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It's a mystery that has plagued scientists for ages: When new puddles and pools form, or a dry and isolated pond suddenly gets revived through rainfall or snowmelt, where do the fish that live in these waterbodies comes from? Now a new study might have finally found the answer: duck poop.

Hungarian researchers fed live eggs of Prussian and common carp—both invasive fish species wreaking havoc in North America’s waterways—to Mallards and looked to see whether any came out the other end. They found that 0.2 percent of eggs endured the journey through the bird’s digestive tract and showed up in their fresh feces. Three of those 18 recovered eggs hatched into baby carps, on June 22 in Proceedings of the National Academy of Sciences, suggesting that Mallards, and possibly other waterfowl, ferry fish eggs between waterbodies near and far.

Because Mallards are abundant and widespread, even with that low percentage, “they can move a lot of eggs because of their huge numbers,” says Orsolya Vincze, an ecologist at Danube Research Institute’s Centre for Ecological Research in Hungary and co-author of the study.

Mallards are ubiquitous across North America, Asia, and Europe. These shallow-water ducks feed on aquatic plants, snails, insects, and larvae, and in spring and early summer they share their ponds, marshes, and lake edges with spawning carp. A single common carp lays up to 1.5 million small, sticky eggs every time it spawns, and it can spawn multiple times per season. These eggs float in the water or sink to the shallows and attach to submerged plants. Then, Mallards feast.

“Birds just love fish eggs,” says Ádám Lovas-Kiss, an ecologist at Danube Research Institute’s Centre for Ecological Research in Hungary and lead author of the study. “If Mallards find these spawning areas, they will go there and eat the eggs until they can’t move. It’s a great resource for protein for them.”

Lovas-Kiss and his team wanted to know if some of these eggs remain alive and intact as they pass through the Mallard’s acidic gut. Previous have shown that some plant seeds can survive this journey; Mallards pooped them out whole as they shuttled from one wetland to the next. “We tried to figure out if they can defecate the eggs viably,” Lovas-Kiss says.

In two experiments, his team fed 500 fertilized eggs of common and Prussian carp to each of eight Mallards housed in captivity. The researchers chose these Eurasian fish species because they’ve been introduced to many parts of the world in the last few decades. Using a plastic tray placed under the ducks’ cage, the team collected bird droppings seven times within a day of feeding the birds to retrieve intact eggs. Turns out, six ducks pooped out 18 intact fish eggs, and 12 of those recovered eggs housed live embryos. (The embryos within the remaining six eggs died as they passed through the Mallards’ gut.) 

Next, the researchers placed the live eggs in aquaria filled with river water to see if they were viable. They found that three of the 12 eggs—one common carp and two Prussian carp eggs—hatched within three days of incubation.

Three baby fish may seem like a small number, Vincze says, but “if you think about how many water birds there are and how many eggs one fish can produce, it’s a lot of eggs that they can move. I think it all adds up.” Mallards can travel up to each day and fly  between stops during migration. That means they could possibly disperse eggs to waterbodies near and far.

"It's a preliminary but fascinating study,” says Peter Sorensen, a fisheries biologist studying invasive carps at the University of Minnesota who was not involved in the research. "I would be interested to know if those fish eggs survive to adulthood.”

Sorensen points out that common carp hatchlings rarely survive in North America’s lakes because predatory fish eat them. “The chances of eggs being spread by waterfowl and surviving through adulthood doesn’t seem especially high to me.” But Prussian carp can reproduce asexually, which means a single egg could establish a new population. “The risk for Prussian carp does seem notable and it would be worth looking at,” he says. 

To further support the theory that ducks help ferry fish to new waterbodies, the researchers are now looking to repeat the experiment with eggs of several other fish species to see if they, like the carp eggs, survive the Mallard’s gut and successfully hatch and develop. If they do, Vincze says, “it would solve the question that’s remained unanswered for centuries.”