One of the most delightful bird species of the Galápagos has almost completely stopped breeding there. According to a new study published this week in the journal Avian Conservation and Ecology, blue-footed boobies (Sula nebouxii) have seen a population drop of more than 50 percent over the past two decades. A series of surveys from 2011 to 2013 found only 134 fledgling birds in an area that less than 20 years ago would have held hundreds if not thousands of nests. The total population, meanwhile, had dropped by more than two-thirds since the 1960s.

"Until 1997, there were literally thousands of boobies at these breeding sites and hundreds of nests full of hatching chicks," principal investigator David Anderson, a professor of biology at Wake Forest University, said in a press release. "Then suddenly, the boobies just weren't there. There were a few cases where we found isolated breeding attempts but most of these didn't produce chicks."

The scientists link—but don't yet conclusively prove—the population drop to a similar drop in sardine populations in the area. Under normal conditions blue-footed boobies eat sardines almost exclusively. The researchers found that sardines now represent only about half of the birds' diet. The lack of the highly nutritious fish may have influenced the boobies' ability to mate and breed.

Although the lack of sardines may be linked to the boobies' population decline, we don't know yet why the fish populations have crashed. The researchers theorize that sardines could be overfished or otherwise affected by climate change.

The race is now on to find out exactly why the boobies aren't breeding and what can be done to save them. As the paper points out, the majority of the birds that now remain in the Galápagos are practically elderly and are now reaching the end of their breeding years (the birds only live 15 to 20 years). "But if humans are in fact contributing to this decline," Anderson said, "we need to get to the bottom of it now rather than five years down the road when you have the equivalent of 75-year-old humans trying to breed."

Blue-footed boobies are not currently considered an endangered species—the IUCN Red List and BirdLife International classify the species as of "least concern," noting that it has a wide range from the U.S. to South America. That said, the Galápagos remains an important stronghold for the species and the birds (and their complex mating dance) are a big draw for tourists. The loss of the birds on the islands could be felt in many ways for years. The lack of sardines, meanwhile, could potentially affect other species in the area, including two other species of boobies, the red-footed booby (S. sula) and the Nazca booby (S. granti). The true impact of this change in the food web remains to be seen.

Photo by Steve Jurvetson via Flickr. Used under Creative Commons license