Provincial police said they first received a call about a transport truck rollover at Main Street and Wellington County Road 124 at about 6 p.m. on Sunday evening.
The trailer was carrying hundreds of live turkeys destined for a London, Ont., processing plant.
Police and firefighters called on a local organization dedicated to rescuing farm animals to help save the birds, who were slowly being asphyxiated by the weight of other birds on top of them inside the crowded trailer.
"The more time that went on the more birds that were lost," Wendy Swackhamer, the owner of Wellington County Farm Animal Rescue said Monday.
Once the trailer was righted, Swackhamer said rescuers worked until 3 a.m. to try to rescue as many birds as they could.
"I would say at least half of the birds came out alive," she said.
The 52-year-old driver of the truck was not hurt in the crash and worked with rescuers to try and get as many birds out of the trailer before they suffocated to death.
The OPP has charged him with one count of careless driving.