during nesting. Cayenne terns, like those on nearby Aruba and occasionally on Bonaire, nest in extremely high density. They do that because they can collectively deter the laughing gulls, which attack their eggs and chicks. [But] with least terns, their usual strategy is to spread themselves out so that they are less obvious to predators."
The new island is nearly 2,000 square metres, about the size of a football field. Del Nevo notes that under normal circumstances only five to seven least tern nests would be found in an area of that size on Bonaire. While Tern Island protects the birds from rats, feral cats and those loud Cargill trucks, there is a potential downside too. Terns concentrated in such unusually large numbers could attract hungry laughing gulls. "One of the problems is that you might eventually create a feed lot for predators," warns Del Nevo. "But studies in other parts of the world have shown that small shelters placed on islands such as this help decrease the predation rate and increase chick survival," he adds.
To protect Tern Island's tiny chicks, Bertuol used small rocks to create inverted V shelters for the birds to hide in. Tropical biologist Dolphi Debrot from IMARES had another idea: using short pieces of PVC pipe, just large enough for the chicks to enter, but too small to allow predatory gulls to squeeze in after them. These shelters provide protection and also some much-needed shade from Bonaire's relentless sun.
Even with these improvements, least terns still face challenges. Laughing gulls will attack chicks that are not quick enough to get to a shelter, and dogs can easily swim the distance that separates Tern Island from the shore. But these are threats the birds would have faced even if they'd remained along the salt harvest roads. For now, Tern Island has eliminated attacks from cats and rats, and harvest truck disruptions are now a problem of the past. There are also plans in the works for a second island before next year's nesting season. That added space would help disperse the least tern nests over a bigger area.
"Remarkably, this project exceeded our expectations," says Rimmey. "I thought we might get a few nests and eventually over time the terns would move to the island. But pretty much all the terns from the north end showed up. Next year, we plan to build another tern island. Maybe having over 150 nests out there shows that we need more."
Top header image: Kenneth Cole Schneider, Flickr
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