Circus Cyaneus Research Articles

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.

Raptors and Red Grouse: Conservation Conflicts and Management Solutions

Abstract: Recovering predator populations may present problems for conservationists if their prey are of economic or conservation value. We address this issue by examining the conflict between raptor conservation and management of Red Grouse ( Lagopus l. scoticus) in Britain. Heather moorland is a distinctive habitat that supports an important assemblage of breeding birds. Large areas of moorland are managed by private landowners for shooting grouse. Although grouse shooting benefits conservation by retaining heather moorland, it is currently unclear whether grouse management directly benefits other upland birds. Human persecution has greatly restricted the range and abundance of most raptor species in Britain. Following the introduction of bird protection laws, the decline in gamekeeping, and the restriction of organochlorine pesticides, raptor populations have started to recover. Persecution of raptors on grouse moors is widespread and limits the range and abundance of Hen Harriers ( Circus cyaneus), Peregrine Falcons ( Falco peregrinus), and Golden Eagles (Aquila chrysaetos). In some circumstances, raptor predation can reduce both the breeding density and productivity of Red Grouse. Limitation of grouse populations through raptor predation is most likely to occur where raptors are at high density because of the abundance of alternative prey, and grouse are at low density either because of poor management or the cyclic nature of some grouse populations. In the long term, habitat management may reduce densities of alternative prey, leading to reductions in raptor densities and their predation on grouse. More active intervention may be required, however, if grouse moors are to remain viable in the short‐term. Current research is focused on manipulating harrier diet through diversionary feeding. Complementary research is needed to investigate methods to reduce raptor numbers locally while ensuring their national status.

Selenium in Blood of Predatory Birds from Kesterson Reservoir and Other Areas in California

Kesterson Reservoir (Merced County, California) is a selenium-contaminated grassland (previously a wetland) that supports a variety of upland birds and other wildlife. Although aquatic birds that occupied a wet Kesterson Reservoir in the early to mid-1980s suffered high mortality and reproductive failure due to selenium exposure, similar effects have not been identified in terrestrial birds. To better characterize selenium exposure and potential effects in terrestrial species, we analyzed blood for selenium concentrations (blood-Se) from predatory bird species at Kesterson and reference areas in California from 1994 to 1998. Except for loggerhead shrikes (Lanius ludovicianus), blood-Se was significantly higher in birds from Kesterson than in birds from other areas. Among species at Kesterson, blood-Se was higher in loggerhead shrikes and northern harriers (Circus cyaneus) than in other species sampled, possibly due to shrikes spending a relatively large proportion of time foraging at Kesterson. In general, data corroborated earlier modeled predictions of selenium accumulation in Kesterson raptors. Calculated (based on blood-Se) but not observed selenium concentrations in eggs of predatory birds from Kesterson exceed a previously established avian toxicity threshold of 10 ppm.

The effects of supplemental perch sites on avian predation and demography of vole populations

We erected supplemental perches to evaluate their effectiveness in attracting perching raptors and to determine if an increase in raptor visitation could affect vole demography. Our model experimental system consisted of six 0.2-ha enclosures containing gray-tailed vole, Microtus canicaudus. The availability of supplemental perch sites had no effect on northern harriers, Circus cyaneus, but resulted in an 11-fold increase in visitation by American kestrels, Falco sparverius. This increased predation pressure did not affect vole population size, growth rate, or adult survival. However, juvenile recruitment and the proportion of reproductive females were lower in treatment sites than in control sites. Male voles had smaller home ranges and were less active than expected, which may have been an avoidance response to predation risk. High densities of voles, adequate cover in treatment sites, and good foraging areas with less cover in surrounding habitat may have negated the effects of the availability of supplemental perch sites. We conclude that adding perches can increase raptor visitation substantially but may have little effect on high-density vole populations.

Breeding success of the hen harrier Circus cyaneus in relation to the distribution of grouse moors and the red fox Vulpes vulpes

