Soil Animals as Vectors of Organochlorine Insecticides for Ground-Feeding Birds

Abstract

The uptake of small sublethal doses of organochlorine pesticides by birds appears now to be quite a common phenomenon, but the pathways have been, for the most part, little examined. This paper discusses some of the ecological factors affecting the uptake and passage of organochlorine compounds in terrestrial ecosystems with particular reference to soil animals and ground-feeding birds such as the Robin (Erithacus rubecula), Skylark (Alauda arvensis), Lapwing (Vanellus vanellus), Starling (Sturnus vulgaris), thrushes (Turdus spp.) and corvids (Corvus spp.) for which animals form a large fraction of the diet. Blackbird (Turdus merula) and the Kestrel (Falco tinnunculus), both of which take large numbers of invertebrates, though the Blackbird is an omnivore with a partiality for fruit and the Kestrel a raptor feeding largely on small mammals. The ppm intervals chosen follow approximately a log scale which is rounded to the nearest convenient figures. The two species cannot be compared in detail since liver alone was used for analysis of the Kestrels and mixed viscera, which consisted of liver, heart and spleen (and possibly gut in some instances), in the Blackbirds. Both species came from a large number of counties but as the birds were generally found dead and the cause of death unknown neither set can be assumed to be a fair sample of the population. However, several features can be deduced. First, that both species had picked up residues of all the common organochlor- ine pesticides used in this country and secondly, apparently in contrast to the American situation, dieldrin and heptachlor epoxide residues are commonly present. DDT itself features very insignificantly but the preponderance of DDE indicates a large original DDT source. The relatively high amounts of heptachlor epoxide and low amounts of BHC contrast with the annual depositions of these compounds (see Strickland 1965) and reflect probably the biased nature of the samples as well as the known properties of relative persistence of these compounds in biological tissues.