DAVIS, Calif. – Researchers have reported that wild bird feces confer little risk to leafy greens.

Foodborne pathogens like E. coli and Salmonella can be spread through the feces of wild animals. Producers often use strategies to deter wildlife from entering agricultural areas to prevent the spread of pathogens, like using fences and rodent traps and removing vegetation that can serve as wildlife habitat.

Still, these strategies do little to keep birds away from crops, as they can easily fly over physical barriers. The vast majority of North American bird species are also federally protected under the Migratory Bird Treaty Act, which prohibits actions that could harm birds. With few options to dissuade birds, growers have been worried about the potential for the animals to contaminate food.

Austin Spence, ornithologist and postdoctoral scholar in the Department of Wildlife, Fish, and Conservation Biology at the University of California, Davis, and his colleagues weren’t convinced that birds were the source of disease-causing pathogens. Spence and his collaborators set out to investigate if birds threaten human health and safety.

“Luckily, the outcome of our work is that (birds) aren’t really a threat (to human health and safety),” Spence said. “Most of the birds that are on farms present very little food safety risk.”

Spence and his team conducted bird consensuses and fecal contamination surveys across 29 organic farms growing leafy greens on the California Central Coast. Leafy green farms were a focus of the study because they are food primarily eaten raw, presenting a higher risk to human health and safety if they are contaminated.

To study wild birds on the farm, field technicians conducted point counts, a process in which each bird seen or heard on the farm was recorded. The surveys were conducted near harvest time. Across 862-point counts, the technicians detected 9,560 birds of 90 species. Technicians also collected fecal samples on the farms, which were later analyzed to determine what species of bird the feces originated from.

The results of the farm studies revealed that of all birds detected, only 1,015 birds of 25 species were observed making direct contact with leafy greens. Of birds observed interacting with crops, 89.6 percent of them were smaller than 100 grams.

The size of birds matters when it comes to understanding the pathogen spread from feces because smaller birds have smaller poops. Through other experiments, Spence and his team discovered that smaller feces were less likely to harbor E. coli, thereby reducing the spread of the pathogen. E. coli is more likely to persist in larger poops, such as those from wild turkeys that are bigger than a quarter.

Additionally, 92 percent of collected fecal samples were on the soil as compared to 8 percent of fecal samples found on lettuce. In greenhouse and field experiments, the research team determined that E. coli populations decline over time at higher rates on soil and plastic mulch compared to lettuce.

Taken together, the results deliver a simple message to farmers: most birds that defecate on farms are small, and small poops have a lower risk of transferring pathogens to food.

Spence also said that in a separate study, birds were collected and tested for E. coli, Salmonella and Campylobacter. Less than 10 percent of birds tested had Campylobacter, and less than 1 percent had Salmonella and E. coli. Spence said that mammals more related to humans, like cows and pigs, are more likely to spread pathogens than birds.

The results of Spence and his team’s work have financial implications for farmers.

“In California, we have a rule that we are hoping to change, in that if there was any sort of wildlife poop (present on a farm), farmers weren’t supposed to harvest a meter or three feet from that fecal sample,” Spence said. “That can be a lot, especially when these small birds are pooping small poops that aren’t a risk. We estimated that it could be up to a 12 percent loss in yield. From our work, we found that if (farmers and policymakers) followed more scientific standards and only not harvesting around these large poops or only not harvesting when (the poop) is actually on lettuce, we could take that 12 percent loss down to 3 percent.”

The results also show that conservation and agriculture can go hand in hand, Spence added. Growers may preemptively remove wildlife habitat that surrounds their crops to reduce the spread of potential pathogens and improve food safety. However, there is no evidence that removing wildlife habitat contributes to food safety. Instead, many scientific studies suggest that habitats around farms can be beneficial, with wildlife aiding in pest control and pollination.

While large birds can pose some risk to food safety, there is little that can be done to deter them from interacting with crops.

“We would classify about 80 percent of birds on farms as low risk (to food safety). It’s really about harvesting around poop,” Spence said. “What we’ve been telling farmers is that if you see (poop) that’s about the size of a quarter, don’t harvest around it. Everything else is pretty low risk.”