STUDIES ON THE UEREAL EUOSYSTEM
166
were chosen so as to minimize “edge effects”-errors inherent in
population counts on farmland.
The methods of calculating the chick survival rates of the grey
partridge from the August counts have been described in detail (Potts,
1973). Mean survival rates have been plotted over the study area for
the five years 1968-1972 (see Fig. 18). There are clear spatial patterns
in partridge chick survival, and these tend to persist from year to year.
The survival rates of the redlegged partridge chick cannot be calculated in the same way, because the female often goes on to complete a
second clutch. Both sexes can then incubate simultaneously (Middleton
and Huband, 1966). However, the overall productivity of the two
species of partridge follows the same geographical pattern in the
study area. The relationship between the number of young produced
per pair of each species seen in March-April is given by the equation
y = 0.73z+ 0.31
(ra = 0.79, P c 0.001)
where y = the number of young produced by each pair of redlegged
partridge and z = the number of young produced by each pair of grey
partridge.
Most of the variation in grey partridge chick survival between and
within the five years could clearly be ascribed to the effect of changes
in the availability of insect food. However, two negative anomalies,
at area (a) (see Plate 3) and at area (b), (see Fig. 18), could not be
explained by differences in the chick food supply. Weather could
readily be eliminated over such short distances; there were no hailstorms or other meteorological phenomena which could possibly account
for the anomalies. There was no correlation with the density of the
breeding stock and none with structural or botanical features of the
habitat. I n fact, area (a) used to be widely known for the quantity and
quality of wild partridge shoots and even now appears to be highly
favourable partridge habitat.
Pesticides were comprehensively monitored but there were no
significant direct effects of any of these on partridge chicks in this
study area (Potts, 1973). Moreover, areas (a) and (b) were not atypical
in this respect.
The gamebird disease known as gapes, which is caused by the
parasitic nematode Syngamua tracheae, was known to be present in the
study area. Partridges which appeared to be unhealthy were therefore
collected whenever possible. Post-mortems were carried out by the
Game Conservancy pathologist. Each year the partridges shot in
October were examined for Syngamus on a drive-by-drive basis (a
drive covers about 100 ha). The percentage occurrence of Syngamus in
the trachea was then mapped and found to be four times higher in
166
were chosen so as to minimize “edge effects”-errors inherent in
population counts on farmland.
The methods of calculating the chick survival rates of the grey
partridge from the August counts have been described in detail (Potts,
1973). Mean survival rates have been plotted over the study area for
the five years 1968-1972 (see Fig. 18). There are clear spatial patterns
in partridge chick survival, and these tend to persist from year to year.
The survival rates of the redlegged partridge chick cannot be calculated in the same way, because the female often goes on to complete a
second clutch. Both sexes can then incubate simultaneously (Middleton
and Huband, 1966). However, the overall productivity of the two
species of partridge follows the same geographical pattern in the
study area. The relationship between the number of young produced
per pair of each species seen in March-April is given by the equation
y = 0.73z+ 0.31
(ra = 0.79, P c 0.001)
where y = the number of young produced by each pair of redlegged
partridge and z = the number of young produced by each pair of grey
partridge.
Most of the variation in grey partridge chick survival between and
within the five years could clearly be ascribed to the effect of changes
in the availability of insect food. However, two negative anomalies,
at area (a) (see Plate 3) and at area (b), (see Fig. 18), could not be
explained by differences in the chick food supply. Weather could
readily be eliminated over such short distances; there were no hailstorms or other meteorological phenomena which could possibly account
for the anomalies. There was no correlation with the density of the
breeding stock and none with structural or botanical features of the
habitat. I n fact, area (a) used to be widely known for the quantity and
quality of wild partridge shoots and even now appears to be highly
favourable partridge habitat.
Pesticides were comprehensively monitored but there were no
significant direct effects of any of these on partridge chicks in this
study area (Potts, 1973). Moreover, areas (a) and (b) were not atypical
in this respect.
The gamebird disease known as gapes, which is caused by the
parasitic nematode Syngamua tracheae, was known to be present in the
study area. Partridges which appeared to be unhealthy were therefore
collected whenever possible. Post-mortems were carried out by the
Game Conservancy pathologist. Each year the partridges shot in
October were examined for Syngamus on a drive-by-drive basis (a
drive covers about 100 ha). The percentage occurrence of Syngamus in
the trachea was then mapped and found to be four times higher in