Toxic chemicals and wildlife in Britain.
The first recorded cases of mortality in birds associated with the use of agricultural chemicals in Britain occurred in 1947. Since that time it has been a recurrent problem, considerably aggravated by the introduction of chlorinated hydrocarbons as cereal seed-dressings in 1956 (Carnaghan and Blaxland 1957). The purposes for which these chemicals are used have been reviewed by Campbell (1962).
The main chemicals having adverse effects - either directly or indirectly - on game and other birds fall into two main categories:-
Taking the insecticides first, there are two groups of importance:
i. The organo-phosphorous group, including malathion, menazon, parathion, schradan, etc., which are characterised by being non-cumulative, and having high toxicity and low persistence.
ii. The chlorinated hydrocarbon group, including aldrin, BHC, DDT, dieldrin, endrin, heptachlor, etc., which are characterised by being cumulative, and having high toxicity and high persistence. These have been largely responsible for the mass-mortalities of recent years.
Secondly, the herbicides, including a large group of selective weed-killers such as 2,4-D, MCPA, 2,4,5-T, etc., and a smaller 'nitro'-group including, for example, DNOC. There are also others. DNOC has been responsible directly for many deaths; the others are more importance for their indirect effects in altering the habitat - the destruction or alteration of 'cover', and the destruction of food-plants.
A third group consists of fungicides, but at present apparently none of these has been implicated either directly or indirectly in either mortality or habitat deterioration. Certain groups, however, such as Thiram, are known to be toxic to birds and to effect egg production under artificial conditions, so that these ought to be watched carefully.
In the following discussion, reference has been confined to the British literature. It should be mentioned that American literature on this subject is vastly greater.