Stephen M. Redpath

Birds of prey as limiting factors of gamebird populations in Europe: a review

Whether predators can limit their prey has been a topic of scientific debate for decades. Traditionally it was believed that predators take only wounded, sick, old or otherwise low‐quality individuals, and thus have little impact on prey populations. However, there is increasing evidence that, at least under certain circumstances, vertebrate predators may indeed limit prey numbers. This potential role of predators as limiting factors of prey populations has created conflicts between predators and human hunters, because the hunters may see predators as competitors for the same resources. A particularly acute conflict has emerged over the past few decades between gamebird hunters and birds of prey in Europe. As a part of a European‐wide research project, we reviewed literature on the relationships between birds of prey and gamebirds. We start by analysing available data on the diets of 52 European raptor and owl species. There are some 32 species, mostly specialist predators feeding on small mammals, small passerine birds or insects, which never or very rarely include game animals (e.g. hares, rabbits, gamebirds) in their diet. A second group (20 species) consists of medium‐sized and large raptors which prey on game, but for which the proportion in the diet varies temporally and spatially. Only three raptor species can have rather large proportions of gamebirds in their diet, and another seven species may utilise gamebirds locally to a great extent. We point out that the percentage of a given prey species in the diet of an avian predator does not necessarily reflect the impact of that predator on densities of prey populations. Next, we summarise available data on the numerical responses of avian predators to changing gamebird numbers. In half of these studies, no numerical response was found, while in the remainder a response was detected such that either raptor density or breeding success increased with density of gamebirds. Data on the functional responses of raptors were scarce. Most studies of the interaction between raptors and gamebird populations give some estimate of the predation rate (per cent of prey population taken by predator), but less often do they evaluate the subsequent reduction in the pre‐harvest population or the potential limiting effect on breeding numbers. The few existing studies indicate that, under certain conditions, raptor predation may limit gamebird populations and reduce gamebird harvests. However, the number and extent of such studies are too modest to draw firm conclusions. Furthermore, their geographical bias to northern Europe, where predator–prey communities are typically simpler than in the south, precludes extrapolation to more diverse southern European ecosystems. There is an urgent need to develop further studies, particularly in southern Europe, to determine the functional and numerical responses of raptors to gamebird populations in species and environments other than those already evaluated in existing studies. Furthermore, additional field experiments are needed in which raptor and possibly also mammalian predator numbers are manipulated on a sufficiently large spatial and temporal scale. Other aspects that have been little studied are the role of predation by the non‐breeding part of the raptor population, or floaters, on the breeding success and survival of gamebirds, as well as the effect of intra‐guild predation. Finally there is a need for further research on practical methods to reduce raptor predation on gamebirds and thus reduce conflict between raptor conservation and gamebird management.

ASSESSING RAPTOR DIET: COMPARING PELLETS, PREY REMAINS, AND OBSERVATIONAL DATA AT HEN HARRIER NESTS

Abstract We compared techniques to assess diet at 17 Hen Harrier (Circus cyaneus) nests. Diet was measured directly from observations and compared to estimates from pellets, prey remains and a combination of pellets and remains. For data pooled across nests, pellets over-represented mammalian prey and under-represented avian prey. Prey remains over-represented large prey and under-represented small prey. Combining pellet and remains data did not eliminate these biases. Pellets gave higher diversity values than direct observations and detected more small prey species. For data analyzed on a nest by nest basis, estimates from pellets were significantly related to estimates from direct observations for three prey types by frequency and all types by biomass. These linear relationships were used to predict frequencies and biomass of prey types in the observed diet at five new nests. Our findings suggest that pellets are useful for estimates of prey diversity and as an index of the frequency of certain prey types in the diet, but direct observations are necessary to help quantify the biases inherent in diet estimates.

