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Birds as Biological Control Agents on Farms

Birds have arguably received far less attention than predatory insects and spiders as regulators of agricultural pests. This is perhaps surprising given that birds are known to be among the most impactful natural enemies of herbivorous insects in forests and other natural systems. Indeed, while somewhat scant, there is evidence in the literature that predation by wild birds can form a key component of natural pest control. Most evidence comes from tropical coffee and cocoa plantations, where researchers deploy an experimental approach: netting is used to exclude birds from particular trees and reveal bird impacts on pests. Studies in these systems have consistently reported higher pest densities and fruit damage when birds are excluded, suggesting a strong role for birds in pest suppression. Similar bird-exclusion experiments in broccoli (Brassica oleracea) plantings in Hawaii, and in corn (Zea mays) fields in the Midwestern US, found that densities of pest caterpillars significantly increased when birds were excluded; on broccoli, birds’ beneficial impacts were sufficiently strong to boost marketable yields.

The apparent benefits of birds for natural pest control raise the obvious question of what individual growers might do to strengthen these impacts. The choice of farming system may be a key factor impacting bird populations: across a broad range of taxa, including wild birds, organic farming is known to increase the number of species while also improving balance among species’ densities (called “evenness”). In turn, increasing both species number and evenness among predators is known to strengthen natural pest control. Beyond the effects of farming system, wild birds’ density and biodiversity are strongly impacted by landscape features such as the nature and extent of forests and other more-natural habitats. This means that the landscape within which a farm is embedded can impact the frequency of bird visits, and perhaps the opportunities for birds to contribute to pest suppression. In addition, there is evidence that smaller-scale landscape features on farms can likewise benefit wild birds. For example, much work in Europe has demonstrated that hedgerows and other conservation practices can significantly increase bird densities in agricultural landscapes. At a smaller scale still, studies in vineyards and apple orchards report that nesting boxes significantly increase songbird densities and hasten removal of sentinel insect prey provided at feeding stations; in the apple study an increase in marketable yield was also noted. These findings suggest that individual growers can modify their farms to attract more song birds and strengthen natural pest control. What is needed is a more-comprehensive examination of wild birds’ pest control benefits in US organic crops, across a broader range of bird species & eco-regions.

Birds as Disease Vectors on Farms

In recent years, several devastating outbreaks of foodborne illness have been linked to raw vegetables. These outbreaks have resulted in thousands sickened, with many thousands-more cases likely unrecognized or unreported, and dozens killed. Particularly damaging are pathogenic Escherichia coli strains (e.g., E. coli O157:H7), which cause bloody diarrhea, vomiting, and abdominal cramps; death usually results from renal failure and loss of red blood cells. When a pathogen outbreak is traced back to a particular farm, resulting lawsuits can bankrupt a grower.

Fig. 1. Kale with bird feces. Photo credit: Olivia Smith

Organic agriculture embraces a vision of healthy, nutritious food grown in harmony with nature. Therefore, disease outbreaks linked to organic farms are particularly troubling. While harmful E. coli are known to be spread by a vast array of wild and domesticated animals, birds are thought to provide unique risks. This is both because deer-fencing or other barriers cannot exclude flying birds, and because feces deposited by birds in flight often land directly on leafy-green foliage (Fig. 1). Native wild birds may be discouraged from visiting farms through the removal of hedgerows and other natural vegetation, but this may only further encourage colonization by the non-native bird species (1) most comfortable in landscapes dominated by humans and (2) most-clearly implicated as vectors of human and livestock pathogens and parasites (see below). Three species of birds that are particularly common on the farming landscape are House Sparrows (Passer domesticus) (Fig. 2.), European Starlings (Sturnis vulgaris) (Fig. 3), and rock pigeons (Columba livia). These non-native species can carry and amplify a range of parasites and pathogens that will infect both humans and livestock. The routes of transmission occur with opportunities for bird contact with animal feces, livestock feed and water, and farming infrastructure. In addition, wild birds carry pathogens that can be transmitted to humans and livestock by blood-feeding insects (vectors). In some cases there is clear, definitive evidence that wild birds and livestock exchange pathogens such as E. coli and Salmonella enterica. However, some studies suggest that parasite and pathogen infections in wild birds are too low, or infrequent to present a significant risk of transmission. Thus, it is not clear how much wild birds influence disease risk on farms. Importantly, studies of parasite/pathogen transmission by wild birds regularly focus on a single disease and do not evaluate the entire spectrum of infectious organisms the birds harbor. Infectious organisms carried by wild birds can cause enteric, respiratory, skin, blood and neurological diseases in diverse livestock. These diseases impair animal health and productivity, reducing yields and incurring veterinary expenses. Furthermore, the pathogens that are infectious to humans often are sustained within bird and livestock populations before entering the food supply.

