Diet specialisation and foraging efficiency under fluctuating vole abundance: a comparison between generalist and specialist avian predators

Specialist species, using a narrow range of resources, are predicted to be more efficient when foraging on their preferred food than generalist species consuming a wider range of foods. We tested whether the foraging efficiency of the pallid harrier Circus macrourus, a vole specialist, and of sympatric Montagu's harriers C. pygargus, a closely related generalist, differed in relation to inter‐annual variations in vole abundance over five years (including two peak‐ one intermediate and two low vole abundance years). We show that the hunting parameters of pallid harriers strongly varied with vole abundance (higher encounter rates, capture rates and proportion of successful strikes in high than intermediate and low vole abundance years, respectively), whereas Montagu's harriers showed stable capture rates and hunting success (proportion of strikes that were successful), irrespective of vole abundance. Encounter rates and capture rates were higher for pallid than for Montagu's harriers when voles were abundant, but lower when voles were scarce. The hunting success of pallid harriers was also lower than that of Montagu's harriers when voles were scarce, and when they had to target alternative preys, in particular birds. Overall, estimated biomass intake rate was 40% higher for pallid harriers than for Montagu's harriers when voles were abundant, but 50% lower when voles were scarce. Our results indicate that specialists predators, like pallid harriers, which evolve specific adaptations or breeding strategies, do better when their preferred prey is abundant, but may face a cost of specialisation, being not efficient enough when their preferred prey is scarce. These results have broader implications for understanding why specialist predators are, in general, more vulnerable than generalists, and for predicting how specialists can cope with rapid environmental changes affecting the abundance or predictability of their preferred resources.     

Disentangling the effects of environmental conditions on wintering and breeding grounds on age‐specific survival rates in a trans‐Saharan migratory raptor

Migratory species are subject to environmental variability occurring on breeding and wintering grounds. Estimating the relative contribution of environmental factors experienced sequentially during breeding and wintering, and their potential interaction, to the variation of survival is crucial to predict population viability of migratory species. Here we investigated this issue for the Montagu's harrier Circus pygargus, a trans‐Saharan migrant. We analysed capture–recapture data from a 29‐year long monitoring of wing‐tagged offspring and adults at two study sites in France (Rochefort‐RO and Maine‐et‐Loire‐ML). The study period covers a climatic shift occurring in the Sahel with increasing rainfall following a period of droughts (Sahel greening). We found that harriers’ adult survival in RO (between 1988 and 2005) varied over time and was sensitive to the interaction between the amount of rainfall in the Sahel and the annual mean breeding success, two proxies of prey availability. The occurrence of adverse conditions on breeding and wintering grounds in the same year decreased survival from 0.70–0.77 to 0.48 ± 0.05. Juvenile survival in RO was slightly more sensitive to conditions in Europe than in the Sahel. Unexpectedly, lower survival rates were found in years with higher mean breeding success, suggesting compensatory density feedbacks may operate. By contrast, adult survival in ML, monitored between 1999 and 2017, was higher compared to RO (0.76 ± 0.03 versus 0.66 ± 0.02), remained constant and unaffected by any proxy of prey availability. This difference seems consistent with the fact that harriers in ML experienced better and especially less variable environmental conditions during breeding and wintering seasons compared to RO. Overall, we showed that survival of a migratory bird is sensitive to the level of variability in environmental conditions and that adverse conditions on wintering grounds can amplify the negative effects of conditions during the previous breeding season on birds’ survival.     

