A lot’s happened in the Gulf of California in the last two decades from a seabird point-of-view: warming waters, shortage of sardines, and starving chicks. Elegant terns (Thalasseus elegans) have been nesting on Mexico’s Midriff Islands in massive aggregations for at least a hundred years. But with abnormally warm gulf temperatures and fierce competition from fisheries, they’re increasingly moving on from their ancestral grounds and heading north to nest, according to findings published in Science Advances this week. 

Isla Rasa in the Midriff region, where the terns return to nest every April, is nestled between the long Baja California peninsula and the Mexican mainland. Since around 1998, the area has endured killer El Niño events and a few anomalously hot spring seasons that forced widespread colony desertion: The terns arrive but leave en masse without nesting. Researchers aren’t sure what’s causing this spike in sea surface temperatures, especially since it’s not happening along the southern California coast. Although, local conditions appear to be related to the global trend of increasing ocean temperatures as well as increasing variation in temperature extremes. 

Now, colony desertion is happening with increasing frequency. Why are they leaving and where are they going? To investigate, a team led by Enriqueta Velarde of Universidad Veracruzana counted nests and measured colony sizes from Mexico to California. They made ground estimates of nests using on-site observations and measured colony shape and size using aerial photography. After all the birds left, they also mapped out the nesting colony by looking at the crust of accumulated guano. 

^{Exodus of elegant terns from nesting colony after breeding failure in Isla Rasa in 2010. E. Velarde}

They found that during the last two decades, elegant terns have expanded from Mexico into Southern California in migration pulses. "Whenever the terns perceive the conditions in the gulf as inadequate to ensure successful reproduction,” Velarde says, “they move to alternative nesting grounds in Southern California.”

The combined effect of warming waters and reduction in sardine populations has triggered an ecological collapse of the Midriff Islands, and the steadfast seabirds are being driven 600 kilometers (373 miles) away from their historic homes. Their new, more northwestern sites include the Bolsa Chica Ecological Reserve in Huntington Beach of Orange County, as well as industrial landscapes like the San Diego salt works and the Pier 400 container terminal at the Los Angeles Harbor.

"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,” study co-author Exequiel Ezcurra from the University of California, Riverside, explains in a statement. “Productivity declines and, with it, the availability of small pelagic fish, on which the seabirds feed, also falls." And that’s further compounded by intense overfishing. 

^{Elegant Tern nesting colony in Isla Rasa 2011, Gulf of California. E. Velarde}