Seventy-five metric tons of poison wasn’t enough to kill every rat on Henderson Island, but it came close, according to a study published today in Royal Society Open Science. Though uninhabited by humans, the 3626 hectares of the South Pacific island are home to several bird species—like the Henderson petrel—that nest nowhere else. To save the birds from invasive rats that were eating their chicks, the Royal Society for the Protection of Birds dropped poisoned rat bait from helicopters in 2011. This strategy, which has been used elsewhere in the South Pacific, succeeds in wiping out rats from islands about 80% of the time. Now, researchers are trying to figure out why it didn’t work on Henderson. Thanks to DNA from the rats and from rats on nearby islands, they now know that the rodents weren’t reintroduced to the island after the drop, always a possibility when poison fails. Instead, the tens of thousands of rats now living on Henderson appear to be descended from about 50 survivors of the original population. The modern rats also aren’t resistant to the poison, suggesting a simpler explanation: Their year-round access to food means that they don’t need to go for the poisoned bait.