Intra- and interspecific agonistic behaviour in sympatric harriers during the breeding season

We analysed the temporal and sexual patterns of intra- and interspecific aggression in sympatric harriers during the breeding season, to determine the main resource defended (food, nest sites, mates) and how factors such as body size or breeding system (territorial versus colonial) influence aggressive behaviour. We predicted that if aggression is (at least partly) related to competition for food, the hen harrier, Circus cyaneus, because of its large size and territorial system, should be more aggressive (both intra- and interspecifically) than the smaller, colonial Montagu's harrier, Circus pygargus. The intraspecific aggression rate of both harriers peaked early in the season, was mainly intrasexual and increased with the number of neighbours. These patterns support the mate competition hypothesis to explain intraspecific aggression in both species. Montagu's harriers were more aggressive towards conspecifics than hen harriers. Their aggression rates towards heterospecifics were high at the start of the season then decreased, supporting the hypothesis that interspecific aggression serves primarily for nest site defence. Hen harriers showed lower interspecific aggression rates in the prelaying period and a gradual increase throughout the breeding cycle, particularly by females, who hunt around the nest for food for the nestlings. These patterns correspond to food competition, as food resources around the nest are probably more important and interspecific intrusion more costly for territorial hen harriers than for colonial Montagu's harriers. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Predator population dynamics under a cyclic prey regime: numerical responses,demographic parameters and growth rates

The consequences of cyclic fluctuations in abundance of prey species on predator continue to improve our understanding of the mechanisms behind population regulation. Among predators, vole‐eating raptors usually respond to changes in prey abundance with no apparent time‐lag and therefore contradict predictions from the predator–prey theory. In such systems, the interplay between demographic traits and population growth rate in relation to prey abundance remains poorly studied, yet it is crucial to characterize the link between ecological processes and population changes. Using a mechanistic approach, we assessed the demographic rates associated to the direct and indirect numerical responses of a specialist raptor (Montagu's harrier) to its cyclic prey (common vole), using long term data from two adjacent study sites in France. First‐year survival rates were weakly affected by vole abundance, probably due to the fact that Montagu's harriers are trans‐Saharan migrants and thus escape the vole collapse occurring in autumn–winter. Recruitment of yearling as well as breeding propensity of experienced adult females were strongly affected by vole abundance and at least partially shaped the trajectory of the breeding population. We argued that the strong density dependent signal detected in predator time series was mostly the phenomenological consequence of the positive direct numerical response of harriers to vole abundance. Accounting for this, we proposed a method to assess density dependence in predator relying on a cyclic prey. Finally, the variation in Montagu's harrier population growth rates was best explained by overwinter growth rates of the prey population and to a lesser extent by previous residual predator density.     

Long-term increase in the fecundity of hen harriers in Wales is explained by reduced human interference and warmer weather

The conservation status of the hen harrier Circus cyaneus in Britain, and its pivotal role in the raptor–grouse moor conflict, has rendered research on the factors influencing its population dynamics of crucial importance. Using over 20 years of data, we examine influences on the reproductive output of the small, but recently expanding, population of Welsh hen harriers. Productivity (fledglings per breeding attempt) has steadily increased with time, being the highest in recent years. Available evidence suggested that food availability could not account for these recent increases. Human interference (apparently, largely or entirely in the form of nest destruction by grouse moor gamekeepers) had a large influence on productivity, despite being restricted to a minority of the breeding population. This negative effect, and a positive effect of May temperature, was most influential on variation in breeding productivity. This study quantifies, for the first time, that cessation of persecution can result in a marked improvement in hen harrier reproductive output. Nest predation by red foxes Vulpes vulpes had no effect on variation in breeding productivity and there was no evidence that control of fox numbers by gamekeepers compensated for their depression of productivity through destroying harrier nests. Our analyses also indicated that gamekeepers probably destroyed an unknown number of nests before they were discovered. We argue that a low proportion of territorial females that apparently breed, in cases when clutch size and fledged brood size are high, may be an indication of persecution. The recent increase in the breeding productivity of Welsh harriers has probably been influential in the recent recovery of the Welsh harrier population and has apparently been due to a combination of cessation of human interference and warmer temperatures.     

