Climate Change: Seabirds are Abandoning Ancestral Nesting Grounds in California

Seabirds are abandoning their ancestral nesting grounds in the Gulf of California in record numbers. Now, scientists may have found out why. Researchers have found that climate change may be to blame.

Over 95 percent of the world's population of Elegant Tern and Heerman's Gulls concentrate on Isla Rasa in the Gulf of California. Yet during some years in the last two decades, the seabirds have arrived to the island in April and have left soon after without ever nesting. This first event was during the 1998 El Niño, when oceanic activity collapsed all along the eastern Pacific coast from Chile to California. Yet this colony desertion happened again in 2003 and since then, has occurred with increasing frequency in 2009, 2010, 2014 and 2015.

Now, researchers have taken a closer look at this occurrence. They examined the nesting Elegant Terns in order to monitor ocean dynamics. They used nest counts in seabird colonies from Mexico and California, and have found that the terns have expanded from the Gulf of California, in Mexico, into Southern California during the last two decades. This expansion, though, fluctuates from year to year.

"Whenever the terns perceive the conditions in the Gulf as inadequate to ensure successful reproduction, they move to alternative nesting grounds in Southern California, including the San Diego Saltworks, Bolsa Chica Ecological Reserve, and Los Angeles Harbor," said Enriqueta Velarde, project leader, in a news release.

So what causes the terns to remain in the Gulf or move to Southern California? The decision is related to the fact that during the last 15 years the Gulf of California has become abnormally warm during some seasons. These peaks in sea surface temperatures have not been equally high along the California current and Southern California Bight in the Pacific coast.

"When the Gulf waters get unusually warm, the sea becomes capped by a layer of warm surface water and the upwelling of cold, nutrient-rich waters fails to reach the surface," said Exequiel Ezcurra, one of the researchers. "Productivity declines and, with it, the availability of small pelagic fish, on which the seabirds feed, also falls."

The findings reveal that warm waters may be the cause behind these shifts. This, in turn, could be a result of globally warming oceans. Whatever the case, seabirds are forced to fly away in search of more suitable environments as their entire food chain shifts.

The findings are published in the journal Science Advances.

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