Feeding behaviour and diet composition of Marsh Harriers Circus aeruginosus in agricultural landscapes

Capsule Diet composition and feeding behaviour of breeding birds differed between areas of high and low agricultural intensification.

Aims To analyze diet composition and feeding behaviour of Marsh Harriers Circus aeruginosus in agricultural landscapes with different degrees of agricultural intensification.

Methods Diet composition and feeding behaviour was assessed by focal observations at nests during breeding seasons in 2001–03 in two agricultural areas within the eastern Ebro Basin (on the northern Iberian Peninsula) which differed in their degree of agricultural intensification: a traditional non-irrigated and non-intensively managed farmland area; and an irrigated and intensively managed farmland area.

Results Marsh Harriers in the intensively managed area took higher percentages of small mammals than harriers in the traditional area managed at low-intensity, where they also had a more diverse diet. Male delivery rates increased throughout the breeding season in the intensively managed area. On the contrary, no significant trend in delivery rates was observed in the traditional area, where harriers turned to larger prey late in the season.

Conclusion Marsh Harriers probably took advantage of different prey types in intensively managed and traditional farmland according to prey availability. Differences in diet composition seem to be related to different foraging strategies adopted by harriers in traditional and intensively managed farmlands. In the traditionally managed farmlands, harriers responded to increases in the energy requirements of the brood over the course of the breeding season by taking larger prey, whereas in the intensively managed farmlands harriers increased feeding rates.     

Mismatches between breeding success and habitat preferences in Hen Harriers Circus cyaneus breeding in forested landscapes

During the past century, the upland breeding areas of Hen Harriers in Ireland have been extensively afforested. There is no evidence that this species avoids breeding in heavily forested landscapes and, indeed, young commercial forests in their second rotation are often selected as nest‐sites. However, Hen Harriers have coexisted with these forested areas for only a few decades and it is possible that such landscapes are suboptimal. We examined the relationship between breeding success and habitat using a dataset spanning three years and four study areas in the south and west of Ireland. We assessed whether nest success and fledged brood size were related to habitat type, both at the nest‐site and in the surrounding landscape. Neither measure of breeding productivity was related to total forest cover or to percentage cover of closed canopy forest in the landscape. However, in a subset of areas, high cover of second‐rotation pre‐thicket (young forests planted on land from which a first rotation has already been harvested) in the surrounding landscape was associated with low levels of breeding success. This may be due to factors related to predation, disturbance or prey availability. The fact that second‐rotation pre‐thicket is a preferred habitat for nesting in Ireland suggests that Hen Harriers may be making suboptimal decisions in the landscapes available to them.     

Survival and causes of mortality among wild Red-legged Partridges Alectoris rufa in southern Spain: implications for conservation

Knowledge of intrinsic and extrinsic factors associated with survival rates and the underlying causes of mortality is essential to understand both population dynamics and the causes of population declines. We studied the influence of habitat variables and body condition on survival of 151 radio‐tracked Red‐legged Partridges Alectoris rufa in four areas of Spain, representing a gradient from natural unmanaged areas to highly managed areas. We examined the effects of differences in game management practices, and seasonal and geographical variations in survival and causes of mortality over 4 years, from 1999 to 2002. Monthly survival rate was consistently over 90% for both sexes in the natural area. However, in two areas managed intensively for both hunting and agriculture, survival was low during the hunting period (72% for females and 79% for males), high during the breeding period for males (99%), and intermediate for females (89%) due mainly to diseases. Hunting was the main cause of Red‐legged Partridge mortality in both hunting areas where driven partridge shooting was performed, affecting 50% of radio‐tagged individuals, and was the main cause of mortality over all areas during both breeding and hunting periods. Disease was the next most common cause of mortality in managed areas, affecting mostly females during the breeding period, whereas predation was the main cause of mortality in unmanaged populations. Finally, we compared the habitat associations and body condition of living and dead individuals with varying causes of mortality. In general, high survival rates were associated with diverse vegetation, habitat edges and a good body condition index. Habitat diversity and a high edge index were also negatively associated with mortality due to predation and diseases. On the other hand, hunting mortality decreased with the proportion of scrubland and increased with the proportion of agricultural land. These results suggest that preventing declines of wild Red‐legged Partridge populations might best be achieved by increasing landscape complexity and connectivity, and promoting game management practices to limit both partridge bags and long‐term densities.     