Migrating Montagu's harriers frequently interrupt daily flights in both Europe and Africa

Time budgets are a powerful but hitherto seldom used way to study how migrants organise their bi‐annual travels. We studied daily time budgets of travelling Montagu's harriers Circus pygargus, based on GPS tracking data, in which we were particularly interested in how time budgets differ between regions and seasons, and are affected by wind. We found that Montagu's harriers used a relatively broad daily time window for travelling by starting daily travels just after sunrise and ending daily travels just before sunset. Occasionally, flights were extended into the night. Montagu's harriers frequently interrupted their daily flights for on average 1.5 h d^–1. These interruptions occurred in all regions and seasons. The tracking data during interruptions suggested two different behaviours: in 41% of all interruptions the birds were moving (presumed foraging,) and in 32% they were stationary (presumed resting; the remaining interruptions could not be classified). The interruptions for foraging indicate that Montagu's harriers have a fly‐and‐forage migration strategy (i.e. combine travelling and foraging on the same day), but the interruptions for resting illustrate that their travels comprise of more than fly‐and‐forage behaviour alone. The large number of interruptions for foraging in the Sahara Desert indicates that this region is less hostile for a migrating raptor than presumed previously. Importantly, harriers spent more time on interruptions for resting on days with stronger headwinds, suggesting that interruptions for resting serve a function of waiting for more favourable weather conditions. Daily variation in time budgets was largely explained by wind; harriers flew more hours per day, and interrupted their flights fewer hours per day, on days they experienced stronger tailwinds. In contrast, time budgets were similar between regions and seasons, suggesting that wind rather than landscape and season shape travel routines of Montagu's harriers.     

Modelling the impact of hen harrier management measures on a red grouse population in the UK

Human‐wildlife conflict is one of the greatest barriers to effective conservation. The recovery of the hen harrier Circus cyaneus in the United Kingdom has been limited due to illegal persecution, a consequence of the raptors’ predation on the economically valuable game‐bird, the red grouse Lagopus lagopus scoticus. To improve management of the system it is necessary to understand the interactions between the two species in their broader community context. We therefore developed a multi‐species model in which the life history and interactions of each of the two bird species are described through linked process models. This model was fit to population data using a Bayesian state–space framework and used to investigate the effectiveness of a conflict–mitigation technique known as diversionary feeding, in which harrier nests are provided with food in an attempt to reduce consumption of grouse chicks. To explore the utility of diversionary feeding we specified four scenarios in which 1) harriers were absent from the system, 2) there was no diversionary feeding of harriers, 3) only a portion of the harrier nests were provided with diversionary food and 4) all nests were provided with diversionary food. The results from fitting the model under the different scenarios were used to determine the strength of harriers’ impact on grouse density, as well as the effectiveness of diversionary feeding. Given the lack of information on other grouse predators and only two years of data on supplementary feeding, our results need to be implanted with caution. However, we found theoretical support for the hen harriers’ suppression of grouse cycle amplitude and average density. Furthermore, our results suggest that on grouse estates where diversionary feeding is the only active management, diversionary feeding is only marginally successful and not sufficient to mitigate the consequences of hen harrier predation on red grouse chicks.     

Field Vole Microtus agrestis abundance and Hen Harrier Circus cyaneus diet and breeding in Scotland

In many parts of the global range, voles form an important part of the diet of Hen Harriers Circus cyaneus , and breeding numbers are correlated with the abundance of these small mammals. In Scotland, however, little information is available on harrier diet in the spring and our understanding of causes of variation in harrier breeding density is complicated by human interference. In this paper we explore the relationship between Field Vole Microtus agrestis abundance and harrier spring diet, density and productivity in southern Scotland. Over three years, voles occurred on average in 67% of pellets, and 79% in years of high and intermediate vole abundance. From 1992, the number of breeding harriers increased following protection from illegal persecution. After accounting for this trend, harrier numbers correlated strongly with vole abundance. Harrier clutch size was also correlated with vole abundance. Although fledging success tended to be greater in years of vole increase than in years of vole decline, fledging success was not significantly correlated with the relative abundance of voles, or with the abundance of Meadow Pipits or Red Grouse chicks.     

