Maintaining a thriving crop is every grower’s top concern. According to Dr. Daniel Carr, an associate professor at UC-Davis, near the top of the list is whether or not birds are bringing foodborne diseases onto their crop fields.
Back in 2006 there was a multi-state outbreak of E. coli. The strain was traced back to bags of spinach that had originated from California’s Central Coast. “The strain was found in the water and the soil, as well as in the feces of cattle and wild pigs,” said Carr. “This led the industry to believe that it was actually the wildlife that were bringing foodborne diseases onto farm fields.” Therefore, growers believed the right thing to do was to keep wildlife away.
In earlier sessions of the “Role of Birds on the Farm” series, Wild Farm Alliance addressed the benefits of building habitats around your farm fields in reference to pest control. What Carr set out to answer in this lesson was the question of whether keeping habitat around your field could lead to food safety risks.
Carr quoted several studies in which he was a participant along with a well-known leafy greens producer. Notably, the results regarding the frequency of detection of pathogenic E. coli in leafy greens showed no significant effects when there were croplands, riparian areas or other natural habitats present.
However, according to Carr, “the more grazed land around a farm field, the more likely you would find a leafy green that tested positive.” This is due in large part to the fact that these areas are often meant for confined animal feedlot operations.
Carr and company conducted a study using 20 strawberry fields and 20 lettuce fields. In both studies the result was similar. Carr noted, “Bird feces only directly touched or contaminated two of the more than 10,000 strawberries we looked at,” demonstrating that finding foodborne pathogens in the birds was rare.
After examining 600 samples of bird feces, the birds appeared to carry zero to a very low percentage each of the pathogenic E. coli strain, Salmonella and Campylobacter, respectively, onto the lettuce fields.
Not satisfied with the findings, Carr wanted to dig deeper and “find out if there were certain characteristics that would create a greater food safety risk,” he said. “We did some transects of the bird feces and noted that pathogenic feces changed based on the natural habitat in question. In areas where there was ungrazed natural habitat, there was a reduced risk of potentially pathogenic feces versus grazed natural habitat which carried a higher risk that birds would be carrying the virulent strain of E. coli or Campylobacter.”
From a food safety perspective, Carr wondered how species of higher conservation concern are less likely to form big flocks. “Growers are really concerned about these big flocks of, for example, blackbirds and starlings, that we know are both more likely to carry foodborne pathogens and more likely to aggregate in large numbers, and defecate in large amounts, on the farm field. So the probability of actually observing a flock declined as you changed from a farm that didn’t have much ungrazed semi-natural habitat to one that had more grazed semi-natural habitat,” he said.
The way insectivorous birds can control the number of insect pests was presented by Megan Garfinkel, a postdoctoral researcher at the University of Illinois-Chicago. According to Garkinkel, there was a study that estimated 400 million to 500 million tons of insect prey are eaten by insectivorous birds every year.
“One really important variable that can affect pest control by birds on farms is the vegetation both within farms and around the farms,” she said.
According to Garfinkel, one study looked at the pattern of birds that use farms fields growing a single crop versus farm fields that were growing multiple crops within a single field. What they found is that more species of birds were in the mixed crop fields than the monocrop fields.
Notably, Garkinkel warned that having more species of birds in the crop field doesn’t necessarily translate to higher level of pest control. However, “the two are often correlated with each other.”
One of the reasons for that, Garfinkel explained, is that “when you have more species of birds you’ve got birds that are foraging in a variety of different ways; some will catch the insects flying through the air while others spend more time closer to the ground, picking the insects off the leaves.”
Another study Garfinkel highlighted looked at farms that were using intercropping to increase the diversity of plant life within a field. “Vegetable fields were compared to those that had intercropped rows of sunflowers and, not surprisingly, more insect-foraging birds were found to be making use of intercropped sunflower fields than birds in the control fields without the sunflowers,” said Garfinkel.
“They also found that the amount of time that these insectivorous birds spent foraging in the sunflower intercropped fields was higher than in the fields without those intercropped sunflowers,” she added.
Garfinkel also spoke about vegetation management surrounding farms and how that affects pest control services. One study she conducted was on a corn field near a prairie. “I placed cages over some of the corn crops to keep the birds off the crops and then compared the crop yield with some nearby plants that didn’t have cages,” Garfinkel said. “By being close to a natural prairie, the crop yield in the corn plants that birds were able to access was much greater than the crops that were in the enclosures.”
After doing a DNA barcoding analysis of some fecal samples from birds that were foraging in the corn field as well as in the nearby edge of the prairie, Garfinkel found that 34.5% of the birds she tested had eaten northern corn rootworm, a major pest of corn in the region.
“What we’re finding across the board is that increasing the diversity of vegetation both on and around the farm is going to help increase your pest control by birds,” Garfinkel concluded.
by Jessica Bern