John Calladine

Impacts of piscivorous birds on salmonid populations and game fisheries in Scotland: a review

Abstract The Scottish populations of salmonids are important both ecologically and economically. Game fisheries for Atlantic salmon Salmo salar, sea trout Salmo trutta trutta and brown trout Salmo trutta fari are all highly acclaimed and generate substantial levels of income for Scotland, but many populations are in decline and efforts are being made to ensure the future sustainability of these species. These declines have led to a focus on the impact of piscivorous bird predation on fish populations. The purpose of our review was to assess the evidence for population-level impacts on salmonid populations, and/or economic impacts on Scottish game fisheries of predation by the four primary UK freshwater piscivorous bird species: cormorant Phalacrocorax carbo, goosander Mergus merganser, red-breasted merganser Mergus serrator and grey heron Ardea cinerea. There is evidence that these birds can, in some situations, remove large numbers of fish from stocked and natural fisheries. However, a lack of information on fish population levels, the numbers and species composition of feeding birds, and robust calculations of fish consumption has hampered the conversion of the results of the existing studies into useful quantitative measures of impact. As a consequence, few studies have demonstrated any reductions in numbers of breeding fish or fish productivity due to predation by piscivorous birds, or direct economic loss to fisheries in Scotland. We support a previous recommendation for a reiterative procedure of modelling, experimentation and remodelling to assess the impacts of piscivorous birds on fisheries. Wide-scale studies of the movements of piscivorous birds, their feeding locations in relation to river characteristics, and the factors that make fish particularly vulnerable to predation are seen as important areas for future research.

The importance of altitude and aspect for breeding Whinchats Saxicola rubetra in the uplands: limitations of the uplands as a refuge for a declining, formerly widespread species?

Capsule The likelihood of an area being occupied by breeding Whinchats was inversely related to altitude, and areas with south- and east-facing aspects were favoured over those with north- and west-facing aspects. Aims To identify habitat features and physical attributes of Whinchat territories in upland areas of the UK. Methods Comparison of habitat features and physical attributes measured at random points within territories and > 100 m from a territory in an area with a relatively high breeding density were used to identify factors associated with Whinchat occurrence in upland Scotland. Further data were extracted from recent independent surveys conducted elsewhere in upland Britain. Results Within a favoured habitat (young sparse shrubs and developing ground vegetation) in the main study site, the probability of an area with south- and east-facing aspects being occupied by breeding Whinchats exceeded 80% below 300 m asl and declined linearly to less than 20% above 500 m. For areas with north- and west-facing aspects, the probability of occupancy at 300 m was 40% and close to zero at 500 m. Territory size increased with altitude. Data from elsewhere in upland Britain supported an association of altitude and aspect with Whinchat territories but suggested that such environmental limitations differed between regions. Conclusion Within marginal uplands, Whinchats could be constrained at their lower altitudinal limit by intensive agricultural land-use and at their upper altitudinal limit by environmental restrictions on their breeding biology. Such limitations need to be considered if such areas are to continue to support viable populations of a declining and formerly widespread species.

Variation in the diurnal activity of breeding Short‐eared Owls Asio flammeus: implications for their survey and monitoring

Capsule The proportion of time during which breeding Short‐eared Owls could be seen during daylight hours was low and assessment of seasonal variation in their detection likelihoods suggests that surveys may only be able reliably to identify pairs that successfully reach the chick‐rearing stage. Aims To identify the most appropriate times of day and stages of breeding for surveying and to contribute towards the development of feasible survey protocols. Methods Fixed point watches sampled the activity of breeding Short‐eared Owls in three extensive study areas in Scotland during two breeding seasons; 2006 and 2007. Results The proportion of time for which breeding owls were visible during daylight hours was low (4.8% of the time for the season March–July) and there was a statistically significant effect of stage of breeding and also of time of day within some stages of breeding on the proportion of time during which owls were visible. There was a greater than 75% likelihood of seeing owls during a combined total of four hours of watching (1) in the evenings during incubation (mid‐April to mid‐May), (2) mornings and evenings during chick‐rearing (June), and (3) early morning and late evening during fledging (July). The likelihood of seeing key territorial behaviours was always low. The mean distance from an observer at which owls were first detected in 2006 was 495 m (95% CI: 390–601 m), and significantly greater in 2007 at 741 m (95% CI: 651–832 m). Conclusion The interpretation of field surveys of breeding Short‐eared Owls should take into account the effect of season and time of day on the likelihood of detection. Surveys may only be able to reliably identify pairs that successfully reach the chick‐rearing stage and, therefore, the most robust survey/monitoring unit may be pairs that successfully hatch young. Distances at which owls could be reliably detected suggest that a survey of any defined area will require vantage points separated by a minimum distance of about 1.5 km with allowance for topography and a 180° field of view. Evidence of a disturbance or deterrence effect of field surveyors on the owls supports the need for unobtrusive survey methods.

