Rats threaten Humboldt penguins, study finds

CHILE – Research has shown that invading rats can be added to the list of threats to the declining Humboldt penguin population on Chile’s coastal islands, the Associated Press reports. Unless the rodents are eradicated, they could push these vulnerable birds towards extinction.

Rats have had devastating impacts on numerous seabird populations, but few studies have been done to show their impact on penguins. This study, published in the Journal of Ornithology in March, shows quantifiably for the first time that rats are important alien predators of eggs at Humboldt penguin colonies.

To look at the effects of rat predation, the researchers placed boiled chicken eggs in empty penguin nests (simulating unattended clutches) in colonies on Pájaros Island in north Chile and Algarrobo Island in central Chile. They found that in both colonies, the eggs were primarily predated by rats; on Pájaros, black rats ate 70% of the eggs, and on Algarrobo, brown (Norway) rats ate 53%. Kelp gulls took 10% of the eggs on Pájaros and 16% on Algarrobo. Significantly more eggs were predated at night, and rates of predation were highest within the first 12 hours.

Humboldt penguins, who face many other dangers such as fishing nets, changing sea currents and their nests being collapsed by nesting pelicans, are classified as ‘vulnerable’ by the International Union for the Conservation of Nature. Study author Alejandro Simeone, director of Andres Bello University’s Ecology and Biodiversity Department, told the Associated Press that the Humboldt population has fallen from hundreds of thousands decades ago to below 45,000.

While the biggest threat to the Humboldts is getting caught in fishing nets, changing sea currents mean that adult penguins are having to travel further to find food, leaving their chicks alone at the nest for longer periods of time. Simeone and co-author Guillermo Luna-Jorquera suggest that the rat presence at Humboldt penguin colonies coupled with this and other events that can cause temporary nest abandonment may impact on the penguins’ breeding success. So to improve the nesting habitat and of the penguins and other seabirds, the rats should be eradicated.

But getting rid of the rats is easier said than done. Simeone said that using toxic bait that is harmless to birds, as has been done in other countries, would be complex and costly in Chile. And while the island in central Chile is one of several penguin sanctuaries established by the Chilean government, there is no budget dedicated to protecting them from rats.

The study was supported by the Zoological Society of Milwaukee, which has provided more than US$200,000 towards Humboldt penguin conservation and research since 1994, including annual population surveys and the building of artificial burrows.

Roberta Wallace, the lead veterinarian at the Milwaukee Zoo, told the Associated Press that eliminating the rats would be a huge logistical challenge, because you would have to pay someone to go to the islands frequently to put out poison in order to break the rodents' reproductive cycle.

"It's not like putting out poison once and everything dies. You'd have to keep at it, because they breed like crazy, and you'd have to make sure you don't do damage to other species,” she said. 

Sources
It's rats vs. penguins on contested Chilean island by Eva Vergara, 12 July 2012, Associated Press

Estimating rat predation on Humboldt Penguin colonies in north-central Chile (Abstract). Alejandro Simeone and Guillermo Luna-Jorquera, Journal of Ornithology, 19 March 2012, DOI: 10.1007/s10336-012-0837-z