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N. W. MOORE
suggest that a number of the prey species of the sparrow-hawk have
become rare in the last ten years, there is no evidence to suggest that
the total bird fauna has been much reduced. In parts of Britain the
sparrow-hawk thrives in country where birds are much less abundant
than in the agricultural districts from which it has disappeared. Despite
a great reduction in hedgerow timber very little of England is completely
treeless: the species occurred in the Fens in the 1940s.
Road traffic undoubtedly kills more birds today than formerly, but
the sparrow-hawk has rarely been found dead on roads. Its habit of
flying parallel and close to hedges makes it particularly unlikely to be
a frequent road casualty.
Some species of birds do show cyclical changes but there is no
evidence to suggest that the sparrow-hawk is one of them. A decline
related to changes in climate should show latitudinal differences, but
no such pattern is shown by the present distribution of sparrow-hawks
which still thrive in relatively uncultivated areas of the north and the
south (Prestt, personal communication).
The decline of the sparrow-hawk has coincided with the introduction
of aldrin and dieldrin and an increase in the use of DDT. It has mainly
declined in those areas where these chemicals have been most used.
Further, organochlorine insecticide residues in sparrow-hawks have
been higher on average than those found in any other bird for which
we have an adequate sample. When all these types of evidence are
taken into account the hypothesis that the decline of the sparrow-hawk
is causally linked with the introduction of the organochlorine insecticides is well supported.
Other birds living in the same areas as sparrow-hawks but containing
much lower residues can be used as a crude control: none of these have
shown the catastrophic decline of the sparrow-hawk except the kestrel
(Fako tinnunculus). The decline of this species has not been so great.
Its average residue is much higher than those of most species but not
quite so high as the sparrow-hawk (Moore, 1965b). If there is a considerable decrease in the use of organochlorine insecticides in the future
both these species should show considerable increases if the pesticide
hypothesis is correct. Rigorously planned field observations can have
some of the features of an experiment.
It is concluded that circumstantial evidence, if extensive and used
critically, can be valuable in assessing effects of a pesticide on a species;
the need to use all types of available evidence in assessing the effects of
pesticides should be obvious. These matters are both theoretically end
practically important because differences and changes in scientific
opinion about the control of pests and pesticides have profound effect
on plant and animal communities in the field.
N. W. MOORE
suggest that a number of the prey species of the sparrow-hawk have
become rare in the last ten years, there is no evidence to suggest that
the total bird fauna has been much reduced. In parts of Britain the
sparrow-hawk thrives in country where birds are much less abundant
than in the agricultural districts from which it has disappeared. Despite
a great reduction in hedgerow timber very little of England is completely
treeless: the species occurred in the Fens in the 1940s.
Road traffic undoubtedly kills more birds today than formerly, but
the sparrow-hawk has rarely been found dead on roads. Its habit of
flying parallel and close to hedges makes it particularly unlikely to be
a frequent road casualty.
Some species of birds do show cyclical changes but there is no
evidence to suggest that the sparrow-hawk is one of them. A decline
related to changes in climate should show latitudinal differences, but
no such pattern is shown by the present distribution of sparrow-hawks
which still thrive in relatively uncultivated areas of the north and the
south (Prestt, personal communication).
The decline of the sparrow-hawk has coincided with the introduction
of aldrin and dieldrin and an increase in the use of DDT. It has mainly
declined in those areas where these chemicals have been most used.
Further, organochlorine insecticide residues in sparrow-hawks have
been higher on average than those found in any other bird for which
we have an adequate sample. When all these types of evidence are
taken into account the hypothesis that the decline of the sparrow-hawk
is causally linked with the introduction of the organochlorine insecticides is well supported.
Other birds living in the same areas as sparrow-hawks but containing
much lower residues can be used as a crude control: none of these have
shown the catastrophic decline of the sparrow-hawk except the kestrel
(Fako tinnunculus). The decline of this species has not been so great.
Its average residue is much higher than those of most species but not
quite so high as the sparrow-hawk (Moore, 1965b). If there is a considerable decrease in the use of organochlorine insecticides in the future
both these species should show considerable increases if the pesticide
hypothesis is correct. Rigorously planned field observations can have
some of the features of an experiment.
It is concluded that circumstantial evidence, if extensive and used
critically, can be valuable in assessing effects of a pesticide on a species;
the need to use all types of available evidence in assessing the effects of
pesticides should be obvious. These matters are both theoretically end
practically important because differences and changes in scientific
opinion about the control of pests and pesticides have profound effect
on plant and animal communities in the field.