Why five small islets off the coast of Aruba host more tern species than any other place on Earth
By Patrick Holian | Published: 2/24/2012
Standing on the white sand shore of Aruba’s Rodgers Beach, we push our kayaks into gin-clear Caribbean waters. The destination? Five unassuming offshore islets that serve as a gathering spot for terns, a place like no other on the planet.
“These reef islands are a unique location,” declares my kayak partner for the day, wildlife population ecologist Adrian del Nevo. “It is the only place that I am aware of in the world where 10 species of terns gather. While other places around the globe may have four or five species converging, nowhere else has 10 species.”
The intrepid ornithologist should know. The head of a Kansas-based ecological consulting firm and a former researcher for Britain’s Royal Society for the Protection of Birds, del Nevo has studied terns for three decades from Scotland to French Guiana, from Africa to the Caribbean. For the past dozen years, he has returned dutifully to the reef islands in San Nicolas Bay off Aruba’s southern shore and their terns.
Nests, not tourists
The islands are not the palm-tree-studded meccas that draw hordes of North American tourists annually to Aruba. That bustling scene is at the other end of the island, where glitter, glamor, and casinos abound. Rather, the spits of sand, craggy rock, boulder coral, and tough shrub barely rise above sea level. They might as well be light years away from hotel row.
That’s fine with del Nevo. Based on his research, the San Nicolas Bay reef islands were designated a Globally Important Bird Area by BirdLife International in 2007 for their large concentrations of Brown and Black Noddies and Roseate, Royal, Least, Common, Sooty, Bridled, Sandwich, and Cayenne Terns.
“The Cayenne Tern is still technically a subspecies of the Sandwich Tern,” del Nevo notes. “However, there have been two recent genetic papers suggesting that while the birds are closely related, they are not enough related to be considered the same species.”
With the wind blowing at our backs, we glide easily over the aqua-blue waters of San Nicolas Bay and arrive at one of the larger reef islands in less than half an hour. To my untrained eye, it is unimpressive.
Del Nevo enlightens me. “The reef islands have open soft sand, compressed sand, rocky gravel, large boulders, and vegetation on the landscape,” he says. “That means there is a niche for many different tern species in one small area. That is important ecologically because there is no competition for space between these species. That is quite unique and quite remarkable.”
More than 40 species of tern are known around the world. The American Birding Association includes 19 on its checklist. Three are not recorded annually in the ABA Checklist Area (Large-billed, White-winged, and Whiskered Tern). The remaining 16 species are seen in North America every year, and 10 of the 16, listed at right, breed on Aruba’s reef islands. Numbers of breeding pairs are 2008 estimates.
Black, Caspian, Gull-billed, and Large-billed Terns have been recorded on Aruba.
*The taxonomic relationship of Cayenne Tern and Sandwich Tern is unresolved.
As I scope from the shore of Island No. 4 toward distant Venezuela, 17 miles to the south, the view is a postcard-perfect expanse of reef and wave. Turning around 180 degrees, facing back toward the Aruban coastline, it’s jarring to see the towering Valero oil refinery just a quarter of a mile away.
Gas flames spew from smokestacks. White emissions drift southwest, missing all but one of the reef islands. The industrial cacophony of steam, flame, and clanging metal competes with the natural sounds of wind and surf. Strangely, the terns appear unbothered by the ruckus.
“The presence of the complex, somewhat ironically, has been of benefit to the terns because the refinery acts as an unofficial guardian of the islands,” explains del Nevo.
“Valero doesn’t want to see boat traffic moving near here because it is very dangerous with their large tankers and tugboats in the vicinity. The facility is far enough away to not disturb the terns. It becomes background noise. It is the sudden, loud, random noises that affect the birds — things like jet skis, speedboats, and people running on the beach. Those could be a big threat. Continual disturbance like that could lead to desertion of the islands by the terns.”
But because Valero’s security force keeps the public at bay, I am able to witness hundreds of Brown and Black Noddies and Sooty and Bridled Terns up close and personal. By moving slowly, I am often within five yards of the birds and get great looks.
In between viewings, del Nevo offers more facts about the terns. They nest here between late March and mid-August, and they account for about one-fourth of the world’s approximately 45 tern species. About 20 percent of the world’s Cayenne Terns breed here, as do 40 percent of the Caribbean’s Black Noddies and 60 percent of its Common Terns. Farther up the coast, in the harbor of the capital city Oranjestad, lie two other islets where a few dozen Common Terns and thousands more Cayennes also breed.
Before I know it, the morning has vanished and we set off again in the kayaks. By now the winds have cranked up to 30 knots, and the white-capped waves hinder our progress. In spite of constant paddling, at times I drift backward. After an hour, we finally reach Rodgers Beach.
The following day, the trip is made substantially easier when the Valero Refinery provides a powerboat. Ten local volunteers have arrived to help band week-old chicks on Island No. 3, a nesting ground for thousands of Cayenne Terns.
