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Elegant Terns changing breeding habits in response to warm water in Mexico

New study has implications for how birds may adapt to climate change.

A recent study into the breeding patters of Elegant Terns shows that the waterbird has been changing the reproductive habits that have served it for millennia in response to recent changes in water temperate at its breeding site in Mexico. The findings could greatly expand our understanding of how birds respond to changes in their environment, particularly climate change.

Researchers from the Universidad Vericruzana in Mexico, along with a team of American scientists, have shown that that Elegant Terns (Thalasseus elegans) will migrate northward to breed when confronted with warm water anomalies in the Sea of Cortez.

Audubon California has a particular interest in Elegant Terns, because the birds are a priority species at the Richardson Bay Audubon Center & Sanctuary in San Francisco Bay. High numbers of Elegant Terns use the bay after breeding, reflecting a regular movement up the California coast where young and adults forage together.

The species is considered to be Near-threatened by the International Union for Conservation of Nature (IUCN) due to the high concentration of breeding individuals in one place and the likelihood of being negatively affected by climate change.

In late summer when adults and young numbers peak in San Francisco Bay, we regularly see 300 to 400 terns foraging in Richardson Bay and resting on the small islands off our shore. On occasion we have documented nearly 700 birds, which is approximately 1% of the global population.  Come November, the terns will head south to the coast of Chile and Peru for the winter before returning to breed again in Mexico—and sometimes, depending on the ocean conditions, southern California!

This map shows the location of Isla Rasa in the Sea of Cortez, where the vast majority of Elegant Terns make their nests.

Elegant Terns have been gambling on conditions staying the same for many centuries now. More than 95% of the entire population nests on one small island in the Sea of Cortez between Mexico’s Baja Peninsula and the mainland. This island (Isla Rasa—a global Important Bird Area) is only about 140 acres (0.22 square miles), but nearly 70,000 terns and 500,000 Heermann’s Gulls nest there when conditions are favorable.

Researchers showed that when ocean temperatures warmed more than 1°C (1.8°F) above normal, fish availability decreased in the Gulf of California resulting in lowered nesting success. During these event years, large numbers of terns were found to be nesting in Southern California, near San Diego and Los Angeles.

Even more interesting is that this behavior of moving northward to breed in more productive waters is relatively new to the terns. Prior to the year 2000, even when the oceanographic conditions were occasionally unfavorable, the species would stay in the area of Isla Rasa (the Midriff Island Region). The authors speculate that the terns are adapting quickly to rapidly changing conditions in the Midriff region in order to increase their opportunities to breed successfully, and that this may buffer them against some of the effects of rising sea temperatures.

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