These bird species are most likely to contaminate our fruits and veggies

A sweeping new study dissects how pathogens get passed on from bird poop to farms to food.
Female house sparrow and brown-headed cowbird, both potential vectors for bacteria that cause food-borne illnesses, feeding on birdseed on the ground
House sparrows (left) and brown-headed cowbirds (right) both rate riskier when it comes to bacteria-spreading wildlife on farms. Deposit Photos

After the E.coli outbreak in spinach in 2006, farms in California’s Salinas Valley had to take action to address food safety. The wave of concern around wildlife contaminating produce resulted in 13 percent of wetlands and river-side habitats being razed over the next few years. And it wasn’t a one-time event: In response to food-borne diseases, habitats are often removed to prevent animals from coming in contact with crops. But according to a team of entomologists and environmental scientists from across the US, the safety of our fruits and veggies doesn’t need to come at the cost of biodiversity. 

Building on previous research, a study published recently in Ecological Applications illustrates the scale of risk associated with birds and pathogens in produce. It also proposes how some farms can deter harmful wildlife interactions—and welcome beneficial ones.

The researchers traced a path to find out how likely it is that birds carry bacteria, which species are most likely to do so, and how frequently they’re able to transmit people-sickening diseases.

In a meta-analysis of the existing data on birds and pathogens, Olivia Smith, a postdoctoral researcher at Michigan State University, found that nearly all past studies were conducted on the West Coast of the US where a majority of domestic produce is grown. In addition, they tended to focus on smaller organic farms that were varied in their production. Like Smith’s previous research, the studies focused on three bacteria that commonly cause food poisoning: E.coli, salmonella, and campylobacter. 

[Related: Compost can help protect us from food poisoning]

To properly establish risk, the authors first looked at whether the birds were actually carrying these contaminants. In their aggregated data from 1,565 studies, they found only 0.22 percent of birds carried E. coli and 0.46 carried salmonella. Conversely, around 8 percent carried campylobacter, which also happens to be the only example of an outbreak that has conclusively been traced to wild birds: a flock of migratory cranes in pea fields in Alaska.

To follow these pathogens back to the responsible bird species, Smith used a previous set of results where researchers collected 1,215 fecal samples from farms across Washington, Oregon, and California and identified them through DNA testing. Out of the 106 species present, nearly 40 percent of all the fecal matter was traced back to 35 kinds of birds. White-crowned, song, and house sparrows were three of the most frequent suspects when it came to the discovered poop deposits. 

Working with Smith’s previous studies and lab data collected by Daniel Karp, an assistant professor in the department of wildlife, fishing, and conservation biology at UC Davis and an author of the paper, the team also used data from 87 farms to see if the birds carrying the pathogens were visiting and interacting with the fruits and vegetables and transferring their bacteria. They then compiled survey data by observing the number of birds in and around the fields and whether they were touching the crops in some way.

For the most part, the total number of contacts observed per species positively correlated to the number of feces that could be traced back to that bird. But while the survey helped identify which feathered visitors were actively shedding their pathogens onto the crops, there were some cases where the abundance of a particular species didn’t match the frequency of its poops. With barn swallows, for example, the correlation was inverted.

“A swallow is flying above the farm field a lot and is dipping down and sometimes interacting with the produce, but it’s not really landing at all,” Karp explains. “We would say that it interacts with the produce a lot, because we see it there a lot. But those birds don’t really poop on the wing much.”

Barn swallow in flight with its forked tail and wings splayed against the blue sky
Insectivorous birds like barn swallows can provide an ecosystem service to farms. Deposit Photos

This means that though they are spotted often, the swallows are probably contaminating the crop much less than they are around it. At the same time, they’re eating pests that are harmful to the crops along the way. In comparison, the three types of sparrows were both present in the field and defecating a lot. 

“For the sparrow, this is a bird that even given its relatively high abundance in these fields, it poops even more often,” says Karp. 

The authors also concluded that the amount of pathogen that a bird species can carry could be determined by the risk of exposure. Species like Canada geese and brown-headed cowbirds that tend to dwell on the ground and around feedlots for livestock are more likely to come into contact with feces that carry E. coli and other bacteria. They might then pick up the those microbes and transport them to produce farms as they move and forage throughout the day.

Meanwhile, insect-eating birds like the yellow warbler that prefer being higher up in the canopies tend to have lower exposure. Smith has found that these species are less likely to carry disease because their natural habitats are relatively cleaner than the ground down below. This could be compounded by the fact that insects aren’t relatively as dirty as the grain that cattle and poultry are contaminating on ground.

Identifying key traits in bird species that serve as vectors for these harmful pathogens makes it easier to utilize safer food-handling techniques without affecting biodiversity and sustainability efforts.

“The exposure trait about livestock tells us something about how these birds are getting the pathogens, but also the kinds of birds that you would expect to be worried about,” says Karp. On the flip side, the diet indicator could suggest that certain avian species are actually beneficial for plants and farms. 

For Karp, this information is useful in creating more targeted schemes to evaluate which birds are harmful and whether their presence should be managed on specific farms. Deterrents such as screamers, sound cannons, or even falconry could be used to shoo away pathogen-carrying species. 

“This kind of targeted management gets us toward a more win-win situation, where we’re sort of benefiting conservation, and then getting some of those pests benefits, but then not worrying too much about the food safety risks,” Karp says. 

[Related: How to clean your bird feeders and baths for salmonella]

Sadhana Ravishankar, professor of food safety at the University of Arizona, stresses that while understanding who the predominant carriers of disease are, being able to deter them is another story. “Farmers can probably take precautionary measures, but nature is so vast. How effective can these controls be?” she says.

Other than scale, a lack of established controls is another food safety issue. “A lot of research needs to be done before you can come up with scientifically sound metrics,” Ravishankar says. Without these guidelines, farmers have an even tougher job of regulating what is or isn’t safe for consumption. 

Karp, however, plans to continue to refine his analysis by focusing on looking at how long a pathogen  lingers once it’s reached the crops. “Does it actually survive in the food?” he says. “Is it there for a couple of hours? Or is it there for weeks, in which case you’d be more worried about it when harvest time comes?”

From a bird lover’s perspective, his co-author Smith is interested in managing agricultural systems in a way that’s good both for people and biodiversity. She hopes to figure out which birds are risky so that US farms can be more sustainable and safe for a kid snacking on an apple and the swallows swooping through the sky.