From The National Farmer's & Housekeeper's Cyclopedia, 1888

The swallow, swift, and hawk are the guardians of the atmosphere. They check the increase of insects that otherwise would overload it. Woodpeckers, creepers, and chickadees are the guardians of the trunks of trees. Warblers and flycatchers protect the foliage. Blackbirds, crows, thrushes, and larks protect the surface of the soil. Snipe and woodcock protect the soil under the surface. Each tribe has its respective duties to perform in the economy of nature, and it is an undoubted fact that if the birds were all swept off the face of the earth man could not live upon it, vegetation would wither and die; insects would become so numerous that no living being could withstand their attacks. The wholesale destruction occasioned by grasshoppers which have devastated the West is to a great extent, perhaps, caused by the thinning out of the birds, such as grouse, prairie hens, etc., which feed upon them. The great and inestimable service done to the farmer, gardener, and florist by the birds is only becoming known by sad experience. Spare the birds and save the fruit; the little corn and fruit taken by them is more than compensated by the quantities of noxious insects they destroy. The long-persecuted crow has been found by actual experience to do more good by the vast quantities of grubs and insects he devours than the harm he does in the grains of corn he pulls up. He is, after all, rather a friend than an enemy to the farmer.

Protect the Swallow - Among insectivorous birds the swallow is worthy of great encouragement. An examination of the stomachs of eighteen swallows killed at different seasons of the year showed that they contained an average of 406 undigested insects each, and not a single grain of corn (of any kind), or the least particle of fruit or a trace of any vegetable..