The Functional Response of a Generalist Predator

Background Predators can have profound impacts on the dynamics of their prey that depend on how predator consumption is affected by prey density (the predators functional response). Consumption by a generalist predator is expected to depend on the densities of all its major prey species (its multispecies functional response, or MSFR), but most studies of generalists have focussed on their functional response to only one prey species. Methodology and principal findings Using Bayesian methods, we fit an MSFR to field data from an avian predator (the hen harrier Circus cyaneus) feeding on three different prey species. We use a simple graphical approach to show that ignoring the effects of alternative prey can give a misleading impression of the predators effect on the prey of interest. For example, in our system, a “predator pit” for one prey species only occurs when the availability of other prey species is low. Conclusions and significance The Bayesian approach is effective in fitting the MSFR model to field data. It allows flexibility in modelling over-dispersion, incorporates additional biological information into the parameter priors, and generates estimates of uncertainty in the models predictions. These features of robustness and data efficiency make our approach ideal for the study of long-lived predators, for which data may be sparse and management/conservation priorities pressing.

Tilting at wildlife: reconsidering human–wildlife conflict

Conflicts between people over wildlife are widespread and damaging to both the wildlife and people involved. Such issues are often termed human–wildlife conflicts. We argue that this term is misleading and may exacerbate the problems and hinder resolution. A review of 100 recent articles on human–wildlife conflicts reveals that 97 were between conservation and other human activities, particularly those associated with livelihoods. We suggest that we should distinguish between human–wildlife impacts and human–human conflicts and be explicit about the different interests involved in conflict. Those representing conservation interests should not only seek technical solutions to deal with the impacts but also consider their role and objectives, and focus on strategies likely to deliver long-term solutions for the benefit of biodiversity and the people involved.

Interactions between intrinsic and extrinsic mechanisms in a cyclic species: testosterone increases parasite infection in red grouse

Field studies of mechanisms involved in population regulation have tended to focus on the roles of either intrinsic or extrinsic factors, but these are rarely mutually exclusive and their interactions can be crucial in determining dynamics. Experiments on red grouse Lagopus lagopus scoticus have shown that population instability can be caused both by the effects of a parasitic nematode, Trichostrongylus tenuis, on host production or by changes in testosterone influencing aggressive behaviour and recruitment. We experimentally tested for an interaction between testosterone and T. tenuis in free-living male grouse. A total of 123 grouse were caught in autumn, treated with an anthelmintic to remove parasites, and then given either testosterone or empty, control, implants. After one month grouse were re-infected with a standard dose of parasites. We show that males with increased testosterone levels had greater parasite intensities than controls after one year. We discuss possible physiological and behavioural mechanisms linking testosterone and increased parasite intensity, and the implications for our understanding of complex, unstable population dynamics.

Low intensity, mixed livestock grazing improves the breeding abundance of a common insectivorous passerine

Abstract Livestock grazing is a major driver of ecosystem change and has been associated with significant declines in various bird species in Britain and worldwide. However, there is little experimental evidence to show how grazing affects bird populations. We manipulated livestock densities in a replicated field experiment and found that mixed sheep and cattle grazing, at low intensity, improved the breeding abundance of a common upland passerine, the meadow pipit Anthus pratensis, after two years. Plots stocked with sheep alone (at high or low density) or not stocked at all held fewer pipit territories. Despite a year-on-year decline in pairs of meadow pipits in intensively grazed plots, we found no effect of sheep number on breeding abundance. Our results support the hypothesis that mixed species of herbivores generate greater heterogeneity in vegetation structure, which modifies prey availability, resulting in a greater abundance of birds. The results of our study should inform the management of grassland areas and enhance the abundance of some bird species, particularly in areas that have seen significant shifts from mixed livestock grazing to grazing dominated by single species of animals.

