The Partridge Survival Project

The grey partridge is (or was) one of the most important of the English game birds. Bag records from long-established estates, some going back for 150 years, show that populations of grey partridges reached their greatest densities during the heyday of traditional game management by intensive predator control. The early records of the National Game Census, for the peace-time years between 1933 and 1960, showed that about 18 to 20 birds on average were shot on every square kilometer of the estates participating in the recording scheme. Then there was a sudden population crash, so intense that the mean bag for 1971-1979 was only 3.7 birds per km2. The reasons for this event were sought in a series of studies of partridge ecology and especially by the Game Conservancy's Partridge Survival Project (Potts 1979, 1986).

Among the prime suspects were, of course, the predators. Among nearly 14,000 partridge nests whose fates were known, 1,993 cases of nests lost to predators (36%) were attributed to foxes (Middleton 1967). Stoats and common weasels appeared to be minor players by comparison (7% of the losses were attributed to stoats, and 0.1% to common weasels), even though many keepers can tell stories about stoats cleaning out eggs from partridge nests.

For example, on one estate in Norfolk in 1944, all the losses in one small area (Figure 12.3) were attributed to the work of a single stoat that was seen at one of the nests. On open farmland, stoats usually move along the field boundaries, which effectively channels predator and prey together (Chapter 8). Stoats can also steal whole clutches of eggs and carry them away to cache for later. Cringle (1968), helping two keepers to dig out rabbits in autumn, found a stoat's larder about a foot underground, comprising two or three dozen partridge eggs packed neatly at the end of the burrow. The keepers knew that the nests that had been robbed were all along the hedgebank up to 100 m away, so the eggs had been carried some distance, but not one of them was damaged. A year later, Cringle saw a stoat carrying a tern's egg by walking on three legs and holding the egg against its chin with the fourth.

Studies of all the predators that kill partridges, and of the presumably serious consequences, were regarded as an important part of the Partridge Survival Project right from the beginning. Potts began the work convinced that this presumption was wrong, and set up the study expecting it to show that predation did not control the population density of partridges. Over an area of 13.1 km2 of the South Downs at North Farm, Sussex, Potts and his team monitored a continuous effort to control all predators of partridges other than the protected raptors. The main predators of partridge eggs, the carrion crows and magpies, were practically eliminated; foxes were removed at an average rate of 3.2 adults per km2 per year, stoats at 3.7 per km2 per year, and feral cats at about 1 per km2 per year (Potts 1980). On other parts of the study area, predators were not systematically controlled.

The team then monitored the reproductive performance of partridges in the areas with and without predators. They also measured various other things, such as the extent of nesting cover and yearly variations in food supplies and in the weather, which could also affect the birds' success.

Figure 12.3 Stoats often raid gamebird nests and eat or remove the eggs. Inset: a single stoat was believed to have been responsible for all the partridge nests lost (filled circles) in one small area of an estate. (Map from Tapper et al. 1982.)

This effort was systematic and intensive enough to achieve considerable success in removing predators, at least temporarily. For example, although on this estate the traps for stoats were set all year round, the most determined effort was made in spring, when the nests of wild game bird are most vulnerable. The records showed (Figure 12.4) a steep decrease in the number of stoats killed per 100 traps over the first 6 months of the year. The team concluded from this that the resident population was largely removed each spring, although it was replaced later in the year as the season's crop of young stoats dispersed.

Tapper (1976) collected 151 common weasels and 46 stoats from the study area during the critical months of May, June, and July of the years 1971-1974. Of all the items he identified in guts, game birds comprised 2.1% and 6.8% of items eaten by common weasels and stoats, respectively. Tapper calculated, from the densities of chicks in his study area (then about 94 per 100 ha) and from the literature on food consumption and density of weasels (see Tables 2.1 and 10.2), that predation could be an important cause of mortality in game bird chicks in cn 8

Jan Feb Mar Apr May Jun

Figure 12.4 The decline in average number of stoats caught per month in the intensive spring campaign at North Farm, Sussex, implies temporary local extinction, but it was usually reversed by immigration within a few months. (Redrawn from Tapper et al. 1982.)