Temperature and hen harrier productivity: from local mechanisms to geographical patterns

Climate is an important factor limiting demography and distribution patterns in many organisms. For species with a broad geographical distribution, the mechanism by which climate influences demography is likely to vary dramatically from one end of the range to the other. In this paper we first assess, in a Scottish population of hen harriers Circus cyaneus , how temperature and rainfall influence adult behaviour and chick mortality patterns at the nest. We then test for associations between harrier productivity and weather across Scotland, towards the northern edge of the range, and Spain, towards the southern edge of the range. We show that during the nestling period, female brooding time increased in cold weather. Male provisioning rate was negatively related to temperature and rainfall. Chick mortality increased in cold temperatures and was most likely to occur at nests where male prey delivery rates were low relative to temperature. Annual values of harrier fledged brood size across Scotland were positively related to summer temperature suggesting that the patterns seen in one population held at a national scale. In Spain, however, the opposite patterns were observed with fledged brood size being negatively related to temperature. This shows that whilst the impact of weather on productivity may be equally strong at two ends of a geographical range, the mechanisms vary dramatically. Large-scale predictive models need to take such patterns into account.     

Testing a novel agri‐environment scheme based on the ecology of the target species,Montagu's Harrier Circus pygargus

Farmland birds are in steep decline and agri‐environment schemes (AES) to counteract these biodiversity losses are expensive and inefficient. Here we test a novel AES, ‘Birdfields’, designed using detailed ecological knowledge of the target species, Montagu's Harrier Circus pygargus. Current AES, such as field margins, that aim to improve foraging conditions (i.e. vole densities) for harriers are inefficient, as prey are difficult to capture in tall set‐aside habitat. ‘Birdfields’ combines strips of set‐aside to boost vole numbers and strips of alfalfa, as voles are accessible after alfalfa has been harvested. We found that vole numbers were generally highest in set‐aside. Montagu's Harriers fitted with GPS‐loggers used ‘Birdfields’ intensively after mowing, preferring mown to unmown strips. Thus, prey availability appeared more important than prey abundance. Thus, ‘Birdfields’, as a targeted AES for Montagu's Harriers, is more effective than previous AES due to increased prey accessibility. An additional advantage of ‘Birdfields’ is that it is considerably cheaper, due to the harvest of alfalfa. We advocate that AES should always include monitoring and research activities, aiming at a more adaptive conservation approach.     

Regional and seasonal flight speeds of soaring migrants and the role of weather conditions at hourly and daily scales

Given that soaring birds travel faster with supportive winds or in good thermal soaring conditions, we expect weather conditions en route of migration to explain commonly observed regional and seasonal patterns in the performance of soaring migrants. We used GPS‐loggers to track 13 honey buzzards and four Montagu's harriers for two to six migrations each. We determined how tailwinds, crosswinds, boundary layer height (a proxy for thermal convection) and precipitation affected hourly speeds, daily distances and daily mean speeds with linear regression models. Honey buzzards mostly travel by soaring while Montagu's harriers supplement soaring with flapping. Therefore, we expect that performance of harriers will be less affected by weather than for buzzards. Weather conditions explained between 30 and 50% of variation in migration performance of both species. Tailwind had the largest effect on hourly speeds, daily mean speeds and daily travel distances. Honey buzzards travelled significantly faster and farther, and Montagu's harriers non‐significantly faster, under better convective conditions. Honey buzzards travelled at slower speeds and shorter distances in crosswinds, whereas harriers maintained high speeds in crosswinds. Weather conditions varied between regions and seasons, and this variation accounted for nearly all regional and seasonal variation in flight performance. Hourly performance was higher than predicted at times when we suspect birds had switched to intermittent or continuous flapping flight, for example during sea‐crossings. The daily travel distance of Montagu's harriers was determined to a significant extent by their daily travel time, which differed between regions, possibly also due to weather conditions. We conclude with the implications of our work for studies on migration phenology and we suggest an important role for high‐resolution telemetry in understanding migratory behavior across entire migratory journeys.     