Comparative breeding performance of Marsh Harriers Circus aeruginosus along a gradient of land‐use intensification and implications for population management

Assessing variation in breeding performance in relation to habitat characteristics may provide insights into predicting the consequences of land‐use change on species ecology and population dynamics. We compared four Marsh Harrier Circus aeruginosus populations subject to similar environmental conditions, but which differed in habitat composition, ranging from natural habitats to intensively cultivated areas. Using a 6‐year dataset, we characterized breeding habitat and diet in these four study sites, and analysed breeding performance in relation to this gradient of land‐use intensification. There was minimal variation in breeding performance between study years but consistent variation between study sites. Unexpectedly, Marsh Harriers breeding in intensively cultivated habitats had higher reproductive success than those breeding in more natural habitats, which, however, hosted higher breeding densities, so overall net population productivity (fledglings per unit area) was similar across sites. This resulted from combined effects of density‐dependence and different predation rates between study sites. The colonization of intensive farmland habitats may not necessarily impact negatively on population sustainability when breeding success and population density are traded against each other. However, our findings should not mask longer‐term conservation issues for populations breeding in these intensively managed areas, and further studies should assess potential long‐term negative effects of occupancy of human‐altered habitat.     

Interspecific and intraspecific differences in habitat use and their conservation implications for Palaearctic harriers on Sahelian wintering grounds

The floodplains of the West‐African Sahel region have experienced extensive habitat transformation during the past four decades, coinciding with an impoverishment of raptor populations. We investigated foraging patterns of Palaearctic migratory Eurasian Marsh Harriers Circus aeruginosus, Pallid Harriers C. macrourus and Montagu’s Harriers C. pygargus on a floodplain system in northern Cameroon to assess species, sex‐ and age‐related habitat preferences. Sex and age have rarely been incorporated into general studies of raptor habitat associations, despite clear evidence of intrasexual and age‐related differences in foraging strategies and diet composition, potentially carrying strong conservation implications. We found evidence of sexual differences in foraging preference related to land use, particularly in the most sexually dimorphic Pallid Harrier, and evidence that juveniles used different habitats to adults. This constitutes the first quantitative documentation of such differentiation by Palaearctic raptors on African wintering grounds, indicating that general patterns of habitat use in wintering raptors may obscure sex‐ and age‐specific preferences. Contrary to expectations, we found limited evidence for interspecific foraging segregation. Food partitioning by prey mass was related to harrier body mass and facilitated by a diverse availability of prey on human‐transformed floodplains. Anticipated further large‐scale conversion of floodplain habitat into predominantly desiccated grasslands raises concerns about the survival of wintering harriers.     

Comparative breeding ecology and hybridization of Eastern and Western Marsh Harriers Circus spilonotus and C. aeruginosusin the Baikal region of Eastern Siberia

New information on the breeding ecology and inter-breeding of Western Circus aeruginosus and Eastern C. spilonotus Marsh Harriers in the Baikal region of Eastern Siberia is presented. Hybridization is relatively common (one of the parents was not 'pure' C. spilonotus in six out of 28 nesting cases) in the Irkutsk area, where nests of a mixed pair were found with chicks in 1999 and 2000. Eastern Marsh Harriers are more similar to Western in size, biology and coloration of young birds, but to the Australasian C. approximans in their displays and mating system, and intermediate between C. aeruginosus and C. approximans in the plumage coloration of females. The systematic status (species' or 'subspecies') of the Western and Eastern Marsh Harriers depends on the systematic approach taken, and existing data seem to be inadequate to resolve this. The Siberian Mongolian zone of contact of these marsh harriers is unique as other marsh harriers of the world are allopatric. More research on ecology, behaviour and molecular systematics is necessary to resolve the situation.     

Grey Partridge Perdix perdix in the UK: recovery status,set‐aside and shooting

The Grey Partridge Perdix perdix is a European Species of Conservation Concern and a priority species under the UK Biodiversity Action Plan. The Game & Wildlife Conservation Trust (GWCT) launched a major programme to help partridge recovery in the UK, built on the GWCT’s Partridge Count Scheme (PCS) and including a demonstration site from 2002. We contrast the national picture of no population change since 1999 from BTO monitoring with a doubling of spring pair density on PCS sites. At the demonstration site, where set‐aside was used for habitat creation, Grey Partridge breeding density increased six‐fold, to 18 pairs/km², then fell back. The drop coincided with bad weather but also with the disappearance of rotational set‐aside when the set‐aside rate fell to zero, which halved the amount of brood‐rearing habitat. Non‐rotational set‐aside remained unchanged, as did the amount of nesting habitat that it provided. Grey Partridge density was significantly linked to rotational set‐aside, especially wild bird cover, but not to non‐rotational set‐aside. The demonstration project also showed that, with appropriate precautions, it was possible to shoot over 60% of Red‐legged Partridges Alectoris rufa while maintaining Grey Partridge losses below 5%. On PCS sites, the annual change in spring density in recent years differed in relation to neither shooting pressure nor intensity of Red‐legged Partridge releasing. Provision of brood‐rearing habitats and game cover increased with the latter, and probably counteracts the shooting losses of Grey Partridges on Red‐legged Partridge shoots when, as on PCS sites, active measures keep those losses below 20%. Targeted personal advice channelled through the PCS has been fundamental to these successes and must be expanded.     