Jan Feb Mar Apr May Jun

Figure 12.4 The decline in average number of stoats caught per month in the intensive spring campaign at North Farm, Sussex, implies temporary local extinction, but it was usually reversed by immigration within a few months. (Redrawn from Tapper et al. 1982.)

some years, but irrelevant in other years. The difference between years was due to insects, not weasels.

Partridge chicks feed entirely on insects for the first 2 to 3 weeks of their lives. More than 70% of the chicks that fail to reach adult size die during this early period, and this mortality is related both to the abundance of insects and to the weather. In a good year for insects, the young partridges that are not killed by predators are likely to survive, because they have enough food at this critical stage. By contrast, in a bad year for insects, it does not matter if most of the chicks are killed by predators, for they probably would not have survived anyway. Hence, the effects of predation are not the same every year, and calculations of the value of removing predators have to take into account umpteen other factors.

Potts identified three sources of density-dependent losses (those whose action is directly related to the number of animals present): (1) the density of breeding female partridges in spring, (2) the proportion of pairs with a brood in early summer, and (3) the proportion of the total population shot in autumn. The first and, especially, the second of these losses can be reduced by effective removal of predators. Losses of adult birds in early spring are not too serious unless very severe, because those killed can be replaced by immigrants during the partridges' annual competition for territories. By contrast, losses of sitting hens are crucial because by that stage they cannot be replaced. If many sitting hens are killed, productivity for the season will be seriously reduced.

The mortality of the chicks is another important loss, but it is density independent, because it is related to the abundance of insects and to the weather, rather than to the density of partridges. Its action does not slacken as the num ber of partridges declines; it acts blindly, and if it continues at a high rate, nothing can stop it driving the population to extinction. Removal of predators that eat chicks does not improve the survival of chicks in poor years, although density-dependent processes acting later can, in some years, compensate for a poor season for production of chicks, especially if predators are removed.

After 10 years of fieldwork, plus endless patient study of an enormous volume of contemporary and historical data on partridges, Potts set up one of the earliest of the computer models, of the type that we now take for granted, to make sense of these complicated interactions. He deduced that the main reason why grey partridges have become so scarce in Sussex was the high mortality of the chicks, due to the destruction of their food supplies by agricultural pesticides. The only hope of restoring numbers, he suggested, is to reduce the effects of pesticide spraying at the field margins. Nevertheless, this policy would be effective only if nesting success was improved at the same time. Why? Because the effects of pesticides were accelerated by predation during the nesting season, which also strongly affects the density of partridges, especially the number of surplus birds available for shooting. Time has shown that Potts' diagnosis was right.

The long-term population decrease in partridges in Britain also coincides with an 80% reduction in gamekeepering since 1911 and a 40% loss of hedges since the 1930s, both of which greatly increase the hazards of nesting for the sitting female partridges. Which of these losses might have disadvantaged the partridges most?

Ever since the work of the influential American ecologist Paul Errington in the 1930s (Errington 1963), game managers have believed that the population density of most animals is set by the extent of their preferred habitat (Chapter 7). Errington was convinced that predation is more often compensatory (substituting for other causes of mortality) than additive (adding to other causes). When predation is compensatory it is relatively unimportant except to "the doomed surplus" individuals unable to find a secure home base. The logical implication of this line of reasoning is that habitat improvement is a much more important means of increasing game than predator control, or even the only means worthwhile. So, Potts' most interesting finding was a completely unexpected interaction between the extent of nesting cover (measured as kilometers of hedgerow per km2) and predator control. Potts argued that, although it is true that predation is not important all year round, it is very important during the nesting season.

