Effects of DDT on Bird Population of Scrub Forest

Abstract

A number of studies have been made by the U. S. Fish and Wildlife Service to determine the effects of DDT on vertebrate life. The work has covered a variety of situations and several dosage rates have been used. As far as birds are concerned, the results have shown that DDT applied to forest areas at a rate of one pound per acre has but slight effect (Hotchkiss and Pough, 1946; Kendeigh, 1947). Repeated applications of small dosages of DDT (0.1 pound per acre) as practiced in malaria control, have been studied for effects on birds by Erickson (1947). He failed to find any measurable effects following 17 weekly applications. Following a twopound-per-acre treatment of a lowland woods, Stewart, et al. (1946) could find no conclusive evidence of mortality in a nesting population, but some changes in habits appeared to have resulted. An experimental treatment of a portion of dense second-growth hardwood forest with five pounds of DDT to the acre was found by Hotchkiss and Pough (1946) to cause a significant reduction in the bird populations. It is becoming increasingly apparent that control of forest insect pests by aerial dispersion of DDT will be practical under some conditions. In order that such control work can be done with a minimum adverse effect on beneficial animal life, the Forest Insect Division (Bureau of Entomology and Plant Quarantine) and the Fish and Wildlife Service are continuing a program of cooperative studies. Effects of DDT at different dosage rates per acre are being determined for vertebrates and invertebrates in both aquatic and terrestrial habitats. A determination of the critical dosage level and conditions of application which create definite hazards to wildlife is urgently needed. The present study details the effects on a bird population of a five-pound-per-acre application where the nature of the cover was such that little screening out of the spray occurred. The absence of a canopy, and resultant deposition of a large percentage of the spray at ground level, also approximated conditions that might be met in comparable control attempts in some agricultural situations. The area chosen for the test is a 90acre tract located on the Beltsville Agricultural Research Center, two miles west of the Patuxent Research Refuge on the Coastal Plain of Maryland. The area was burned over in 1942 and is now covered with a scrub and sapling growth. Numerous dead pine and oak trees of the former forest are still standing. The commonest woody plants in the sprayed area, all in the form of saplings, sprouts, or shrubs, are sweet gum (Liquidambar styraciflua), red maple (Acer rubrum), black gum(Nyssa sylvatica), and dwarf sumac (Rhus copallina). Large-toothed aspen (Populus grandidentata), willow oak (Quercus phellos), white oak (Q. alba), tuliptree