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Elegant and Royal Terns

An Elegant Tern lands at its nesting site in a mixed Elegant and Royal Tern colony in Roasa Island in the Midriff Island Region of the Gulf of California. This exceedngly rich marine region is the breeding area for sevearl million seabirds, and Rasa Island has almost half a million of them. Photo by Alberto Montaudon.

Isla Rasa:
Seabird Grand Central

By Exequiel Ezcurra, Ph.D.

The Gulf of California’s Midriff is an exceptional place, like few others in the world. The tidal currents that flow from the south into the Gulf’s waters pick up speed as they approach the narrow channels between the islands of Tiburón and Ángel de la Guardia, often gushing into a torrential flow.

The force of the fast-moving waters creates turbulence, and deep ocean layers come to the surface, cool and loaded with nutrients. The water also creates a cool, stable, and arid atmosphere. It seldom rains here, but winds, pushed by the turbulent changes in seawater temperature, can blow treacherously. One minute, the water looks like a mirror; the other, gushes of 40 miles an hour are threatening to capsize your panga.

Impetuous as they may be, these are also some the most productive ocean waters on Earth. Plankton blooms here like almost nowhere else, and a frenzy of life feeds upon it. Blue whales filter tons of small crustaceans with their mammoth pouches, and sperm whales dive deep into the channels looking for squid. Low in the food chain, feeding on the plankton, huge schools of small pelagic fish flourish here in amazing density. Sardines, anchovies, and mackerels are the basic link between the microscopic plankton and the larger predators. They are the forage fish of the ocean, the stuff that keeps all the rest going.

Including millions of seabirds.

Rasa, in Spanish, means flat. And indeed, Rasa Island is the flattest island of the Midriff, hidden behind the curvature of the Earth and almost impossible to see until you sail right into it. And yet, it is the favorite nesting ground for three important species of seabirds– the Heerman’s Gull (Larus heermanni), the Elegant Tern (Sterna elegans), plus a smaller number of Royal Terns (S. maxima). The flat, unencumbering island provides a perfect site for nesting, with a circular, almost fish-eye view of the surrounding environment that allows the birds to see approaching predators. Almost all the world’s populations of these two seabird species congregate every year between April and June on Rasa Island to breed and rear their young. It is one of the most fascinating spectacles on Earth: half a million birds squeaking and screaming and flying and feeding and courting and bonding and endlessly feeding their chicks with sardines—tens of millions of them, brought from the rich surrounding waters, one fish at a time.

Because of its sheer simplicity and the grandeur of its scale, the ecosystem of Rasa Island has fascinated researchers for decades. Sardines, gulls, terns, and ocean upwellings tell the story of life on Earth. They are the best indicators of the Gulf ’s ecosystem health, and even of planetary health. If coastal pollution hurts the gulls or the terns during their migrations along the coast of the Pacific Ocean, it will be noted in Rasa. If sardines are overfished by the industrial fleet, Rasa will tell. If invasive cats or rats are introduced to the island, then two entire species of seabirds will be put at risk. Since the 1950s, Museum researchers have devoted time and effort to understand the ecosystem of Rasa Island and ensure its preservation. Lewis Wayne Walker, the taxidermist at the San Diego Natural History Museum, was the first to publish in 1951 a detailed paper in National Geographic magazine on the reproductive aggregation of seabirds on Rasa Island. In the following years, George Lindsay, at that time Director of the Museum, worked intensively with Walker and Dr. Rodolfo Hernández Corzo, the Mexican Director of Wildlife, to protect the island from egg poachers who were decimating the nesting populations. Their efforts were successful; in May 1964, Mexican President Adolfo López Mateos signed a decree establishing Rasa Island as a migratory waterfowl sanctuary.

Their pioneering work was later followed by Bernardo Villa, a zoologist at Mexico’s National University, and in 1979 the torch was passed on to Dr. Enriqueta Velarde, at that time a young student of Dr. Villa. Dr. Velarde, with many of her students, has been working on the island since, in what constitutes one of the most wonderful examples of in-the-field conservation in the world.

For 28 years, she has visited the island every spring, and year after year she has followed the nesting success of banded individuals along their entire lifetime. During these years, she has amassed one of the best and most comprehensive databases on seabird reproduction in the world. She also promoted a successful rat eradication program, carried out by Jesus Ramírez, eliminating in one season a dire threat to successful seabird reproduction. Perhaps more importantly, without any weapons or any form of violence, she has been able to dissuade the egg poachers, completely stopping the trade in tern and gull eggs. As a result, the three populations that totaled 290,000 in the late 1970s now have reached almost half a million.

Now on sabbatical at the Museum, Dr. Velarde is preparing her next campaign to Rasa Island and writing scientific papers based on her lifelong data on Rasa Island. Her work has started to turn around an amazing ecosystem. The recovery of Rasa Island is now on its way, and the dreams of Lewis Wayne Walker and George Lindsay have achieved their promise.

Exequiel Ezcurra, Ph.D., is the San Diego Natural History Museum Provost and Director of the Museum’s Biodiversity Research Center of the Californias.

Joseph Wood Krutch, Nancy Bechtel, Charles A. Lindberg, Kenneth Bechtel

Joseph Wood Krutch, Nancy Bechtel, Charles A. Lindbergh, and Kenneth Bechtel near Bahía de los Ángeles in 1969, during a flying-boat expedition to the Gulf islands. Lindbergh’s commitment to the islands led the Mexican government to seriously consider the protection of Isla Rasa. Behind the camera: George Lindsay (image: SDNHM archives).

SAN DIEGO NATURAL HISTORY: FIELD NOTES,  May 2008

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