The impact of raptors on the abundance of upland passerines and waders

The issue of predator limitation of vertebrate prey populations is contentious, particularly when it involves species of economic or conservation value. In this paper, we examine the case of raptor predation on upland passerines and waders in Scotland. We analysed the abundance of five wader and passerine species on an upland sporting estate in southern Scotland during an eight-year period when hen harrier, peregrine and merlin numbers increased due to strict law enforcement. The abundance of meadow pipit and skylark declined significantly during this time. Golden plover also showed a declining trend, whereas curlew increased significantly and there was a near significant increase in lapwings. Contrasting the local population trends of these species with trends on nearby areas revealed higher rates of decline for meadow pipit and skylark at the site where raptors increased, but no differences in trends for any of the three wader species. There was a negative relationship between the number of breeding harriers and meadow pipit abundance the same year and between total annual raptor numbers and meadow pipit abundance. Predation rates of meadow pipit and skylark determined from observations at harrier nests suggested that predation in June was sufficient to remove up to 40% of the June meadow pipit population and up to 34% of the June skylark population. This 'quasi-natural' experiment suggests that harrier predation limited the abundance of their main prey, meadow pipit, and possibly the abundance of skylark. Thus, high densities of harriers may in theory reduce the abundance of the prey species which determine their breeding densities, potentially leading to lower harrier breeding densities in subsequent years. We found no evidence to suggest that raptor predation limited the populations of any of the three wader species. We infer that concerns over the impact of natural densities of hen harriers on vulnerable upland waders are unjustified.     

Broad wintering range and intercontinental migratory divide within a core population of the near‐threatened pallid harrier

Aim To identify the migration routes and wintering grounds of the core populations of the near‐threatened pallid harrier, Circus macrourus, and highlight conservation needs associated with these phases of the annual cycle. Location Breeding area: north‐central Kazakhstan; Wintering areas: Sahel belt (Burkina Faso to Ethiopia) and north‐west India. Methods We used ring recovery data from Kazakhstan and satellite tracking data from 2007 to 2008 on six adults breeding in north‐central Kazakhstan to determine migration routes and locate wintering areas. In addition, one first‐year male was tagged in winter 2007–2008 in India. Results Data evidenced an intercontinental migratory divide within the core pallid harrier population, with birds wintering in either Africa or India. The six individuals tagged in north‐central Kazakhstan followed a similar route (west of the Caspian Sea and Middle East) towards east Africa, before spreading along the Sahel belt to winter either in Sudan, Ethiopia, Niger or Burkina Faso. Spring migration followed a shorter, more direct route, with marked interindividual variation. The bird tagged in India spent the summer in central Kazakhstan. Half of the signal losses (either because of failure or bird mortality) occurred on the wintering areas and during migration. Main conclusions Our study shows that birds from one breeding area may winter over a strikingly broad range within and across continents. The intercontinental migratory divide of pallid harriers suggests the coexistence of distinct migratory strategies within the core breeding population, a characteristic most likely shared by a number of threatened species in central Asia. Conservation strategies for species like the pallid harrier, therefore, require considering very large spatial scales with possibly area‐specific conservation issues. We highlight urgent research priorities to effectively inform the conservation of these species.     

To change or not to change experimenters: caveats for repeated behavioural and physiological measures in Montagu's harrier

Animals facing anthropogenic disturbances may exhibit different anti‐predator responses depending on their perception of the risk. Experimental measures of behavioural and physiological traits may be impacted by the disturbance due to experimenter handlings and activities. In this study, we assessed the behavioural and physiological responses of Montagu's harrier chicks Circus pygargus from 14 nests, visited four times from hatching to fledgling by either the same (group I) or different (group NI) experimenters. Escape, aggressiveness and stress‐induced corticosterone concentration increased for both groups over time but chicks from group I showed a higher increase in their behavioural and physiological responses than chicks from group NI. This increase occurred after the second visit when experimenters took the first blood sampling on nestlings. Handling by the same experimenters throughout the study did not result in habituation but in sensitization. Our results suggest that Montagu's harrier nestlings may discriminate between the people who visit their nest and respond to experimenters’ identity. Therefore, we urge researchers working on behaviour and/or the physiology of stress including repeated measurements (such as animal personality studies for instance) to consider whether the experiments should or should not be conducted by the same experimenters (as usually done to minimize bias) as it can influence the outcomes of the experiments.     

