Perhaps the most productive segment of Mexico’s Sea of Cortez is the Midriff Island region, where great Isla Rasa tourism is found. This small, flat-topped island is only about 142 acres, yet it attracts approximately 95 percent of all the world’s Heermann’s gulls and elegant terns. The nesting gulls may number 260,000, along with 180,000 elegant terns. One other species, the royal tern, also nests here, with approximately 15,000 individuals.
Although very quiet during much of the year, Isla Rasa comes to life in spring. Heermann’s gulls are in all their glory, stepping proudly with lipstick-red beaks. Terns crowd tightly together, just out of reach of their neighbors’ sharp orange bills. They nest on no more than a scrape of soil where they raise their chicks.
Today this critical nesting colony is protected, but it has not always been that way. Egg collectors in the mid-20th century caused seabird numbers to drop to only a few thousand. Guano was also gathered here, resulting in tremendous disruption to nesting birds. House mice and black rats became established as a result of human activity, and these non-native rodents destroyed both eggs and young birds.
Fortunately, through tremendous efforts, these pests have been eradicated, and the birds of Baja California have made an amazing recovery. Their breeding success varies from year to year depending on the abundance of their food source of small anchovies and sardines, but the populations are doing extremely well in light of the challenges that these birds have faced over time.
One can take a Sea of Cortez cruise past these communities to hear the din of squabbling and greetings and to watch the constant coming and going of adults in their ceaseless quest to find fish for their growing youngsters.
Continuing research by dedicated individuals will provide insight into not only the status of the birds of Baja California but also into the overall health of the surrounding marine environment within the Sea of Cortez.
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