STUDIES ON THE CEREAL ECOSYSTEM
183
Taken together we conclude that all the circumstantial evidence points
to very high predator pressure on partridges in area (a) over the three
years 1970-1972.
Although there have been no directly comparable studies, several
investigations have demonstrated that predator control can increase
the productivity of gamebirds and wildfowl (Bump et al., 1947;
Chessness et al., 1968; Balser et al., 1968; and others). Frank (1971)
found that the removal of 0.1 1 predators per ha per year (Mustela spp.,
42%; carrion crow and magpie, 30%; feral cat, 22%; fox, 6%) doubled
the bag of grey partridges in a flat treeless area of arable land in north
Germany. Other species, especially the pheasant and hare, also
benefited from the predator control.
7. Conclusions
Direct quantitative evidence of significant losses of partridge chicks
to predators, even in area (a), is lacking, but we consider that we have
accumulated sufficient evidence to recognize the probable impact of
predators in certain areas. For example, the chick survival rates in two
parts of our North Yorkshire study area were very different, even though
the areas were within the same farm. These differences were not due to
variations in insect availability or to gamebird disease but did reflect
differences in the degree of predator control in each of the four years
studied (Fig. 20).
During the period 1962-1969 the rate of change of unmanaged (in
most cases) grey partridge populations, as measured by the British
Trust for Ornithology Common Birds Census, was - 13% per annum.
The comparable estimate for managed populations, estimated in the
National Game Census, is - lay0, i.e. virtually the same. Thus, the
factors causing the decline, which began in 1953-1954 (Potts, 1970a),
are operating independent of traditional management, such as predator
control, and of density, since the density of the breeding stock in the
managed population is four to five times that of the unmanaged. In
view of the apparent importance of predation it seems reasonable to
conclude that the low density of unmanaged populations is due
primarily to a higher predator pressure. In many areas it may be that
predator control has become more important since myxomatosis.
Therefore, although the decline of the grey partridge was primarily due
to a shortage of insect food in cereal crops, an additive effect of predation
seems likely (and further evidence for this conclusion was obtained in
1973). Further work will show the relative importance of food shortage
and predation.
It appears at present that predation is exaggerated by shortage of
food, especially in poor weather (see also Potts, 1973).
183
Taken together we conclude that all the circumstantial evidence points
to very high predator pressure on partridges in area (a) over the three
years 1970-1972.
Although there have been no directly comparable studies, several
investigations have demonstrated that predator control can increase
the productivity of gamebirds and wildfowl (Bump et al., 1947;
Chessness et al., 1968; Balser et al., 1968; and others). Frank (1971)
found that the removal of 0.1 1 predators per ha per year (Mustela spp.,
42%; carrion crow and magpie, 30%; feral cat, 22%; fox, 6%) doubled
the bag of grey partridges in a flat treeless area of arable land in north
Germany. Other species, especially the pheasant and hare, also
benefited from the predator control.
7. Conclusions
Direct quantitative evidence of significant losses of partridge chicks
to predators, even in area (a), is lacking, but we consider that we have
accumulated sufficient evidence to recognize the probable impact of
predators in certain areas. For example, the chick survival rates in two
parts of our North Yorkshire study area were very different, even though
the areas were within the same farm. These differences were not due to
variations in insect availability or to gamebird disease but did reflect
differences in the degree of predator control in each of the four years
studied (Fig. 20).
During the period 1962-1969 the rate of change of unmanaged (in
most cases) grey partridge populations, as measured by the British
Trust for Ornithology Common Birds Census, was - 13% per annum.
The comparable estimate for managed populations, estimated in the
National Game Census, is - lay0, i.e. virtually the same. Thus, the
factors causing the decline, which began in 1953-1954 (Potts, 1970a),
are operating independent of traditional management, such as predator
control, and of density, since the density of the breeding stock in the
managed population is four to five times that of the unmanaged. In
view of the apparent importance of predation it seems reasonable to
conclude that the low density of unmanaged populations is due
primarily to a higher predator pressure. In many areas it may be that
predator control has become more important since myxomatosis.
Therefore, although the decline of the grey partridge was primarily due
to a shortage of insect food in cereal crops, an additive effect of predation
seems likely (and further evidence for this conclusion was obtained in
1973). Further work will show the relative importance of food shortage
and predation.
It appears at present that predation is exaggerated by shortage of
food, especially in poor weather (see also Potts, 1973).