Effects on bird abundance and species richness of edge restructuring to include shrubs at the interface between conifer plantations and moorland

Capsule Bird species richness and (for most species) abundance were positively related to the extent of shrub cover at the interface between conifer plantations and moorland, but it appears that responses to shrub development vary between different bird guilds. Aims To assess the bird assemblages in both winter and breeding seasons at the interface between managed conifer plantations and open moorland, where that interface had been restructured to include a mosaic of shrubs and open ground. Methods Timed point counts were used to sample the birds at restructured plantation – moorland interface areas and also in neighbouring plantations (post- and pre-thicket age classes) and neighbouring moorland. Associations between species richness and abundances with measures of shrub cover and composition were assessed using GLMMs. Results A total of 60 bird species were recorded including 29 on lists of conservation concern, most of which were associated with shrub interface habitats. Species richness and, for most species, abundance were positively related to the extent of shrub cover. Positive relationships between shrub cover in interface areas and the abundance of some species in neighbouring plantations and open moorland suggested a resource subsidy to birds in neighbouring habitats. In contrast, some birds tended to be less abundant in plantations next to areas with more shrub cover. These species were more abundant in the shrub itself, suggesting redistribution by species with a preference for early successional shrub habitats. Conclusions The long-term management of shrub, especially with regard to successional development, is a challenging aspect of forest and landscape management that deserves further study.

The influence of survey frequency on population estimates of moorland breeding birds

Capsule A minimum of four constant‐effort‐search survey visits are required to generate reliable population estimates of breeding birds on moorland that are not subject to biases associated with varying levels of detectability through the season. Aims To investigate the influence of the number and the combination of survey visits on the population estimates of breeding birds on moorland. Methods Four constant‐effort‐search surveys (80–100 minutes per km2 per visit) of moorland in southwest Scotland were undertaken in each of six years, 2003–2008. Using standard protocols, the numbers of apparent territories that would have been identified for each possible combination of survey visits were determined.glms were used to assess the influence of the frequency of survey visits, and different combination scenarios on the derived population estimates for Red Grouse, European Golden Plover, Common Snipe, Eurasian Curlew, Sky Lark, Winter Wren and Stonechat. Independent assessments of population density were made by transect sampling for Red Grouse and Sky Lark. Results Robust population estimates were possible from three survey visits for European Golden Plover, Eurasian Curlew and Stonechat. However, there were differences between species in the seasonal variation of their detectability. Four survey visits would underestimate the populations of Red Grouse (probably by 67–91%), Sky Lark (probably by 31–61%) and Winter Wren (by an undetermined proportion). Common Snipe were also likely to be underestimated after four survey visits, but the value of the derived estimate as an index of population density deserves further investigation. Conclusions If there is a need to carry out a multi‐species survey on moorland, we suggest that a minimum of four survey visits is required to ensure the derivation of reliable population estimates for a suite of the most readily detectable species. Population estimates derived from three or fewer survey visits risk biases through uneven sampling in periods of differing detectability. With evidence for changes in the breeding phenology of birds associated with changing climate or weather patterns, it arguably becomes more important to ensure that surveys sample an adequately broad period of the breeding season.

Forestry thinning in commercial conifer plantations has little effect on bird species richness and breeding abundance

Capsule Levels of thinning practised in upland Scotland are insufficient to permit the development of ground vegetation and shrub layers that might enhance some breeding bird populations.

Changes in breeding wader assemblages, vegetation and land use within machair environments over three decades

