Why are Gulf of California seabirds heading north?

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In 1856, the Italian researcher Federico Craveri visited the Midriff Islands in Mexico’s Gulf of California and spent five days on Isla Rasa. Though he was there during the nesting period for the gulf’s Elegant Terns (Thalasseus elegans), his detailed notes included no mention of the fluffy white-and-grey black-capped seabirds. That’s despite the fact that Isla Rasa is where some 95 percent of the world’s Elegant Terns come to build their nests and rear their young.

The explanation for the inconsistency is pretty straightforward: while Rasa is home to the terns’ nests most years – tens of thousands of them – occasionally they have to nest elsewhere. But in recent years, Rasa has become less of an Elegant Tern stronghold as the population has begun to move north, and their brief jaunts away from Rasa have become less irregular and more predictable. But why? The answer involves long-term oceanographic trends, fishing, and avian decision-making.

To understand the contemporary movements of Elegant Terns, it helps to know a bit about their natural history. Sometime in the late 1800s, black rats and house mice were introduced to Rasa, probably thanks to guano miners. While mature terns can fly away from the rodents, eggs and nestlings aren’t so lucky, so the tiny mammals slowly began to eat away at the population. When guano mining stopped in the early 1900s, egg harvesting began. At the peak of the harvest, half a million eggs could be harvested over a single breeding season. The practice was outlawed in the 1960s but didn’t truly cease until the 1980s, probably because that’s when researchers started becoming a visible presence there. Between 1993 and 1995, an eradication program successfully rid the island of its rodent infestation. Combined with the cessation of egg harvesting, the population could finally begin to recover. In 2004 and 2008, the population grew large enough that some pairs began nesting on other nearby islands within the Gulf’s Midriff region, though those nesting grounds never became permanent, like Rasa.

Conventional wisdom regarding seabirds is that even in years of poor food availability, the birds tend to stay put. But two strong El Niño events in the 1990s resulted in a different nesting pattern for Elegant Terns. In 1992 and 1998, most of the birds built their nests in the Upper Gulf, in the Colorado River delta. Together with observations like Craveri’s, that suggests the birds look elsewhere if there isn’t enough food to eat.

It’s not that the delta was particularly attractive to the terns; most researchers at the time suspected that El Niño made the Midriff area highly unattractive instead. During the atmospheric anomaly, the ocean gets a bit warmer, which decreases the intensity of upwellings, undersea currents that send nutrients towards the surface from below. Weaker upwellings mean fewer fish for the birds to eat, which leads to reductions in egg laying and in chick survival.

The birds’ northward expansion didn’t end at the Upper Gulf, but spread all the way into California. In 1959, they were in the San Diego Bay, and a few hundred stayed there year after year. A population explosion in 1993, perhaps owing to the rodent eradication in Rasa, meant that by 2014 more than 22,275 nests could be found in San Diego. Other terns found their way to Bolsa Chica Ecological Reserve in Orange County, and still others to the Los Angeles Harbor. In 2014, there were more than 50,000 Elegant Tern nests in California.

Isla Rasa is so tiny it doesn't actually show up on Google Maps.

Isla Rasa is so tiny it doesn’t actually show up on Google Maps.

What this means is that it’s not just El Niño that’s sending the birds northward, but something else. Since the terns in the Gulf have been continuously monitored since the 1980s, and those in California since 1991, Universidad Veracruzana biologist Enriqueta Velarde and her colleagues now think they know why. Their results were published last week in the journal Science Advances.

The tern population in California slowly grew starting in 1991, with brief explosions during El Niño years, coinciding with severe decreases in nesting success at Isla Rasa. But after 2000, “the numbers of successfully nesting terns in Rasa became un-coupled from large-scale El Niño indices,” say the researchers. Instead, the better predictor of nesting success at Rasa was the local sea surface temperature. When the temperature ocean surface surrounding Rasa gets a bit too warm, more than 70% of the total Elegant Tern nesting population shows up in California, and especially in San Diego. The researchers used oceanographic data to confirm that the water in the Southern California Bight remained nice and cool in the years when the terns escaped their Gulf of California hot tub. In years in which the Gulf water was comfy, less than 20% of the tern population nested in California.

Then there’s the fishing. Terns and humans both like to eat the same sardines, but humans leave a much bigger mark on sardine schools than do the birds. The nesting birds in Rasa can scarf down around 100 tons of sardines every day. That seems like a lot until she explains that’s only a bit more than half the amount of sardines that a single fishing vessel can haul in on that same day. But the birds only feast upon the silvery baitfish during their four-month breeding season, while the fishing vessels are out eleven months of the year. During the 2012-2013 fishing season, humans took in 54 times more fish than those harvested by the nesting seabirds in Isla Rasa.

Taken together, the northerly movement of the Elegant Tern population can be explained by three factors. First, conservation efforts to protect the species in recent decades have been largely successful, both in Mexico and in California. The rapid increase in the Rasa population, especially following the eradication of rodents on the island, pushed breeding pairs to find room to nest in California. But on top of that background rate of expansion, warming seawater in the Gulf drives nesting pairs towards California – 600 kilometers away – in search of better fishing grounds. Their push northward is only compounded by the depletion of fish due to human harvests, which drives even more of the birds towards the Golden State.

If warmer oceans are making it harder to find fish, then humans and seabirds will increasingly be relying on the same dwindling supply of food to survive. While the researchers acknowledge that there are no good estimates for fish stocks in the Gulf, it seems reasonable to infer that current harvests are unsustainable based on the terns’ behavior.

Still, the trend shows how remarkably resilient the birds are, and demonstrates their rapid decision-making abilities. Both are essential if they are to thrive in a rapidly changing global environment. – Jason G. Goldman | 03 July 2015

Source: Velarde, E., Ezcurra, E., Horn, M. H., & Patton, R. T. (2015). Warm oceanographic anomalies and fishing pressure drive seabird nesting north. Science Advances, 1(5), e1400210. DOI: 10.1126/sciadv.1400210.

Header image: Elegant Tern nesting colony in Isla Rasa in 2010. Credit: E. Velarde