Fig. 2. Male House Sparrow. Note the distinct black bib. Photo credit: Mathias Appel
Fig. 3. European Starling. Photo credit: Olivia Smith

Given ever-increasing food-safety concerns and regulations, it is perhaps remarkable that a visionary group of vegetable growers is actively integrating livestock into their farming operations. There are a broad range of ecological and economic factors that underlie this decision. Key among these, livestock feces provide a direct-delivered source of manure to be used as fertilizer, while animal products draw new customers and increase revenue. Despite their many ecological and economic benefits, integrated farming systems also could change the risk/benefit equation for on-farm populations of wild birds. Livestock barns, stalls, coops, and other structures involved in livestock production provide shelter for the non-native starlings and sparrows often implicated as pathogen/parasite vectors. In turn, this brings livestock and wild birds into the close contact needed for transmission. These factors might heighten conflicts between wild birds and livestock on integrated farms.


Landscape Predictors of Avian Community Structure

The cornerstone of our project is an exploration of how farming practices, and the farm environment, impact wild bird communities and these birds’ benefits and harms to farm ecology. We are focusing our comprehensive bird-ecology studies on a group of 40 geographically, physically, and philosophically diverse mixed-vegetable farms across WA, OR, and CA (Fig. 1). While differences between “integrated” and “vegetable-only” farms may be most obvious, these farms also differ in a wide variety of other management practices (e.g., pest management regimes, use of flowering hedgerows, etc), landscape settings (urban, rural, natural) and scales (0.38 ha to 272 ha [mean 29.95 ha ± 8.55 SE]) likely to impact wild-bird populations. As part of farm characterization, we have mapped farm features, and surrounding habitats, using GIS approaches. We are also in the process of conducting rigorous, repeated surveys of bird populations on each farm. Finally, we are developing a survey for a broader group of western growers for their experiences with bird management, while interviewing our focal growers about their farming practices. The end product will be an integrated outreach component will develop powerful, convenient, mobile-web-based tools to allow growers to better understand and manage wild-bird populations on their farms.

Fig 1. Map showing locations of farms in the study










In cooperation with the Nature Conservancy and the Cornell Laboratory of Ornithology, our project will adapt and modify the Merlin Bird ID Mobile App and online mapping tools of the Habitat Network Citizen Science Program to provide growers with practical, easily-accessible, and grower-targeted information about bird ecology and management. Together, these outreach products will pair easy bird identification with a simple and intuitive tool for growers to map, simulate and predict the effects of various bird-management practices on their own farms. These electronic outreach efforts will be paired with more-traditional farm-walk, print, and video-media outreach efforts.

We recently published a series of outreach materials on identification, diet, and management of avian insectivores common on West Coast organic farms that are available online through eOrganic’s site here, here, and here. Previous pieces from the lab about IPM can be found hereherehere, and here.


Previous Outreach Activities:

Spotting Birds and Bugs on the Farm, October 7, 2017, Full Belly Farm, Guinda, CA

Costs and Benefits of Birds in Agro-Ecosystems, February 17, 2017, Pasco TRAC, Pasco, WA

Creating Bird Habitat in Agriculture, October 1, 2016, Full Belly Farm, Guinda, CA

Integrated Biological Pest Management Practices for Oregon Farms, August 23-24, 2016, Persephone Farm, Lebanon, OR