Potts concluded that the most urgent and effective way to conserve partridges is to curtail the spraying of toxic chemicals at field margins, since no other measure can have more than a temporary effect if chick mortality remains high. After that, habitat improvements such as the provision of more and better nesting cover will increase the number of partridges only if predators are controlled as well, at least in the nesting season (Figure 12.5).

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Figure 12.5 G.R. Potts' analysis predicted that the highest numbers of grey partridges in autumn (the shooting season) would be found on estates where nesting cover has been augmented and predators have been removed. (Redrawn from Potts 1979.)

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Nesting Cover, km Hedgerows/km 2

Figure 12.5 G.R. Potts' analysis predicted that the highest numbers of grey partridges in autumn (the shooting season) would be found on estates where nesting cover has been augmented and predators have been removed. (Redrawn from Potts 1979.)

The effect of predator control is most decisive where nesting cover is already sufficient to support nests at a density at or above 25 nests per kilometer of hedgerow (Potts 1979). Without gamekeepering, the success of these nests is only about 10%, mostly because the more sitting birds there are, the more easily can predators find and kill them. Gamekeepering in addition to plenty of cover increases nesting success to between 60% and 70%, and results in up to three times more birds in the autumn population. In nature, these extra birds would be the 'doomed surplus' which would be removed by over-wintering mortality or by emigration of birds next spring; but on a game estate nature is managed so as to direct the largest possible proportion of the doomed surplus into the shooters' bags.

The Sussex study area has now been monitored for over 30 years, and provides one of the longest running data sets on the cereal ecosystem in the world (Tapper 1999). The longer it goes on, the more amply it confirms Potts' early conclusions on the interaction between pesticides and predation on farmland birds. It also led to an interesting experiment, done in Wiltshire over 6 years (1984 to 1990).

This research used a classic paired plot design done on two areas of about 5 km2 each, and separated by 6 km (Tapper et al. 1996). The common predators were removed every spring for 3 years from one plot, and from the other for the next 3 years. Predator control focused mainly on foxes and corvids, but also removed stoats at an average rate of four to seven per month during each

4-month nesting season. Partridge stocks on the controlled area increased by 75% in the late summer, and were still 36% higher by the start of the following breeding season. Most of this effect was due to the removal of foxes, even though they depended mainly on rabbits and took partridges only incidentally. Stoats played a minor part in this drama, as elsewhere (Chapter 7), but their contribution was still counted.

Common weasels take only chicks, so their effect is short term, and often too minor to be damaging, especially during the years that voles are abundant. On the other hand, artificially reared game birds are especially vulnerable to weasels, because these little predators can run underground through mole tunnels (see Figure 8.4) and come up on the inside of supposedly predator-proof wire enclosures designed to protect the young birds.

Most gamekeepers have a fund of eloquent eyewitness accounts of this problem. For example, one keeper had been unable to prevent continuing losses of pheasant chicks from their wire-mesh rearing coops, or to find what had happened to them, until one morning when he was moving the coops. He went to fire a heap of old straw, but caught a slight movement and, instead, began to turn the heap over while his mate, a first-class shot, stood by with a gun. Under the straw was a mole's nest containing a score of dead chicks, each of which must have been carried or dragged from inside a coop, some 150 m across the grass or through a mole run. While they stood there, a common weasel made a dash for freedom, but the keeper's mate missed it with both barrels.

Stoats also destroy many eggs and chicks from pheasant nests, and they can also kill the sitting hen. Predation by stoats can therefore make a contribution to the problem in the long term, since losses of sitting birds are more directly damaging than losses of eggs or chicks. This is what makes them a pest, less serious than foxes and corvids (Tapper et al. 1996), but more serious than polecats, rats, hedgehogs (Packer & Birks 1999), and common weasels (McDonald & Harris 1999; McDonald & Birks 2003). Studies of upland game birds such as grouse and curlew have reported similar conclusions (McDonald & Murphy 2000).

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