Warming seas, overfishing drive Elegant Terns away from primary nesting site

6/26/2015 | 0

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Elegant Terns crowd together on Isla Rasa in the Gulf of California. Photo by Enriqueta Velarde

It appears that we can add Elegant Tern to the list of birds being negatively affected by climate change.

For decades, more than 95 percent of the tern’s world population nested on Isla Rasa, a small island in Mexico’s Gulf of California. So much of the population has nested there that Elegant Tern was said to have the most restricted breeding distribution of any tern in North America. The island is also home to significant numbers of nesting Royal Terns and Heermann’s Gulls. The video below shows the thrilling chaos of the place.

Isla Rasa: A Seabird Sanctuary

Egg collectors and predation by introduced rats and mice took their toll on the birds, but egging ended in the early 1980s and the rodents were eradicated in 1993. Elegant Tern numbers rebounded rapidly, to about 150,000 breeding pairs in 2012, and the species was considered a conservation success.

Most of the credit was due to Enriqueta Velarde, a seabird ecologist with the Institute of Marine Sciences and Fisheries at the Universidad Veracruzana. (She is featured in the video.) For more than three decades, she has lived on the island during the breeding season and studied and protected the birds with the assistance of graduate students and fishermen.

Today, Velarde and three California researchers report on new causes for concern for Elegant Tern. Writing in the journal Science Advances, they say that in the last 13 years, the colony has been abandoned or not established four times: in 2003, 2009, 2010, and 2014.

In each year, sea surface temperatures in the gulf were abnormally warm. At the same time, sardines in the gulf have been depleted, leaving little for the birds to eat and feed their young.

The combined effects of warming waters and overfishing have pushed many of the birds away from the gulf to nesting grounds about 375 miles (600 km) northwest in California.

In southern California, the species had nested in relatively low numbers at three sites: in San Diego Bay, since 1959; at Bolsa Chica Ecological Reserve in Huntington Beach, since 1987; and in Los Angeles Harbor, since 1998. Recently, their numbers have skyrocketed in the years that the colony on Rasa has failed. Last year, the researcher say, more than 50,000 pairs nested in California.

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Elegant Terns depart their colony on Isla Rasa after the failure of their breeding season in 2010. Photo by Enriqueta Velarde

In years with warm seas, as much as 70 percent or more of breeding terns have been found in California, especially in San Diego Bay. Velarde and her co-authors say the fact that some birds shift to distant nesting sites “suggests that Elegant Terns can make fast decisions and dynamically adapt to rapid changes in the global environment.”

However, when sea temperatures rise, says Velarde, only about 20 percent of birds from Rasa establish nests at the California sites. The rest apparently skip the breeding season and disperse.

The study examined data through 2014, but this year’s nesting season on Rasa fits the recent pattern. The colony in 2015 “did not even start,” Velarde tells me. Several hundred birds were flying around the area at night, but “before sunrise, they were gone. This went on for several weeks, and I noticed the number of terns flying about in the day and the level of noise at night decreased, so I believe they were leaving the area.” — Matt Mendenhall, Managing Editor