Comparative breeding biology of Hen Harrier and Montagu's Harrier: an 8-year study in north-eastern France

Hen and Montagu's Harriers breed in the same cultivated areas of eastern France. We present data from an extensive study conducted in three adjacent areas where 757 nests of the two harriers were monitored between 1993 and 2000, with the aim of comparing the breeding ecology of these two species and to evaluate their possible future trends. Breeding habitat for harriers consisted nearly exclusively of winter cereals, causing great conservation concern in this intensively farmed region. The Hen Harrier was almost absent from two of the study areas. This species showed significantly larger clutch size and higher breeding success than the Montagu's Harrier, and an earlier breeding phenology. It was thus less adversely affected by harvesting activities than Montagu's Harrier. Both species showed a reduced breeding success with increasing laying date. There was a large diet overlap between the two species, possibly leading to competitive interactions. Overall, the Montagu's Harrier should be considered as the more vulnerable of the two species, necessitating conservation measures, such as protection of nests from early harvesting activities. Nevertheless, to maintain both species in agricultural habitats, farming practices that preserve sufficient food should also be promoted.     

Habitat selection and foraging success of Hen Harriers Circus cyaneus in west Scotland

The foraging behaviour of Hen Harriers breeding in west Scotland was studied in a variety of afforested and moorland habitats. Time budget information from sample areas, stratified by habitat type and controlled for nest distance, was used to investigate habitat selection. Preferences were then related to measures of foraging performance in each habitat. Hen Harriers foraged preferentially over young first rotation coniferous forests, and selected heathland and grassland habitats ahead of closed canopy woodland. Harrier preference for the various habitats was positively correlated with the frequency with which they struck at, and captured, prey. Prey strike success was much lower than that reported for moorland managed for Red Grouse Lagopus lagopus scoticus. The results explain many of the reported changes in local Hen Harrier populations. The extent of young first rotation forestry is currently in decline. It is predicted that this will lead to a reduction in numbers of Hen Harriers breeding in west Scotland, and an increase in the proportion of the British harrier population occupying moorland managed for Red Grouse.     

Food-niche differentiation in sympatric Hen Circus cyaneus and Montagu's Harriers Circus pygargus

Hen Harriers Circus cyaneus and Montagu's Harriers Circus pygargus are medium-sized raptors that differ in size (Hen Harrier being slightly bigger than Montagu's Harrier) and breeding system (Montagu's Harriers are semi-colonial and Hen Harriers defend nesting–hunting territories). In contrast, the diets of the two species when in sympatry are very similar. We evaluated food-niche differentiation among these coexisting raptor species and how between-species differences in body size and social system influence interspecific relationships. We present data from a study conducted in 1997 and 1998 in northeastern Madrid province (central Spain). Diet of the two species largely overlapped (55–95%) during the breeding season, but Hen Harriers preyed more often on larger species. This segregation was observed both in the average size of the primary prey (lagomorphs) and in the alternative prey (birds for Hen Harriers vs. insects for Montagu's Harriers), and was particularly apparent late in the season. Accordingly, feeding frequency of Montagu's Harriers, but not of Hen Harriers, increased later in the season. Size differences between species in prey brought to the nest were apparent for both males and females. Foraging behaviour also differed, as Hen Harriers spent more time hunting close to the nest than did Montagu's Harriers. This implies that segregation in foraging areas may also exist. Observed niche partitioning may relax the potential for competition between these species.     

Factors influencing Hen Harrier Circus cyaneus territory site selection and breeding success

ABSTRACT

Capsule: Our findings regarding Hen Harrier Circus cyaneus territory site selection and breeding success in Ireland offer an opportunity for the development of initiatives and conservation actions aimed at enhancing the suitability of upland areas for breeding Hen Harriers and ensuring the long-term persistence of the species.