Interactions between population processes in a cyclic species: parasites reduce autumn territorial behaviour of male red grouse

The causes of population cycles fascinate and perplex ecologist. Most work have focused on single processes, whether extrinsic or intrinsic, more rarely on how different processes might interact to cause or mould the unstable population dynamics. In red grouse (Lagopus lagopus scoticus), two causal mechanisms have been supported: territorial behaviour (changes in autumn aggressiveness) and parasites (parasite induced reduction in fecundity). Here, we report on how these two regulatory processes might interact, by testing whether the parasite suspected to cause the grouse cycles, the nematode Trichostrongylus tenuis, reduces male autumn territorial behaviour. We either treated males with an anthelmintic, to remove parasites (dosed or D-males), or challenged them with infective T. tenuis larvae, to increase parasite intensity (challenged or C-males). We first show that dosing was effective in removing T. tenuis parasites, while parasite intensities increased in challenged birds during the autumn. Because old males initially had more parasites than young males, the treatments generated greater differences in parasite intensity in old than in young males. We also show that various aspects of territorial behaviour (increase in testosterone-dependent comb size in autumn, territorial call rate, likelihood of winning territorial interactions and over-winter survival) were significantly higher in dosed than in challenged males, but in old birds only. Our data thus supported the hypothesis that parasites reduce male aggressiveness during the autumn territorial contests, and could thereby influence recruitment. Our results also highlight that the territorial behaviour of young males, which have fewer parasites, is not as limited by parasites as that of old, previously territorial males. We discuss the implications of these findings for our understanding of the processes regulating red grouse populations and causing their complex, unstable population dynamics.

Separating behavioral and physiological mechanisms in testosterone-mediated trade-offs.

Testosterone often mediates trade‐offs between reproduction and other life‐history traits, which are usually investigated using testosterone implants. However, this approach does not distinguish between the physiological and behavioral effects of testosterone. We studied a wild game bird, the red grouse Lagopus lagopus scoticus, and took a new approach to investigate mechanisms linking elevated testosterone to increased parasite intensity. We caught males in autumn, removed their parasites, implanted them with the antiandrogen flutamide in combination with an aromatase inhibitor (FA males) or with empty implants (control males), and challenged them with parasites. The FA treatment increased testosterone concentration and physiological stress, but without enhancing testosterone‐dependent behaviors, because testosterone receptors were blocked. FA males ended up with more parasites than the control males the following autumn, an effect similar to that of a testosterone treatment reported elsewhere. However, and unlike the testosterone treatment, the FA treatment did not affect home range, pairing, or breeding success. The results supported a physiological mechanism (increased susceptibility) linking elevated testosterone and increased parasite intensity. The FA treatment provided a new way of investigating testosterone‐mediated trade‐offs whereby testosterone concentration was increased while the effects on behavior were blocked, resulting in physiological costs without phenotypic benefits.

The diet and breeding density of Common Buzzards Buteo buteo in relation to indices of prey abundance

The diet and breeding density of 19 pairs of Common Buzzard Buteo buteo¸ were studied in relation to indices of lagomorph and vole abundance during June July 1993 in a range of habitats in southern Scotland. Lagomorphs, voles and birds formed over 70% of the food items analysed from prey remains and pellets collected at nest sites. Indices of prey abundance differed significantly between habitat types. The percentage of lagomorph in Buzzard diet in different localities was significantly correlated with the index of lagomorph abundance; no such correlation was found between the percentage of vole in Buzzard diet and an index of vole abundance. The mean nearest-neighbour distance between Buzzard nests was 1.9 km. There was a significant negative correlation between nearest-neighbour distance and lagomorph abundance. It appears that lagomorphs were the primary prey which influenced the diet and breeding density of Buzzards in the study area.

Livestock grazing affects the egg size of an insectivorous passerine.

Livestock grazing is a major driver of ecosystem change, and has been associated with significant declines in various bird species worldwide. In Britain, there is particular concern that severe grazing pressure is deleteriously affecting vegetation and birds in upland regions. However, the mechanism by which grazing affects birds is unclear. Here, we report for the first time, to our knowledge, that sheep grazing pressure affects the egg size of a common upland passerine: the meadow pipit Anthus pratensis. We manipulated sheep stocking densities in a replicated field experiment, and found that plots with the highest stocking density contained nests with the smallest eggs, and that plots with low stocking density contained nests with the largest eggs. However, eggs laid in ungrazed plots were also small, suggesting that either too many sheep or their removal from upland areas might have a detrimental effect on pipit egg size. We found no significant effect on fledging success but the reduced post-fledging survival of young from smaller eggs, as seen in other studies, could partly explain declines in upland birds.