Studies show that birds provide biological control services worth millions of dollars in farmlands and forests, and are encouraged in some plantations through the provision of nest-boxes
In the Sacramento Valley, California, Western Meadowlarks Sturnella neglecta were reckoned to require 193 tons of insects daily during the breeding season, and in an area north-west of Wichita, Kansas, some 15–20 million Red-winged Blackbirds Agelaius phoeniceus were estimated to have fed their young, from hatching to independence, the equivalent of 4,260 tonnes of insects or 6.3 billion cutworms (moth larvae notoriously harmful to plants) (Henderson 1927). All the birds of Texas were conservatively estimated to consume around 35,500 “bushels” (1.25 million litres) of insects a day (Henderson 1927). During outbreaks of western spruce budworm Choristoneura occidentalis birds, in particular the Evening Grosbeak Coccothraustes vespertinus, provide biological control services worth, in 1980s value, $1,820 per square kilometre (Diamond 1986). In 1921 it was estimated that insects annually destroyed one billion dollars’ worth of forest and agricultural products in the USA, and that without the services of the birds that cost would have risen by another US$444 million. Eighty years ago the birds were thus saving the US farmland and forest economy 28% of its losses (Henderson 1927). Across Europe and northern Asia the same considerations apply. The European Pied Flycatcher Ficedula hypoleuca, for example, is a major suppressor of insects harmful to forest vegetation, taking many pest insects, especially moths and caterpillars, and is often catered for in plantations and economically important stands of oak trees through the provision of nest-boxes. Indeed the use of nest-boxes not only for flycatchers but also for titmice is a fairly standard management tool in forests throughout Europe.
BirdLife International (2008) Birds control insect pests in farmlands and forests. Presented as part of the BirdLife State of the world's birds website. Available from: http://www.birdlife.org/datazone/sowb/casestudy/99. Checked: 28/11/2015