Aims: To investigate landscape-scale associations between habitat composition and Hen Harrier territory site selection, and to explore the influence of habitat and climate on breeding success.

Methods: We used multi-model inference from generalized linear models and Euclidean distance analyses to explore the influence of habitat, topographic, anthropogenic and climatic factors on Hen Harrier territory selection and breeding success in Ireland, based on data from national breeding surveys in 2010 and 2015.

Results: Hen Harrier territories were associated with heath/shrub, bog and pre-thicket coniferous forests. Comparisons between territories and randomly generated pseudo-absences (upland and lowland) showed that breeding pairs preferentially select for these habitats. Breeding success was negatively influenced by rainfall early in the breeding season and by climatic instability, and was positively influenced by the presence of heath/shrub and bog.

Conclusions: The results suggest that Hen Harrier breeding success is compromised by the synergistic effects of climate, landscape composition and management. Effective conservation of Hen Harriers in Ireland will therefore rely on landscape-scale initiatives.     

The direct and indirect effects of predation by Hen Harriers Circus cyaneus on trends in breeding birds on a Scottish grouse moor

Two phases of an experimental reduction in management at Langholm Moor in southwest Scotland provided an insight into factors that determined trends in the abundance of moorland birds. In 1992 the historical control of breeding Hen Harriers Circus cyaneus stopped and in 2000 grouse moor management was discontinued. Golden Plover Pluvialis apricaria , Lapwing Vanellus vanellus , Curlew Numenius arquata and Red Grouse Lagopus lagopus scoticus declined, whilst Carrion Crow Corvus corone and Snipe Gallinago gallinago increased. Hen Harriers increased from two to 20 breeding females, then declined back to two. Lapwing abundance was positively associated with that of Hen Harriers, and moorland passerines (Skylark Alauda arvensis , Meadow Pipit Anthus pratensis and Stonechat Saxicola torquata ) were negatively associated. Golden Plover, Lapwing, Curlew, Red Grouse, Skylark and Hen Harrier were more abundant when the moor was managed for grouse, whilst Carrion Crow, a common predator of clutches of ground-nesting birds, increased during the second half when management ceased. Increased Crow numbers, together with an increase in Red Foxes Vulpes vulpes , probably contributed to the observed bird declines.     

Global dispersion of nesting Hen Harriers Circus cyaneus; implications for grouse moors in the U.K.

In the U.K., a full recovery of Hen Harrier Circus cyaneus breeding numbers is prevented by illegal culling by some gamekeepers who fear the species threatens the future of grouse moors. This study's main purpose was to estimate how many more Hen Harriers there would be in the U.K. if this culling were to cease. A review of 33 studies of the Hen Harrier in nine countries showed that the densities of nesting females decreased strongly as the size of the areas studied increased and also that such information could not be used to estimate the potential density over a large area, such as the U.K. Correcting for the size of the areas studied showed that the potential density of nesting Hen Harriers on grouse moors could be predicted from studies overseas. Estimates of the densities of Hen Harriers in occupied areas globally were therefore used to estimate the density appropriate for the U.K. If all potential habitats were occupied, present numbers could more than double, to an estimated 1660 nesting females. This estimate represents an average of one nesting female per 2 5 km² of habitat, a density which would cause little or no significant economic damage on grouse moors. However, because Hen Harriers tend to aggregate, they would not spread out evenly but would nest in relatively high densities on a number of moors. The economic impact on Red Grouse Lagopus lagopus would not be a function of overall numbers, rather it would arise from the uneven dispersion of nesting Hen Harriers.     

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.     

Hen Harrier Circus cyaneus population trends in relation to wind farms

Capsule: The data presented here demonstrate a considerable spatial overlap between wind farms and the breeding distribution of Hen Harriers in Ireland, but evidence for a negative impact of wind farms on their population is weak.

Aims: To assess the extent of the overlap between wind farms and breeding Hen Harriers and to investigate their potential impact on Hen Harrier population trends.

Methods: Data on Hen Harrier breeding distribution in 10?km?×?10?km survey squares from national surveys were used in conjunction with information on the location of wind farms to examine whether, and to what extent, changes in Hen Harrier distribution and abundance between 2000 and 2010 were related to wind energy development.

Results: Of the 69 survey squares holding Hen Harriers during the 2010 breeding season, 28% also overlapped with one or more wind farms. Data from 36 of the squares with breeding Hen Harriers during the 2000 survey revealed a marginally non-significant negative relationship between wind farm presence and change in the number of breeding pairs between 2000 and 2010.