Banding allows del Nevo to track the birds over time, providing data essential to understanding the so-called swallows of the sea. Because terns can live for 20 or 30 years, del Nevo says long-term research is key to unlocking their secrets.
“You really start to gain an understanding for the terns’ biology and requirements only over successive years. The same holds true for the natural variability in their biology. Even after 12 years of study, we still don’t have that because of the birds’ longevity. That’s why I keep on coming back.”
In 2010, the study was cancelled due to lack of funding. Fortunately, in 2011 the Dutch Caribbean Nature Alliance and Vogelbescherming Nederland (the Netherlands Society for the Protection of Birds) supplied the necessary funds. Since Aruba is an island nation within the Kingdom of the Netherlands, it receives support from Dutch-focused conservation organizations.
Once our group arrives on the island, a worktable is set up, hundreds of Cayenne chicks are gently guided into a makeshift pen, and the banding begins. In the meantime, I notice wave after wave of adult Cayenne Terns returning to the island with small fish firmly clasped in their beaks. They hover over the holding pen, searching for their particular chick so it can be fed. The endless procession of food-bearing terns is another sign of Aruba’s appeal to the birds.
“It’s a tough life out on the sea,” says del Nevo, deftly banding chicks while we talk. “This relatively tiny organism makes its living in one of the most hostile environments on this earth. The adults fly a long way, 30 miles or more, to nab just one fish. The energetic cost for them is quite high. If you want to carry a fish that far, you want to optimize the size of the fish. So there is a trade-off between what you can carry, what you can get past the various predators like frigatebirds, Laughing Gulls, or other terns, and what your chick needs. We’re talking evolutionary adaptation at its best.”
I have a new appreciation for the hardy birds as I watch them return from Aruba’s fish-rich waters. They hunt the upper 12 inches of the sea in a quest for squid, young flying fish, and other fish species. I learn that in spite of the thousands of squawking chicks on the beach, an adult can recognize the unique call of its own offspring and deliver the food. It is a wondrous aerial ballet to witness.
Volunteer Jean Wade and I chat while surrounded by an untold number of the birds. “It’s like being inside a snowflake globe,” she says, “but instead of snow, the blue sky is peppered with thousands of terns. It is just breathtaking.”
Even after completing 10 seasons of banding terns, Wade is amazed that the reef islands attract so many species. “The terns huddle in difficult terrain, sitting on their eggs,” she exclaims. “Why do they choose such a demanding location? It is a mystery to me.”
By early afternoon, the group has banded more than 700 Cayenne Tern chicks, and many birds have been weighed and measured. The important information goes into del Nevo’s database for further analysis.
Protecting Aruba’s terns
The government of Aruba owns the reef islands in San Nicolas Bay and the Oranjestad harbor, where thousands of terns breed annually. The sites have been designated Globally Important Bird Areas by BirdLife International but are not formally protected.
Aruba does not have a government agency focused solely on environmental policies, and wildlife-conservation efforts are not well coordinated. The coast guard, local police, and oil-refinery personnel patrol the islands and help to keep people away and disturbance to the birds at a minimum.
A more formal protection policy may be forthcoming, however. Government officials, including Prime Minister Mike Eman, visited the San Nicolas Bay islands in July 2011 and met with ecologist Adrian del Nevo. Eman said the protection of the islands and birds is necessary and mentioned that a marine park may be created. Stay tuned.
A single egg
The next day, my last with del Nevo and the terns, he and I visit Island No. 2, a place full of dense buttonwood ideal for nesting Brown Noddies. The biologist shows me several nests hidden among the thick branches. I observe juveniles still sitting in their nests, preparing for their eventual departure to a life at sea. At another tree, I spot a single egg in an elevated maze of twigs and small stones reminiscent of a Japanese Zen garden. I am amazed by its feng shui appearance.
“You know, not many get to see this remarkable place like you have,” remarks del Nevo, “but that doesn’t mean that birdwatchers can’t enjoy it. They can’t visit the islets due to disturbance, but there are promontories along Aruba’s coast, such as Colorado Point, where one could watch the terns flying past as they head north to feeding grounds.
“A birdwatcher could stand on those high points and see all 10 species that gather here and not make any disturbance at all. There’s a real opportunity for birders to come here between the months of April and July. After that, the terns are back out at sea.”
While I ponder my time on the reef islands, Bridled Terns watch my every move just yards away. Del Nevo has showed me why so many birds are here: niche habitats, lack of disturbance, protected shores, and abundant fishing grounds. But he admits that an unscientific intangible is at work.
“These reef islands are magical,” he says. “It’s that threshold between land and sea — the color of the water, the salt in the air, the pervasive calls and cries of the terns. They are just a glorious group of animals. I have been studying them for decades. I do not tire of the intricacies of their social system, the complexities of their world, nor the beauty and poetry of their movements and behavior.”
Patrick Holian is a freelance writer, photographer, and documentary filmmaker based on Bonaire. He writes often about environmental issues facing the Caribbean. He blogs at worldkid66.wordpress.com.