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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.