Conclusions: A considerable overlap exists between Hen Harrier breeding distribution and the location of wind farms in Ireland, particularly in areas between 200 and 400?m above sea level. The presence of wind farms is negatively related to Hen Harrier population trends in squares surveyed in 2000 and 2010, but this relationship is not statistically significant, and may not be causal. This is the first study to assess the influence of wind energy development on Hen Harriers at such a large geographic and population scale.     

The status of the Hen Harrier,Circus cyaneus,in the UK and Isle of Man in 2010

Capsule The fourth national survey of Hen Harrier showed that the population in the UK and the Isle of Man declined significantly between 2004 and 2010.

Aim To estimate the size of the breeding Hen Harrier population (with associated 95% confidence intervals) in the UK and Isle of Man, constituent countries and Scottish regions, in 2010 and calculate population change since previous surveys in 1998 and 2004.

Methods Complete surveys were made of 10-km squares likely to be occupied by breeding Hen Harriers in England, Wales, Northern Ireland and the Isle of Man, using standard methods developed for previous national surveys. In Scotland, a ‘census’ area was non-randomly selected for survey by volunteers, and randomly selected squares were surveyed in two strata covering the rest of the known range.

Results The UK and Isle of Man Hen Harrier population was estimated at 662 territorial pairs (95% confidence interval (CI): 576–770), a significant decline of 18% since 2004. Scotland holds the bulk (76%) of the population (505 territorial pairs; 95% CI: 417–612), with smaller numbers in Northern Ireland (59 pairs), Wales (57 pairs), the Isle of Man (29 pairs) and England (12 pairs). Declines of 49% and 20% were observed in the Isle of Man and in Scotland, respectively, whereas the Welsh population increased by 33%. A significant decrease was recorded in numbers of pairs using young and mature plantation forest in Scotland.

Conclusion The breeding population of Hen Harriers in the UK and Isle of Man declined between 2004 and 2010. Notable decreases in Scotland and the Isle of Man may be related to habitat change and illegal persecution. Illegal persecution continues to limit the population size of harriers in England to very low levels.     

The importance of pre-thicket conifer plantations for nesting Hen Harriers Circus cyaneus in Ireland

Hen Harriers Circus cyaneus are threatened across much of their range and their conservation requires appropriate habitat management. The locations of 148 Hen Harrier nests found in the Republic of Ireland during national breeding surveys in 2000 and 2005 were used to assess nest-site selection. The distribution of these nests was compared to distributions of randomly located points to investigate selection at the scale of the nest-site and landscape. The main nesting habitats selected were pre-thicket stage of first and, particularly, second rotation plantations, mostly of exotic conifers. There was no evidence that the area of post-closure plantations negatively affected Hen Harrier nest distribution. There was a positive correlation across study areas between changes in numbers of Hen Harrier nests between 2000 and 2005 and changes in the area of pre-thicket second rotation plantations over the same period. The overall effect of plantation forests on breeding Hen Harriers in Ireland therefore appears to be positive. However, this study did not consider the effects of plantation habitats on breeding success. Improved grassland was strongly avoided as a nesting habitat. Furthermore, after controlling for the influence of nesting habitat on nest location, landscapes with a high percentage cover of improved grassland were also avoided. Further agricultural intensification of grassland in areas where Hen Harriers breed is likely to have a negative impact on this species. These results are required for the development of management strategies for the conservation of this species.     

Managing the UK Grey Partridge Perdix perdix recovery: population change,reproduction, habitat and shooting

In 1995, the UK Government designated the Grey Partridge Perdix perdix as a priority species under its Biodiversity Action Plan (BAP). The Game Conservancy Trust (GCT), nominated as lead partner for the species, launched a programme to help Grey Partridge recovery by (1) raising awareness among the shooting and farming communities, (2) encouraging land managers and local BAP groups to conserve Partridges by setting numerical targets and offering feedback from counts on how to increase numbers, and (3) motivating by example by setting up a site demonstrating management techniques and the increase in Partridge abundance that ensues. Building on past research, we quantify the amount of habitat management required to achieve the BAP targets of halting the decline and achieving population recovery. Using three major GCT databases, (1) the National Gamebag Census, which collates data on shooting bags and gamebird releasing in the UK, (2) the Partridge Count Scheme, which monitors national Partridge density and reproduction, and (3) the Sussex Study, which monitors local Partridge population dynamics over 37 years, we emphasize the importance of good breeding success for increasing densities, review the evidence that management can be successful and evaluate the impact of shooting on Grey Partridge conservation.     

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.     

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.