Summary 1. A recent analysis showed that the survival and breeding success of female hen harriers were much lower on grouse moors than on other upland land management classes in Scotland, excluding the Orkney islands: a difference attributed to persecution of hen harriers by humans on grouse moors. However, this study did not look for possible beneficial effects on hen harrier breeding success of the control by moorland gamekeepers of other predators, particularly red foxes. 2. These and other recently published data were re‐examined to see whether there was regional variation in hen harrier nest success associated with the presence or absence of foxes and the control of foxes and other predators by moorland gamekeepers. 3. The main finding of the previous analysis, that nest success was much lower on grouse moors than on moorland that was not managed for grouse or in young conifer forests, was even more striking when comparisons were made among land management classes within the same region. 4. The mean number of broods fledged per female hen harrier per year on two grouse moors at which gamekeepers prevented human interference with harrier nests and continued to control other predators was compared with the average for moorland in Scotland. Harrier productivity on these moors was much higher than the average for grouse moors, but similar to the average for moorland not managed for grouse shooting. 5. Nest success of hen harriers breeding on moorland not managed for grouse averaged 19% higher in regions where there were, on average, more grouse moors in the vicinity, but in young conifer forests success was 34% lower in regions with more grouse moors nearby. Neither of these differences was statistically significant. 6. Nest success within a particular land management class was not significantly different inside and outside the range of the red fox. 7. Even a generous assessment of the magnitude of a supposed beneficial effect of the control of foxes and other predators by moorland gamekeepers on hen harrier nest success indicated that its effect on the population trend of hen harriers would be small relative to the large negative effect of persecution of harriers on grouse moors.

Numerical and functional responses in generalist predators: hen harriers and peregrines on Scottish grouse moors

Summary 1. The functional and numerical responses of two generalist raptors (hen harrier and peregrine) were studied on one moor for 6 years and on five other moors for 5 years. 2. Neither species showed numerical responses to grouse abundance. Harrier densities were highest in areas and years where their small prey (meadow pipits and small mammals) were most abundant. Peregrine densities were highest on southern study moors, probably in association with high abundance of racing pigeons. 3. For harriers preying on grouse chicks, the available data fitted a sigmoidal or type III functional response. Peregrines showed a type II response to adult grouse densities around eyries. 4. The proportion of grouse chicks taken by harriers was estimated to have been highest at densities of 67 chicks km–2 (equivalent to a mean of about 12 broods km–2). The proportion of adult grouse taken by peregrines appeared to be inversely density dependent, such that an increasing proportion of grouse was taken at grouse densities below 20 km–2. 5. In the absence of persecution, the impact of harriers on grouse populations is most likely to be greatest on moors where alternative prey and thus harriers are abundant. The question of whether harriers may dampen grouse population cycles at low grouse density is discussed.

Response of Northern Harriers and Short-Eared Owls to Grassland Management in Illinois

We studied habitat selection by northern harriers (Circus cyaneus) and short-eared owls (Asio flammeus) on 2 intensively managed grassland sanctuary complexes in southeastern Illinois to determine the influence of grassland management on nest placement. Northern barriers showed a tendency (P < 0.001) to locate their nests in fields not disturbed by recent management activities (within the last 12 months). Overall, northern harrier nest placement was not influenced by whether fields were dominated by native or nonnative grasses (P = 0.307), Short-eared owls were irruptive breeders on these grasslands, and habitat selection could only be assessed in 1 year (1990). In 1990, all short-eared owl nests were located in fields disturbed by grassland management activities within the last 12 months. Although northern harriers are believed to be area-sensitive in the Midwest, grassland area did not influence nest-site selection for northern harriers on our sanctuaries (P = 0.622). Northern harriers and short-eared owls have generally contrasting habitat preferences. To provide appropriate habitats for both species, a mix of both managed and idle grasslands must be provided.

Effects of Habitat Area on the Occurrence of Grassland Birds in Illinois

Abstract We documented the occurrence of 12 bird species within Prairie Ridge State Natural Area, Jasper County, Illinois, a matrix of nine grassland tracts (range 7–120 ha, totaling 489 ha), from 1994 to 1997 and analyzed the presence of these species relative to the area of the nine grassland tracts. Grassland tract area was correlated with the occurrence of greater prairie-chickens (Tympanuchus cupido) (P = 0.016), upland sandpipers (Bartramia longicauda) (P = 0.012), savannah sparrows (Passerculus sandwichensis) (P = 0.009) and Henslow's sparrows (Ammodramus henslowii) (P = 0.005). Minimum area requirements varied from 12 ha for grasshopper sparrows (Ammodramus savannarum), 55 ha for northern harriers (Circus cyaneus), 65 ha for greater prairie-chickens and upland sandpipers, to 75 ha for Henlsow's and savannah sparrows. Therefore, grasslands larger than 60 ha may be required to attract nine breeding grassland bird species. Each grassland tract was divided into 2- to 6-ha management units (mode = 2.8 ...

Respiratory nematodiases in raptors in Quebec.

