Volcanic eruptions have been big penguin killers in the Antarctic over the past several thousand years, according to a new study that used penguin poop to track population booms and busts in the region.

Gentoo penguins, which weigh 12 pounds and stand almost three feet high, are well known for the prodigious quantities of poop they produce. In fact, they even spread their feces across their breeding sites — making massive skidmarks on the icy Antarctic landscape that help scientists count the inhabitants of penguin colonies from space. Now, scientists have discovered another new and creative way to use penguin poop to count the filthy birds. In this latest study, published today in the journal Nature Communications, researchers led by Stephen Roberts with the British Antarctic Survey analyzed guano-rich layers of sediment that accumulated over millennia in an Antarctic lake.

Each bird poops 84.5 grams of guano per day, the study says. (For scale, an adult human reportedly produces between 100 and 200 grams of (wet) poop per day.) On Ardley Island in the Antarctic, there are about 5,000 breeding pairs of Gentoo penguins. The authors calculate that every breeding season, these penguins release about 139 tons of dry guano (that’s 306,443 pounds) onto Ardley Island. Some of that washes out to sea, and the rest accumulates in soil and in lakes like Ardley Lake, which sits at the center of the island.

That made Ardley Lake a perfect time capsule. To figure out whether there were changes in temperature or sea ice that tracked with the size of the penguin colony, the researchers used geochemistry to identify five points in time with lots of poop in the sediment record. (The official scientific language for peak poop is “elevated guano flux.”)

It turns out that the biggest penguin killer didn’t seem to be changes in temperature or sea ice, but volcanoes — specifically, a volcano on nearby Deception Island. The study authors called the volcano “exceptionally explosive,” producing between seven and 14 cubic miles of volcanic ash during its early, caldera-forming eruptions. These eruptions contain glass, poisonous gases, and toxic metals that could have killed or injured nearby penguin colonies.

Eruptions of the volcano on Deception Island appear to have ended at least three of the five phases of “elevated guano flux” identified by the researchers. “Even relatively minor volcanic eruptions can be potentially devastating to the ecology at locations far from the eruption source,” the study authors note, because ash blanketing the region could disrupt both nesting and foraging for food. After an eruption on Deception Island killed off the second big penguin colony on Ardley Island, it took another 800 years to fully recover.