Capsule Changing agricultural activity and soil chemistry appear to be associated with some changes in breeding wader assemblages on the machair of the Outer Hebrides, Scotland, between the 1980s and 2000s. Aims To identify potential factors in addition to nest predation by Hedgehogs that could have driven changes in machair breeding wader assemblage of the Western Isles. Methods Surveys of breeding waders in the 1980s, 1990s and 2000s and of vegetation in 1976 and 2009 were carried out on machair habitats on the west coast of Uist (four study sites). Generalized linear mixed models were used to identify associations between: (1) breeding wader densities in 2007–2010 and vegetation metrics (Ellenberg indicator values, axes determined by detrended correspondence analyses and structural measures) derived from the 2009 vegetation survey and (2) changes in breeding wader densities between 1983–1987 and 2007–2010 and changes in vegetation metrics derived from surveys in 1976 and 2009. Reported changes in agricultural use were assessed from interviews with local land managers to assess the likely influence of land-use changes on vegetation and breeding waders. Results Changes in population densities of breeding waders differed between species and sites over the three decades. Overall, Eurasian Oystercatcher and Common Redshank increased. Common Ringed Plover, Dunlin and possibly Common Snipe decreased and there were no significant changes for Northern Lapwing. Changes in vegetation indicated relatively small decreases in soil fertility of machair grassland cultivation, increased soil salinity and a decrease in overall vegetation cover. Higher breeding wader densities were associated with higher values of soil moisture, pH, fertility, salinity, vegetation height, vegetation density and extent of cultivation with inter-specific differences mostly associated with vegetation structure. Where soil fertility declined, Ringed Plover and Dunlin tended to decline while Oystercatcher increased. Temporal changes in Lapwing density were positively associated with changes in soil moisture. Ringed Plover density changes were positively associated with indicators of vegetation shade intolerance, soil acidity, soil fertility and reduced soil salinity. Changes of Redshank were positively associated with vegetation height. Reported changes in land use included a reduction in the number of people practising cultivation, though not necessarily in the area cultivated, reductions in numbers of grazing cattle and increases in numbers of sheep, a switch from using seaweed and farmyard manure to using compound fertilizers and deeper ploughing. Conclusions Although pointing to some interesting changes in machair systems associated with soil fertility, moisture and salinity that may have affected breeding waders, possibly acting through alterations to food supplies, further understanding of how waders use the mosaic of habitats is required to understand potential mechanisms. The importance of vegetation changes relative to, and interacting with, predation deserves further attention.

Changes in the breeding wader populations of the machair of the Western Isles, Scotland, between 2000 and 2007

Capsule The machair of the Uists continues to support one of the most important assemblages of breeding waders in Europe, but there have been major variations in population change across the archipelago, the causes of which are poorly understood.

Conservation management of moorland: a case study of the effectiveness of a combined suite of management prescriptions which aim to enhance breeding bird populations

Capsule A specific suite of moorland management prescriptions had no clear positive impact on breeding birds over ten years. Aims To test the expectation that a combination of grazing, and burning/cutting manipulations, the blocking of drains (affecting habitat quality and foraging resources) and the control of some predators (affecting survival and breeding success) will lead to an increase in numbers of breeding birds on 9.5 km2 of moorland in Scotland. Methods General Additive Modelling was used to compare bird population trends at the managed site against a ‘control’ of moorland-specific background trends (spatially and temporally smoothed) derived from an extensive UK-wide bird monitoring programme. Changes in vegetation, the effectiveness of predation control and influences of disturbance were also examined to attempt to identify potential mechanisms for population size changes. Results Breeding populations for most species showed either a more negative trend at the managed site (Red Grouse, Skylark and Meadow Pipit) or no significant difference was detected (European Golden Plover, Eurasian Curlew, Common Snipe and Winter Wren) when compared to moorland-specific background trends. Stonechats and Carrion Crows increased, despite the latter being actively removed as part of predation control measures. No general change in the vegetation was detected and no reasonable habitat-based cause for the observed declines in some species was apparent. There was also no convincing evidence for a negative effect on moorland birds of disturbance from a nearby mine. As crow control failed to stem their increase and with supportive evidence for a decline in the breeding success of Red Grouse, it is suggested that the control of predation was ineffective and may have contributed to some species declines. Conclusion There has been wide advocacy of, and considerable resources (e.g. through agri-environment schemes) devoted to, moorland habitat management for birds and yet declines continue. However, there are practical difficulties of establishing management that is effective in increasing moorland bird abundance. It appears likely that in areas where those birds are vulnerable to predation and its control is not effective, expectations of increasing bird populations in response to habitat management alone may be unrealistic.

The decline of a population of farmland breeding waders: a twenty-five-year case study

ABSTRACT Capsule: Changes in sward height, driven by changes in management, were associated with a large decline in a population of farmland breeding waders. Aims: To examine the relationship between changes in habitat and numbers of breeding wader on an area of Scottish farmland over the last 25 years. Methods: Nesting waders in a core survey area of 7.5 km2 were monitored annually from 1990 to 2015. An additional 10.3 km2 were monitored less frequently. Habitat characteristics of each field were recorded and breeding success by Lapwing determined in a sample of fields. Results: All species showed large declines over the study period, Oystercatcher Haematopus ostralegus by 95%, Lapwing Vanellus vanellus by 88%, Curlew Numenius arquata by 67% and Redshank Tringa totanus by 87% from peaks of 125, 365, 57 and 53 pairs, respectively. Changes in spring sward height, considered to be due to changes in crop type, were associated with changes in the numbers of breeding waders over time. Productivity by a sample of Lapwings was unchanged through the study period. Conclusion: Short swards, especially bare till, in spring appear to have been important in contributing to the maintenance of an assemblage of breeding waders in mixed arable-pasture farmland. The decline appeared greater than could be accounted for by losses of preferred habitats alone.