This is a retrospective study on wild raptors submitted to the Université de Montréal (Quebec, Canada) from 1989 to 1996. Cyathostoma spp. (Nematoda: Syngamidae) adults and/or eggs were found in air sacs, lungs, bronchi, and trachea of 12 raptors (Falconiformes and Strigiformes) from Quebec, Canada, belonging to eight different species, five of which are first host records for this parasite: barred owl (Strix varia), snowy owl (Nyctea scandiaca), northern harrier (Circus cyaneus), northern goshawk (Accipiter gentilis), and broad-winged hawk (Buteo platypterus). The infection was considered fatal in four birds, while no significant clinical signs were observed in the other cases. Major pathologic changes included diffuse pyogranulomatous air sacculitis, pneumonia, and bronchitis. A few unidentified larval nematodes embedded in a granuloma were found in the lungs of an additional Coopers' hawk (Accipiter cooperii); they were not considered clinically significant. A dead nematode, surrounded by necrotic inflammatory cells, was found in the air sac of a northern goshawk. The presence of nematodes in air sacs or lungs should be considered in wild raptors demonstrating respiratory problems.

Effects of Military Training Activities on Shrub-steppe Raptors in Southwestern Idaho, USA.

/ Between 1991 and 1994, we assessed relative abundance, nesting success, and distribution of ferruginous hawks (Buteo regalis), northern harriers (Circus cyaneus), burrowing owls (Athene cunicularia), and short-eared owls (Asio flammeus) inside and outside a military training site in the Snake River Birds of Prey National Conservation Area, southwestern Idaho. The Orchard Training Area is used primarily for armored vehicle training and artillery firing by the Idaho Army National Guard. Relative abundance of nesting pairs inside and outside the training site was not significantly different from 1991 to 1993 but was significantly higher on the training site in 1994 (P &le 0.03). Nesting success varied among years but was not significantly different inside and outside the training site (P > 0.26). In 1994, short-eared owl and burrowing owl nests were significantly closer to firing ranges used early in the spring before owls laid eggs than were random points (P < 0.001). In 1993, distances from occupied burrowing owl nests to firing ranges used early in the year were similar to those from random points to the same firing ranges (P = 0.16). Military activity contributed to some nesting failures from 1992 to 1994, but some pairs nested successfully near military activity. KEY WORDS: Distribution; Military impacts; Nesting success; Raptors; Relative abundance; Shrub-steppe

Estimating the cause and rate of mortality in red grouse Lagopus lagopus scoticus

We assessed biases in the techniques used to investigate the cause and rate of mortality in two red grouse Lagopus lagopus scoticus populations in Scotland during 1985–96. Comparison of the field signs left on grouse carcasses by known predators suggested that whilst it was usually possible to distinguish between grouse killed by mammals and by raptors, it was not possible in most cases to further distinguish between grouse killed by peregrine falcons Falco peregrinus and hen harriers Circus cyaneus. Similar estimates of the cause and rate of grouse mortality were derived from systematically counting grouse and searching for carcasses and by radio‐tagging. Searching for carcasses may provide a useful technique for identifying major causes of mortality in grouse populations inhabiting open habitats.

Nest site selection by Hen Harriers in Scotland

The aim of this study was to examine nest site selection by Hen Harriers Circus cyaneus at two spatial scales in Scotland and to assess whether breeding success was influenced by choice of nest site. At the landscape scale, we compared availability and utilization of habitats in 610 km2 of Argyll, and at the local scale, we compared vegetation and topography at 52 harrier nests and random points within three areas of heather moorland covering a total area of 462 km2. At both scales, harriers showed a clear preference for nesting in heather. Within heather moorland, harriers nested in taller heather (average height 46cm) and nearer streams than expected by chance. More nests were on northwest-facing slopes than expected by chance. We found no evidence that breeding success was influenced by habitat or topography. Heather moorland is declining in the uplands due to overgrazing and afforestation. The association of harriers with heather suggests that their future may become increasingly dependent on moorland, where heather is maintained for grouse.

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.

The hen harrier and the grouse

The hen harrier is a scarce bird of prey which is today mainly found on moorland in Scotland, much of which is grouse-shooting territory. A population survey of the hen harrier shows that, because it preys on grouse, it is subject to severe persecution on grouse moors. This highlights a conflict of interests in conservation policy. On the one hand, the hen harrier needs protection; on the other, management of moorland estates for shooting has helped save large areas of the British uplands from overgrazing, or disappearing under conifer plantations, and the economic reasons for hen-harrier harassment are understandable.

