It doesn’t require a degree in ornithology, a lab test or even an app for most growers to determine whether bird poop near their crops presents a food safety risk. They just need to ask themselves a simple question: How big is it?
That’s according to a study from the University of California, Davis, published today in the Journal of Applied Ecology. The study said an informed, nuanced view of food-safety risks and wild birds could help growers avoid crop losses and manage farms for food safety, biodiversity conservation and crop production.
Since 2006, when an E. coli outbreak devastated the U.S. leafy greens industry, growers have been pressured to remove natural habitat to keep wildlife — and the foodborne pathogens they sometimes carry — from visiting crops. Growers are often advised not to harvest crops within a roughly three-foot radius of any wildlife feces, lest they risk failing a food safety audit or losing a buyer contract. Growers often cite such concerns as a barrier to implementing conservation actions they would otherwise consider taking on their farms.
“We wanted to find out the true risk of wild birds to food safety,” said lead author Austin Spence, a postdoctoral researcher in the UC Davis Department of Wildlife, Fish and Conservation Biology. “Which birds have pathogens, which birds are spending time on farms, and if a bird has a pathogen, does that pathogen survive long in bird poop? Our findings indicate that we can co-manage our areas for both agriculture and conservation.”
Where pathogens persist
Through field and greenhouse experiments, bird surveys, point counts and fecal transects, the authors assessed food-safety risks from nearly 10,000 birds across 29 lettuce farms on California’s Central Coast. They spent hours following turkeys, bluebirds and other wild birds at the UC Davis Student Farm and nearby Putah Creek, collecting hundreds of fecal samples. They compared E. coli survival in bird droppings on lettuce, soil and plastic mulch to measure pathogen persistence.
After all of these efforts, they landed on a simple finding: Smaller poops from smaller birds carry very low risk of foodborne pathogens, which were rare in birds overall. If a bird were to become infected, pathogen survival depends heavily on the bird’s size.
“Birds that are large produce really big feces, and that’s where pathogens are more likely to survive,” said Spence. “Birds that are small have tiny feces, and the pathogens die off quickly. So farmers don’t have to know the species of bird it came from. They just need to know the size. If it’s the size of a quarter, don’t harvest near that. If it’s a tiny white speck, it’s very low risk and probably fine.”
Balancing conservation, crop yields, and food safety
Beyond fecal size, what the birds pooped on — be it the crop, soil or plastic — made a difference for pathogen survival. In the study, E. coli survived longer on lettuce itself than on soil or plastic mulch.
Fortunately, about 90% of birds observed on the farms were small and tended to poop mostly on soil, where pathogens perish quickly. By avoiding no-harvest buffers when food-safety risks are low, growers of leafy greens could harvest about 10% more of their fields.
“Birds generally present really low food-safety risks,” said senior author Daniel Karp, a UC Davis professor in the Department of Wildlife, Fish, and Conservation Biology. “The industry has been concerned about birds for a while. But pathogenic E. coli and Salmonella are vanishingly rare in wild, farmland birds. And we now know E. coli tends to die off quickly in most bird poop.”
The study opens up new strategies for growers to better balance conservation and food-safety risks. For example, growers could erect nest boxes to attract small, beneficial insect-eating birds, like bluebirds and swallows, that help control pests without risking food safety.
The work also contributes to a growing body of research that suggests growers do not need to remove habitat to improve food safety.
“There have been no studies to date that suggest habitat removal improves food safety,” Karp said. “Habitat around farms is actually likely to favor the small, insect-eating birds that are unlikely to carry pathogens. Studies like ours are giving farmers science-based permission to conserve habitat — and many species of wild birds — on their farms again.”
The study’s additional co-authors are Jeffery McGarvey and SangIn Lee at the USDA Agricultural Research Service, Olivia Smith at Michigan State University, Elissa Olimpi at Conservation Science Partners, and undergraduates Wentao Yang and Meirun Zhang at UC Davis.
The study was funded through the Center for Produce Safety, the California Department of Food and Agriculture and the U.S. Department of Agriculture. — By Kat Kerlin, UC Davis
Kathryn Deschenes (Colorado) is appointed to the Handler seat. She is the Director of Regulatory Affairs at Danone, which produces dairy, plant-based products, and infant food, including the Happy Family Organics brand. She holds both master’s and bachelor’s degrees in Food Science and Technology.
Amanda Felder (California) is appointed to the Handler seat. She is the Organic Integrity and Environmental Compliance Manager at Taylor Farms, a leading producer of organic salads, where she oversees the organic compliance of a network of farming, food processing and manufacturing operations. She holds a bachelor’s degree in Business Administration.
Andrea Hatziyannis (Arizona) is appointed to the Retailer seat. She is the Category Strategy Manager for Sprouts Farmers Market, a supermarket chain that specializes in organic foods and fresh produce. She brings 20 years of industry experience in product development, retail, and research. She holds both master’s and bachelor’s degrees in Food Science.
Cathleen McCluskey (Wisconsin) is appointed to the Environmental Protection and Resource Conservation seat. She is the Policy and Advocacy Director for Organic Seed Alliance, a national organization that conducts research and training with seed producers and develops policies for organic seed, food, and farming systems. She holds a Doctorate in Environment and Resources and a bachelor’s degree in Agricultural Systems Analysis.
Corie Pierce (Vermont) is appointed to the Farmer seat. She’s the owner and operator of Bread & Butter Farm, a 600- acre diversified organic vegetable and grass-fed beef and pork farm in Vermont. She also teaches organic farming at the University of Vermont. She holds a bachelor’s degree and a certificate in AgroEcology.