Factors influencing diet variation in a generalist predator across its range distribution

Factors linked with intraspecific variation in trophic diversity are still poorly understood in generalist species like the Montagu’s harrier (Circus pygargus) but may have important implications for conservation management at a wide scale. We described geographic patterns of Montagu’s harrier diet across Eurasia, gathering diet data from 30 studies in 41 areas from 11 countries. We grouped prey as invertebrates, reptiles, small mammals, large mammals, eggs, small birds and large birds, and calculated the contribution of each prey type to the diet (as % biomass) and Shannon’s Diversity Index for each study site. We analysed qualitative estimates of prey abundance in relation to latitude and longitude, then diet composition in relation to habitat of the study area and prey abundance estimates. Diet diversity of Montagu’s harriers increased from north to south, while abundance of all prey groups other than small mammals showed the opposite trends. Agricultural areas in northern latitudes seemed to hold high densities of small mammals, but low densities of alternative prey. Overall, birds were the main prey in most of Montagu’s harrier’s distribution range, although the relative importance of each prey type in the diet was significantly explained by its local abundance and habitat, confirming the opportunistic foraging strategy of this raptor species. Consumption of mammals was an exception to this trend, being negatively associated with the abundance of alternative prey, suggesting that this prey is not preferred. Trophic diversity in this species could be influenced by land-use changes through variations in the abundance and availability of prey, which could impact its population dynamics. This may be particularly important for northern populations of Montagu’s harriers breeding in agricultural habitats, where trophic diversity is already low.     

Philopatry and natal dispersal of Montagu's harriers (Circus pygargus) breeding in Spain: a review of existing data

Natal dispersal is an important component of bird ecology, plays a key role in many ecological and evolutionary processes, and has important conservation implications. Nevertheless, detailed knowledge on natal dispersal is still lacking in many bird species, especially raptors. We review and compile existing information from five tagging programmes of juvenile Montagu's harriers (Circus pygargus) in different Spanish regions, with PVC rings or wing tags, to provide an assessment of philopatry and natal dispersal of the species in Spain. Only 7% of all tagged harriers were observed as breeders in subsequent years. The percentage of philopatric (i.e. breeding within 10?km of the natal site) males and females was lower that 5%. Overall, there were no sexual differences in percentage of philopatric birds or dispersal distances, but we found study area differences. The low philopatry observed suggests a high capacity for natal dispersal in this species, for both sexes, and therefore high genetic mixing between populations. Differences in philopatry between study areas may be influenced by the different observation effort or detectability, or else reflect different philopatric strategies among populations. Finally, we found no significant differences in philopatry rate or dispersal distances related to tagging method, suggesting that tagging technique has a smaller effect than monitoring effort or observation ease on observation probability. Developing tagging programmes at a small scale and without procuring very large-scale and intensive subsequent monitoring is not worthwhile for evaluating philopatry and natal dispersal in this species.     

Aggressive behaviour in Montagu’s harrier Circus pygargus during the courtship period

Aggressive behaviour of Montagu’s harrier was observed during the pre-laying period in the 1992–1995 seasons on the calcareous marshes of Chelm in Eastern Poland. In total, 435 flights performed by 24 pairs of individually marked harriers were analysed. All flights were performed in relation to the territory of defence. Males performed 61% of aggressive interactions while females performed 39%. Intraspecific and interspecific aggression comprising direct attacks (58%), escorts (13%), pursuits (28%) and “mirror behaviour” (1%) were described for the first time. Interspecific aggressive behaviour occurred only near territory boundaries. The main intruders were marsh harriers Circus aeruginosus breeding on the same marshes. Occasionally, harriers attacked curlews Numenius arquata, short-eared owls Asio flammeus, magpies Pica pica, hooded crows Corvus corone cornix, common buzzards Buteo buteo or hobbies Falco subbuteo. Four cases of communal mobbing were observed. Three to five males from the neighbourhood attacked the intruders together. Intraspecific aggressive behaviour was observed in the pair territory, near the boundary or at a distance of up to 100 m from the defended area. Aggressive interactions performed by both sexes were more frequent towards birds of the same sex. Females defended their territories more aggressively against females. Similarly, males were more aggressive against males. All cases of aggressive behaviour were observed near harrier territories. Aggressive relations between birds outside breeding territories or when foraging around marshes were not observed.     