The Effects of Illegal Killing and Destruction of Nests by Humans on the Population Dynamics of the Hen Harrier Circus cyaneus in Scotland

Breeding productivity, natal dispersal and survival of hen harriers Circus cyaneus were studied between 1988 and 1995 on moorland managed for sport shooting of red grouse, other heather moorland and young conifer forests in the uplands of Scotland. Nest success was much lower on grouse moors than on other land management classes. Annual productivity was 0.8 fledglings breeding female -1 year -1 on grouse moors compared with 2.4 on other moorland and 1.4 in young conifer forests. Human interference was recorded on half of the grouse moor estates studied and accounted for at least 30% of breeding failures in this land management class. It was much less frequent in the other land management classes. Annual survival of female hen harriers which bred on grouse moors was about half that of females breeding on other moorland. On grouse moors, survival of females which bred unsuccessfully was much lower than that of females which reared at least one fledgling. Survival of breeding females on other moorland was high and unrelated to breeding success. The difference in survival of breeding females between grouse moors and other moors was attributed to killing by humans. On average, 55-74 females were killed each year, 11-15% of the total population of breeding females in Scotland, excluding Orkney. The population of breeding females on grouse moors was estimated to decline rapidly without immigration. Harriers breeding on the other habitats were producing a surplus of female recruits approximately sufficient to compensate for the losses on grouse moors. Most females started to breed at I year old and most males at 2 years old. The percentage of breeding males which were I year old was higher on grouse moors than on the other land management classes. The median natal dispersal distance of both sexes exceeded 10 km. Harriers fledged from one land management class were often found breeding in another. Natal dispersal resulted in net movements of 1-year-old females between land management classes which were sufficient to reduce the differences in population trend which would otherwise have occurred. Moorland managed for grouse shooting was a sink habitat which received two-thirds of its female recruits from other habitats. The difference in productivity and survival between grouse moors and other habitats was attributed to illegal human interference. It is speculated that, without persecution, the hen harrier population in Scotland would increase, initially by about 13% year -1 , until a new, but unknown, equilibrium level was reached.

Impact of agricultural intensification of pastoral systems on bird distributions in Britain 1970–1990

The distribution of six species of birds known to depend on extensive pastoral systems, as published in atlases of breeding birds, was mapped in Britain for two time periods: 1968–1972 and 1990–1992. The species chosen (corncrake Crex crex, golden eagle Aquila chrysaetos, hen harrier Circus cyaneus, stone curlew Burhinus oedicnemus, chough Pyrrhocorax pyrrhocorax and red-backed shrike Lanius collurio) are known to be declining or vulnerable across large parts of their European range. Summed species distribution was compared with available data on the distribution of low intensity agricultural land in Britain. In 1990, bird and land-use distribution was similar. Thus it is concluded that it may be possible to use summed species distribution to predict areas where ecologically beneficial farming systems exist elsewhere in Europe. Most coincidence in species distribution now occurs in Scotland, northern England and Wales. However, large parts of mid-west Scotland have less selected pastoral species than in 1968–1972. The most obvious changes have occurred throughout East Anglia, the south-east and south-west of England. There has been significant fragmentation of coincidence along the north Norfolk coast and along the coasts of Suffolk and Essex. The extent of summed distribution of the indicator bird species chosen declined and fragmented dramatically between 1970 and 1990. There was a significant negative relationship between livestock units per hectare (both sheep and total), and mean species coincidence (average number of species occurring in each 10km 2) for six regions in Scotland and northern England in 1990. In each of these six regions, mean species coincidence declined as sheep livestock units per hectare increased between 1970 and 1990. The need for changes to agricultural policies at the level of the European Community in order to reverse this serious conservation problem, is discussed.

A comparison of the feeding ecology of wintering Hen Harriers Circus cyaneus centred on two heathland areas in England

