Near the southern tip of South America, thousands of ladies — wives, mothers, anchovy enthusiasts — are vanishing from their nests.

The females in question are Magellanic penguins — a mid-size species of black-and-white bird native to South America's Patagonia region. When not breeding in the latter part of the year, both male and female members of the species migrate north toward Uruguay and Brazil to hunt for the tasty anchovies that call those waters home. Over the last decade, however, scientists have observed an upsetting trend: some penguins are swimming too far north — sometimes hundreds of miles away from their breeding grounds — and getting stuck there.

According to a new study published today (Jan. 7) in the journal Current Biology, every year, thousands of Magellanic penguins fail to return home from their migrations. Some become stranded on the shores of Uruguay, Argentina and Brazil. Others wash up already dead, their stomachs empty or polluted with plastic waste. Strangely, about two-thirds of the stranded birds are female. [Photos of Flightless Birds: All 18 Penguin Species]

Takashi Yamamoto, lead author of the new study and a researcher at the Institute of Statistical Mathematics in Tokyo, wanted to find out what was happening, and why female penguins were disproportionately afflicted. So, he and several colleagues tagged a small group of 14 Magellanic penguins (eight males and six females) with GPS ankle monitors, then watched where the birds strayed after their breeding period ended in early 2017.

After several months of observations, the team saw a clear pattern. During their spring and summer migrations, male penguins tended to dive deeper and stay closer to their Patagonian breeding grounds; female penguins swam closer to the water's surface, but migrated significantly farther north than their male counterparts.

There, in the waters near Uruguay and southern Brazil, the penguins approached known penguin-stranding hotspots. According to the researchers, these stranding sites — such as the riverfront near the city of Buenos Aires, in northern Argentina — likely trap the penguins through a mixture of strong currents that prevent smaller-bodied birds from swimming home and man-made threats. "These [threats] include water pollution caused by oil development and marine transport as well as fishery-associated hazards, such as bycatch and depletion of prey species," Yamamoto said in a statement.

The reason that female penguins seem to be disproportionately stranded compared to males might be as simple as body size. According to the researchers, female Magellanic penguins are smaller than males, which could make it harder for them to compete for food in crowded southern waters, or to fight against strong currents in the north. A smaller body also means a greater sensitivity to ocean temperatures, Yamamoto noted. This could give the smaller-bodied females a preference for chasing warmer waters northward toward the equator, and for avoiding deep dives into the cold, dark ocean.

This small study is just to first step toward understand the cause and scale of the mysterious bird strandings. But, according to Yamamoto, this much is clear: if fewer and fewer females return to their breeding grounds every year, the viability of the entire Magellanic penguin population could soon be at risk.

Originally published on Live Science.