Warming and Overfishing Sent Seabirds Flocking to California

Warming and Overfishing Sent Seabirds Flocking to California

Mexico's elegant terns have begun nesting farther north in years when their traditional food is scarce


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The aptly named elegant tern. (drferry/iStock)
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By Sarah Zielinski
smithsonian.com
June 26, 2015 2:00PM

The Mexican seabirds called elegant terns have had a rocky history. Thanks to invasive species gobbling up their eggs and El Niño events depleting their food supply, the birds have seen their numbers rise and fall for decades. But they weathered those trials and steadfastly held to their traditional nesting grounds off Mexico's Pacific coast.

Now, however, a one-two punch of climate change and overfishing has the elegant terns moving into California by the thousands, scientists report today in Science Advances.

“The elegant tern is unique to the eastern Pacific,” notes Enriqueta Velarde of the University of Veracruz in Mexico, who has studied the birds since 1979. The terns migrate on a long path from wintering grounds in Chile and Peru north to their breeding grounds on Isla Rasa, in the Gulf of California. Dependent on small fish such as sardines, the birds' reproductive success has been an indicator of the gulf’s health, accurately predicting food availability for species such as blue whales and sea lions.

In the late 19th century, people came to Isla Rasa to mine the island for guano—bird poop—to be used as fertilizer. With the people came rats and mice, and the invasive rodents began eating the terns' eggs and chicks, putting a dent in the population. When the guano mining ended in the early 20th century, people began harvesting tern eggs by the thousands, taking as many as 500,000 eggs in a single year. But egg hunting was outlawed in the 1960s, and the rodents were finally eliminated by 1993, and the elegant tern population began to rebound. The number of nesting pairs increased from 15,038 in 1980 to more than 100,000 in 2013.

Read more: http://www.smithsonianmag.com/science-nature/warming-and-overfishing-have-seabirds-flocking-california-mexico-180955715/#d9KdOiOC3MODvHFj.99