Small passerines were the principal prey in pellets from Hen Harrier Circus cyaneus communal winter roosts in Breckland and the New Forest. Skylark Alauda arvensis remains predominate in the pellets from Breckland. Dunnock Prunella modularis and Greenfinch Carduelis chloris were also important. The contents of New Forest pellets reflected the position of each roost. In pellets from a roost on the northern edge of New Forest heathland and close to farmland, Skylark was by far the predominant prey. Wren Troglodytes troglodytes and Linnet Carduelis cannabina were the next most important passerines. Pellets from the more extensive heather heathland in the south of the New Forest showed greater dependence on the heathland community of insectivorous passerines, with Wren, Dartford Warbler Sylvia undata and Meadow Pipit Anthus pratensis occurring as prey in that order of frequency. Pellets from a roost in northwest New Forest heathland with less heather and closer to farmland contained mainly a mixture of farmland‐and heathland‐feeding passerines. The directions of arrival and departure at roosts indicated that the Breckland‐roosting harriers hunted mainly in arable fenland and the New Forest edge‐roosting harriers in arable downland. Counts of the principal passerine prey species in roost catchment areas indicated that Greenfinch and Linnet were most abundant in arable fenland and arable downland squares, respectively, and the Wren was most abundant in the New Forest catchment area as a whole. The influence of the distribution of weed species on the species of seed‐eating passerines in prey was investigated. Proportions of passerines in pellets from the Breckland and New Forest edge roosts were positively correlated with the proportions of “grey” males. Proportions of finches in both the Breckland and the New Forest edge‐roost pellets declined sharply after January. There was a shift toward lagomorphs in the diet after January and a suggestion that this was positively correlated with the proportions of ringtails at the Breckland roost. Factors affecting prey composition and selection of foraging habitat, the effect of harrier predation on Dartford Warbler numbers and the implications for differential distribution of the harrier sexes in winter are discussed.

Importance of a bird migration hot spot: proportion of the Swedish population of various raptors observed on autumn migration at Falsterbo 1986–1995 and population changes reflected by the migration figures

The importance of Falsterbo, Sweden, as a concentration site for migrating raptors was assessed by recording total numbers of various species during ten autumns 1986–1995. Population estimates and number of fledged young per pair were taken from the literature to estimate the Swedish autumn population. These estimates were compared with the average numbers of migrants at Falsterbo to see how large proportion was recorded there and if there was any difference between adults and juveniles. The proportion observed at Falsterbo varied from below 1% in Goshawk Accipiter gentilis to 38% in Red Kite Milvus milvus. Species with more southerly distribution (breeding closer to Falsterbo), like Red Kite and Marsh Harrier Circus aeruginosus were recorded to a higher degree compared to northerly species like Northern Harrier Circus cyaneus and Rough-legged Buzzard Buteo lagopus. Thermal migrants like Honey Buzzard Peris apivorus, Red Kite and Common Buzzard Buteo buteo were more concentrated at Falsterbo compared to active flyers like harriers and falcons, which are less inclined to follow leading lines. In most species a higher proportion of juveniles was recorded. This may be due to adults generally wintering further north or being less inclined to follow leading lines. Three species (Honey Buzzard, Rough-legged Buzzard and Peregrine Falco peregrinus) showed a higher percentage of adults. In these species I suggest the adults use their previous experience to follow established safe and efficient routes to their known winter quarters. Systematic autumn counts of migrating birds at Falsterbo have been conducted since 1973. The correlation of annual numbers with these standardised counts is significant in all species, although the average figures are between 38 and 214 percent higher in this study due to a better coverage. Most species are at the moment stable or increasing. Recent population increases in Red Kite, Marsh Harrier and Peregrine are very well reflected by the counts. Standardised migration counts at Falsterbo are presently the best way to follow long-time changes in the Swedish raptor populations.

The influence of reproductive season stage on nest defence by meadow pipits (Anthus pratensis)

Meadow pipits were tested for their responses to the nest predator (stuffed hen harrier placed close to the nest). The predator was mobbed by groups of birds consisting of 1–6 individuals. A single pipit distracted the predator less intensively than groups of two or more birds. However, distraction displays by a pair and larger groups did not differ in respect of the number and proportion of the most risky displays. These findings suggest that one or two birds (probably parents of the threatened offspring) actively mobbed the harrier whereas other individuals were fairly passive. The level of defence by birds displaying to the predator was constant during the breeding cycle and breeding season. The variance in the intensity of nest defence resulted from differences in the number of active mobbers. They tended to ignore the predator at the beginning of breeding cycle and late in the breeding season. As a result, the intensity of nest defence significantly increased in the breeding cycle and declined as the breeding season progressed. We propose that late breeders mobbed the predator less willingly because, in comparison with early breeders, they had higher probability of survival to the next breeding season and lower expected fitness benefits from the current brood. We hypothesise that the response to the nest predator according to an “allor- nothing” rule (“defend the offspring with the constant intensity or avoid mobbing”) might be the result of a long coevolution of the nest attacker and the nest defender.

First Nest Discovered for Alaska Subspecies of Marbled Godwit

First Nest Discovered for Alaska Subspecies of Marbled Godwit