A SYNOPSIS OF THE PESTICIDE PROBLEM
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their relevance is slight, than on consideration of relevant circumstantial
evidence derived from field observations. The limitations and value of
circumstantial evidence in interpreting the effects of pesticides is illustrated by the following examples which concern the possible effects of
pesticides on three avian species in Britain.
1. The chaffinch (Fringilla coelebs) has probably declined considerably
in Britain during the last twenty years. Many ascribe this decline to
the use of organochlorine insecticides.
In ordinary farmland chaffinches are largely dependent on hedgerows and small woods. Recent studies by Moore, Hooper and Davis (in
press) show that hedgerow destruction has occurred throughout Britain,
and in at least one area 70% of total hedge length was destroyed during
the last twenty years. In this area a 70% reduction in habitat would
have led to a comparable reduction in the number of chaffinches quite
irrespective of other factors. In this case, where habitat reduction must
have caused a population reduction, it would be wrong to conclude that
the decline of the chaffinch was solely or principally due to pesticides.
Pesticides may have been a contributing factor, but in this case it
would be quite impossible to evaluate the different factors from the
data available.
2. The red-backed shrike (Lanius collurio) has declined catastrophically in Britain since 1940 (Peakall, 1962). From being a common bird
there are now probably only about 100 pairs left in the country. Its
decline has been ascribed to the use of insecticides.
Records show that the decline of the red-backed shrike started long
before 1940. It is also occurring in countries in which organochlorine
insecticides are relatively little used. As with the chaffinch, organochlorine insecticides may well be affecting this species, but it is quite
certain that other factors are also involved and again no evaluation of
the importance of the pesticide factor is possible on existing evidence.
3. The sparrow-hawk (Accipiter nisus) has declined catastrophically
in much of England during the last 10-15 years (Prestt, 1965 and
unpublished). Its decline has been attributed to increased game preservation, destruction of habitat, increased road traffic, unexplained
cyclical changes and organochlorine insecticides.
There is evidence to suggest that game preservation increased immediately after the war, but none to suggest that there was a further
increase in the rnid-l950s, when the sparrow-hawk began to die out.
There is much evidence to show that before the war this species survived
well in areas in which there was intensive local persecution. It has
been legally protected since 1964. The principal requirements of the
species are trees to nest in and small birds to feed upon. It is catholic
in both nest,ing and feeding habits. While there is some evidence to
119
their relevance is slight, than on consideration of relevant circumstantial
evidence derived from field observations. The limitations and value of
circumstantial evidence in interpreting the effects of pesticides is illustrated by the following examples which concern the possible effects of
pesticides on three avian species in Britain.
1. The chaffinch (Fringilla coelebs) has probably declined considerably
in Britain during the last twenty years. Many ascribe this decline to
the use of organochlorine insecticides.
In ordinary farmland chaffinches are largely dependent on hedgerows and small woods. Recent studies by Moore, Hooper and Davis (in
press) show that hedgerow destruction has occurred throughout Britain,
and in at least one area 70% of total hedge length was destroyed during
the last twenty years. In this area a 70% reduction in habitat would
have led to a comparable reduction in the number of chaffinches quite
irrespective of other factors. In this case, where habitat reduction must
have caused a population reduction, it would be wrong to conclude that
the decline of the chaffinch was solely or principally due to pesticides.
Pesticides may have been a contributing factor, but in this case it
would be quite impossible to evaluate the different factors from the
data available.
2. The red-backed shrike (Lanius collurio) has declined catastrophically in Britain since 1940 (Peakall, 1962). From being a common bird
there are now probably only about 100 pairs left in the country. Its
decline has been ascribed to the use of insecticides.
Records show that the decline of the red-backed shrike started long
before 1940. It is also occurring in countries in which organochlorine
insecticides are relatively little used. As with the chaffinch, organochlorine insecticides may well be affecting this species, but it is quite
certain that other factors are also involved and again no evaluation of
the importance of the pesticide factor is possible on existing evidence.
3. The sparrow-hawk (Accipiter nisus) has declined catastrophically
in much of England during the last 10-15 years (Prestt, 1965 and
unpublished). Its decline has been attributed to increased game preservation, destruction of habitat, increased road traffic, unexplained
cyclical changes and organochlorine insecticides.
There is evidence to suggest that game preservation increased immediately after the war, but none to suggest that there was a further
increase in the rnid-l950s, when the sparrow-hawk began to die out.
There is much evidence to show that before the war this species survived
well in areas in which there was intensive local persecution. It has
been legally protected since 1964. The principal requirements of the
species are trees to nest in and small birds to feed upon. It is catholic
in both nest,ing and feeding habits. While there is some evidence to