Diet composition and foraging success in generalist predators: Are specialist individuals better foragers?

Factors affecting individual diet specialization in generalist populations and the relationship between diet and foraging success remain poorly studied, particularly in terrestrial wide-ranging predators. We studied whether individual variations in diet in Montagu's harrier males (determined through a combination of direct foraging observations and pellet analysis) were associated with patterns of foraging habitat selection and foraging success of 12 radiotracked males during the breeding period. We found important differences in diet composition and breadth between individuals. Diet diversity was negatively related to hunting success: the most efficient individuals in terms of hunting success had the most specialized diet. This study also suggests an important role of individual foraging habitat selection in explaining individual diet, as the proportion of different prey types in the diet was associated with habitat composition within the home range, with higher proportion of those habitats that held higher abundances of their more frequent prey. This study thus provides evidence of individual diet specialization having a knock-on effect on foraging efficiency in a wide-ranging raptor and highlights the role of individual behaviour as a driving force of intra-population niche variation.     

Hen harriers and red grouse: moving towards consensus?

1.  The presence of predators may lead to conflict between different stakeholders. Finding ways to resolve such conflicts is a challenge to all involved.
2.  Within the UK a long and, at times, acrimonious conflict has developed around the conservation of hen harriers Circus cyaneus on moorland managed for red grouse Lagopus lagopus scoticus . This paper follows our original forum article and the replies from colleagues in the Royal Society for the Protection of Birds (RSPB) and the Game and Wildlife Conservation Trust (GWCT).
3.  Whilst differences of opinion remain, there are encouraging signs that both sides are striving for consensus. There is broad agreement that evidence is required on three fronts to address: (1) the ecological, economic and social consequences of alternative forms of land management in the uplands; (2) the efficacy of diversionary feeding of hen harriers; and (3) the utility of a brood management scheme to improve harrier conservation status and reduce the extent of the conflict.
4.   Synthesis and applications : The conflict between hen harrier conservation and red grouse management has existed for decades with little sign of progress. For the first time, stakeholders, researchers and policy advisors are engaged in effective dialogue and are striving for solutions. There are encouraging signs that all sides are now willing to listen to alternative viewpoints and appear prepared to seriously consider compromises. It is to be hoped that a solution to the harrier–grouse conflict can be found as this will have considerable positive ramifications for management of heather moorlands in the UK and wider implications for the conservation of predators around the world.     

Carotenoid limitation and allocation priorities in asynchronous raptor nestlings

The evolution and maintenance of conspicuous animal traits and communication signals have long fascinated biologists. Many yellow–red conspicuous traits are coloured by carotenoid pigments, and in some species they are displayed at a very young age. In nestling birds, the functions and proximate mechanisms of carotenoid‐pigmented traits are probably different and not as well known as those of adults. Here we investigated how Montagu's harrier (Circus pygargus) nestlings within structured families used a limited resource, carotenoid pigments, and whether they used these for increasing coloration (deposition in integuments) or for mounting a response to a phytohaemagglutinin (PHA) challenge, which measures pro‐inflammatory potential and aspects of cellular immune responsiveness. We manipulated carotenoid availability, using dietary carotenoid supplementations, and show that when supplemented, nestlings primarily allocated supplemental carotenoids to increase their coloration, irrespective of their sex, but depending of their position within the brood. Responses to PHA challenge were condition‐dependent, but depending on carotenoid availability. Moreover, how nestlings allocated carotenoids depended on their rank within the brood, which in turn influenced their level of carotenoid limitation (first‐hatched nestlings being less constrained than later‐hatched nestlings). We discuss why nestlings would use supplemental carotenoids for increasing bare parts coloration rather than for responding to a PHA challenge, and the potential benefits for doing so in a parent–offspring communication context. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 13–24.