The Effects of Raptor Predation on Grey Partridges Perdix perdix.

Abstract

The effect of raptors on the survival rates and compensatory anti-predation behaviours of grey partridges was studied across 20 sites in England. The study aimed to provide one of the first empirical examples of how individual behavioural decisions determine predation risk over a sufficiently large area so that the effects of variation in predation risk across populations could be tested. The study also involved a declining species valued as a game quarry, where raptor predation may be a causal factor: this study therefore aimed to determine if predation risk was sufficient to limit grey partridge populations, and if so, how its effects might be mitigated.
The starting point of the study was a long-term study site where declining partridge populations were found to be negatively correlated with the presence of raptors. Although 12% of grey partridge overwinter losses were to raptors and in the hypothetical absence of shooting, these losses were calculated as 17% of the autumn stock, shoot bag data showed that areas where grey partridges declined rapidly to extinction were areas of high shooting pressure that exceeded the maximum sustainable yield. When shooting pressure was restricted or abandoned, grey partridge decline was arrested, even in the presence of raptors. In order to draw more general conclusions about the effects of raptors on grey partridge populations a total of 20 separate sites that varied in raptor densities were then studied and 150 partridges were radio-tagged to measure overwinter survival rates, habitat use and anti-predation behaviour. Data from radio-tagged partridges showed that 18% (N = 150) were killed by raptors, adjusted for 30% fox scavenging. The mean overwinter survival rate from tags censoring signals lost, tags failed and stress deaths was 0.50 (s.e. = 0.06). The mean survival rate calculated from counts was 0.55 (s.e. = 0.04). Variation in the kill rate of bird prey including grey partridges was explained by sparrowhawk density across 20 sites. Partridge mortality to raptors was linearly density dependent and a simulation model suggested that raptor predation was only likely to cause a significant proportion of mortality below a very low density of 5 birds/km². Four sources of data showed that partridge mortality to raptors occurred mainly in the late winter, suggesting that the investigation of raptor predation effects on habitat use and anti-predator behaviour should focus on the change in size of social unit from coveys to pairs that occurred in late January. Choices in partridge anti-predator behaviour were found to have consequences for survival. Partridges selected the same micro-habitats in the covey and pair periods despite strong site effects. They used taller cover much more frequently relative to its availability in both periods. Artificially provided game cover was only 1% of available habitat, but was used seven times more than its proportional availability. The principal difference between covey and pair periods lay in a two-fold increase in the use of tall cover, mainly hedgerows. In the covey period, partridges were at higher density, with smaller home-range sizes at sites that had a greater hedgerow density and correspondingly smaller fields. In the pair period, grey partridge density best explained variation in home-range size. Emigration appeared minimal at the range of densities studied here. Overall, partridges dispersed further when at low densities. Partridges responded to increased raptor predation risk by increasing vigilance. Being in a larger group allowed vigilance to be reduced, and gave more time for foraging . Raptor predation risk was also mediated by habitat in two main ways. Firstly, partridges used cover more often at sites that had higher levels of predation risk. Second, after pairs were formed, partridges could be less vigilant when further from edges. Generally, partridges increased vigilance with vegetation height within the range of heights at which this could be observed (0-30 cm), suggesting that vegetation hindered ability to detect attacking raptors at least up to this threshold. Survival was higher in larger group sizes and higher for pairs at sites where they could remain further from the edge. Partridges could use shorter vegetation more at sites where survival was higher, because risk of predation was lower. The change in partridge vigilance levels between covey and pair periods was a consequence of group size. Because pairs' use of edge cover (for nesting) was twice that of coveys and partridges were in small group sizes (pairs) predation risk was higher in the pair period and partridges could probably not increase vigilance sufficiently high enough to compensate. This may explain the reason for most raptor losses occurring during late winter after pairs had formed. The study shows clearly that grey partridges are well adapted to minimising predation risk under variable raptor threat and that their populations are not greatly affected by it, but this depends crucially on availability of habitat and reasonably high starting densities of partridges.