:It really depends on what you mean by control, and how you classify " harmful insects . " For the agricultural side of things, here's some refs : " Sustainable pest regulation in agricultural landscapes : a review on landscape composition, biodiversity and natural pest control " which says " In 74 % . . . of the studies reviewed . . . natural enemy populations were higher and pest pressure lower in complex landscapes versus simple landscapes . " ( Natural enemy includes insectivorous birds ) There's also a review article on the topic, " Past and Current Attempts to Evaluate the Role of Birds as Predators of Insect Pests in Temperate Agriculture " [ http : / / link . springer . com / chapter / 10.1007 / 978-1-4615-5881-1 _ 5.
