Smaller droppings from less sizable birds pose a minimal risk of foodborne pathogens on farms, reveals a recent study.
For most farmers, determining if bird droppings near their crops pose a food safety threat doesn’t require expertise in ornithology, laboratory testing, or specialized apps. They just need to ask a straightforward question: How large is it?
This finding comes from a study conducted at the University of California, Davis, which was published today in the Journal of Applied Ecology. It suggests that a more informed and nuanced understanding of the food safety risks related to wild birds could help farmers minimize crop losses while also promoting food safety, biodiversity conservation, and crop production.
Since an E. coli outbreak significantly impacted the U.S. leafy greens industry in 2006, farmers have felt compelled to eliminate natural habitats to deter wildlife—and the pathogens they sometimes harbor—from accessing their crops. Farmers frequently receive guidance to avoid harvesting crops within a three-foot radius of any wildlife droppings, fearing food safety audits or losing buyer contracts. Such anxieties are often cited as reasons for not pursuing conservation initiatives on their farms.
We aimed to uncover the actual risk that wild birds pose to food safety,” stated lead author Austin Spence, a postdoctoral researcher in the UC Davis Department of Wildlife, Fish and Conservation Biology. “We investigated which birds carry pathogens, which species frequent farms, and how long pathogens survive in bird droppings. Our results suggest that it is possible to manage areas for both agricultural productivity and conservation simultaneously.”
Where pathogens remain
By conducting field and greenhouse experiments, bird observations, point counts, and fecal sampling, the researchers evaluated food safety risks associated with nearly 10,000 birds from 29 lettuce farms on California’s Central Coast. They dedicated considerable time observing turkeys, bluebirds, and other birds at the UC Davis Student Farm and nearby Putah Creek, collecting hundreds of fecal samples. They measured E. coli survival in bird droppings on lettuce, soil, and plastic mulch to assess pathogen persistence.
Their cumulative efforts led to a straightforward conclusion: Smaller droppings from smaller birds carry a very low risk of foodborne pathogens, which are already uncommon among birds. If a bird does become infected, the likelihood of pathogen survival is mainly influenced by the bird’s size.
“Larger birds produce big droppings, which are more likely to retain pathogens,” explained Spence. “Conversely, smaller birds emit tiny droppings where pathogens tend to die out swiftly. Consequently, farmers don’t need to identify the specific bird species; they just need to assess the feces size. If the droppings are the size of a quarter, it’s best to avoid harvesting nearby. If they are tiny white specks, the risk is very low and likely acceptable.”
Balancing conservation, yields, and food safety
Beyond the size of the droppings, the surface on which the birds poop—whether it’s crops, soil, or plastic—also affects pathogen survival. The study found that E. coli lasted longer on the lettuce itself compared to soil or plastic mulch.
Encouragingly, around 90% of the birds studied on the farms were of smaller varieties, mainly defecating on soil, where pathogens die off more quickly. By not imposing no-harvest zones when food-safety risks are low, leafy greens producers could potentially harvest about 10% more from their fields.
“Overall, birds present a very low risk for food safety,” stated senior author Daniel Karp, a UC Davis professor in the Department of Wildlife, Fish, and Conservation Biology. “The industry has been monitoring birds for quite some time. However, pathogenic E. coli and Salmonella are exceedingly rare in wild birds found on farmlands. Additionally, we now understand that E. coli usually breaks down quickly in most bird droppings.”
This study offers new avenues for farmers to better balance conservation efforts with food safety concerns. For instance, farmers could install nest boxes to attract small, beneficial insect-eating birds, like bluebirds and swallows, which aid in pest management without compromising food safety.
The research augments a growing body of evidence suggesting that farmers do not need to remove habitats to enhance food safety.
“No studies thus far have shown that habitat removal leads to improved food safety,” Karp remarked. “In fact, the areas surrounding farms likely favor small, insect-eating birds that are less likely to harbor pathogens. Studies like ours provide scientific backing for farmers to retain their habitats—and the diverse wildlife inhabiting them—on their farms.”
The study’s other coauthors include Jeffery McGarvey and SangIn Lee from the USDA Agricultural Research Service, Olivia Smith from Michigan State University, Elissa Olimpi from Conservation Science Partners, and undergraduate students Wentao Yang and Meirun Zhang from UC Davis.
This research was supported by the Center for Produce Safety, the California Department of Food and Agriculture, and the U.S. Department of Agriculture.
