They once numbered in the billions, and travelled in flocks so large they blocked the sun for days.  But in 1914, the last of the passenger pigeons died at the Cincinnati Zoo.

Passenger pigeons were a staple of the North American diet in the 19^th century and were easy to hunt, especially after the telegraph made it easier to locate enormous pigeon flocks, and railroads made it easier to ship their meat to growing cities.

But was overhunting what killed the passenger pigeon? It is one of two prevailing theories on how this once-abundant species declined to extinction in a mere 40 years. Habitat loss has also been considered as a possible cause of the passenger pigeon’s decline. They subsisted primarily on tree mast—nuts and seeds—from the mature hardwood forests that once covered much of eastern North America, but had mostly been cleared by the early 20^th century.  If passenger pigeons were unable to adapt to a changing food supply, that could have decimated their numbers.

In research recently published in the scientific journal Quaternary Science Reviews, Trent University’s Dr. Eric Guiry shows that passenger pigeon had more flexible diets than was previously known, eating more than just tree mast.

Stable isotope analysis shows that passenger pigeons likely ate corn
All plants photosynthesize using carbon (C) from the atmosphere, but not all carbon is identical.  When most plants, including hardwood trees, photosynthesize they use a C₃ photosynthetic pathway that produces a characteristic carbon isotope composition. In contrast, some plants, including key agricultural crops like corn, use a C₄ photosynthetic pathway that produces highly distinctive carbon isotope compositions that differ significantly from C₃ plants.

Using stable isotope analysis–a technique that identifies the signature of non-radioactive elements at a molecular level – Dr. Guiry examined the bone remains of passenger pigeons from museums and archaeological collections in Ontario and Quebec. He knew that C₃ would be present, but if C₄ was also there, it would indicate that passenger pigeons had been eating more than just tree mast.

“Our study found evidence of systematic use of C₄ plants in the pigeon diets, and because there are few plants that produce this distinctive isotopic composition in northern climates, we believe that it likely comes from corn,” says Dr. Guiry.

Environmental change likely not primary cause of extinction
As mature forests were cleared, they were replaced with farms. While tree mast would no longer have been abundant, corn would have been.

“This demonstrates a level of dietary flexibility that would be inconsistent with habitat decrease hypothesis,” says Dr. Guiry, a SSHRC Banting postdoctoral fellow.

“It suggests that environmental change alone likely isn't the primary cause of the passenger pigeon’s extinction – overhunting was the main thing. They should have been able to switch to eating something else.”

Read more about Dr. Eric Guiry’s research on passenger pigeons in The Conversation.