Elegant terns roam far and wide during the course of a year: the seabirds, dapper despite their raggedy black mohawks, migrate as far south as northern Chile during the (North American) winter, then range up the U.S. Pacific coast after breeding during the early summer.
During the breeding season, however, almost the entire worldwide population of elegant terns congregates in one place – Isla Rasa, a tiny island (just one quarter of a square mile in size) in the middle of Mexico’s Gulf of California.
At least, that’s what used to happen.
In recent decades, as the population of elegant terns has grown, scientists have noticed a shift – during some years, elegant terns have abandoned Isla Rasa in favor of breeding colonies in southern California. In a study recently reported in the journal Scientific Advances, a group of researchers details their investigation into a few possible explanations for the move.
The researchers monitored elegant tern nests on Isla Rasa as well as three breeding colonies in southern California, starting in 1980 at Isla Rasa and 1991 at the California sites. After the year 2000, there were four years during which the sea surface temperature was anomalously high around Isla Rasa – and during each of those years, more than 70 percent of the nests the scientists found were in California. During yeas without particularly high sea surface temperatures around Isla Rasa, less than 20 percent of the elegant tern nests were located in California.
Higher sea surface temperatures may make fishing more difficult for the terns, driving them away from Isla Rasa to the north; the researchers also found some evidence that more intense human sardine fishing may push the birds away from Isla Rasa, too. (Sardines and other small fish are elegant terns’ main source of food.)
The shift in where elegant terns are nesting is not necessarily a bad thing for the species – in fact, it’s probably at least partially due to a recent population boom, which, as the scientists note, “seems to be pushing reproductive pairs onto new nesting grounds in California.”
The ability of elegant terns to shift where they nest from year to year suggests that they “can make fast decisions and dynamically adapt to rapid changes in the global environment,” a useful skill for any species.
A group of elegant terns on a fishing boat in Chile.
(Original image by tk-link via Flickr/Creative Commons license)