Seabirds had abnormally low breeding success this year at the Farallon Islands National Wildlife Refuge, where biologists with Point Blue Conservation Science observed one of the five worst breeding seasons on record for most species.
A variety of weather conditions in California brought on by El Niño made incubating an egg and raising a chick difficult this spring, especially for species that lay a single egg and eat a limited diet. Researchers observed chicks starving, mothers abandoning their nests and eggs being sun-baked, flooded or eaten at the largest seabird breeding colony in the contiguous United States, located 20 miles south of Point Reyes.
Cassin’s auklets, which feed primarily on krill, usually succeed in about 75 percent of their breeding attempts, but this year biologists observed a success rate of 11 percent—the third lowest since 1970, when data begins. Cruises around the island noted a high abundance of krill—which forms the basis of the food chain—but not until later in the breeding season.
“There was a mismatch of when the food was available, and when [the birds] needed those resources,” said Mike Johns, a Point Blue biologist who spends his summers at the refuge. Likewise, common murres and rhinoceros auklets, which primarily feed on anchovies and juvenile rockfish, had drastically lower breeding success rates than normal. Both species succeeded in just one-third of their breeding attempts. The birds—reliant on what is close to their nest while they incubate their eggs—had trouble finding fish that were small enough for their babies to eat.
Other species that thrive only in a narrow range of environmental conditions, like the pigeon guillemot and the pelagic cormorant, also experienced well below average breeding success because of El Niño’s effects.
In general, El Niño brings warmer temperatures, more extreme rainfall and reduced upwelling—the process through which cold, deep, nutrient-rich water rises to the ocean’s surface. This year, the climate phenomenon was weaker than average overall, yet its impacts were concentrated in California. June saw an extremely intense heat wave, and rainfall came late during bird incubation periods. Although these weather conditions are typical for El Niño years, they forced birds to abandon their eggs or to incubate dead ones.
On the Farallones, adult birds would fly out to sea to cool off, and their eggs would either cook or be eaten. Large puddles also formed in breeding colonies, flooding eggs. And, once the chicks hatched, researchers observed them gaining weight very slowly or disappearing.
“As scientists, we try our best to be detached observers of ecological processes,” Pete Warzybok, the Farallon Islands program manager with Point Blue, said in a press release. “That said, as conservationists, we want to see a thriving ecosystem and it’s heartbreaking to watch seabird chicks starve due to a lack of suitable food.”
The findings on the Farallones echoed observations from ocean research cruises conducted by Point Blue and the National Oceanic and Atmospheric Administration. Off the coast of Northern California and within the Gulf of the Farallones, researchers observed just a handful of Cassin’s auklets and very few common murre chicks.
Generally, bird populations crash after an El Niño then recover during off-years, so this year’s poor breeding season doesn’t necessarily indicate a long-term impact. Yet given that climate models forecast more frequent El Niño winters in the future, researchers are alarmed.
Meanwhile, western gulls and Brandt’s cormorants—birds that have more general diets and lay multiple eggs—did well this breeding season.