British Breeding Population Research Articles

From a research study to a conservation partnership: Developing approaches to restoring common scoter populations

Abstract Like many sea‐duck populations, the British breeding population of common scoters Melanitta nigra has declined markedly. In 2009, a study was established to measure factors affecting lake use by breeding scoters, to inform conservation measures. That study, published in Aquatic Conservation: Marine and Freshwater Ecosystems (AQC) in 2016, showed that lakes used by scoters had relatively more shallow water and large invertebrates. Furthermore, lakes with abundant large invertebrates had relatively few brown trout Salmo trutta. These results probably reflect the importance of foraging habitat (shallow water), prey abundance (large invertebrates), and competition for prey (trout abundance) in influencing scoter lake use. To develop this study, additional research, and scoter conservation measures, we formed a long‐term partnership, comprising nature conservation non‐governmental organizations, statutory agencies, fisheries managers and the regional hydropower company. The partnership meets regularly to coordinate and review research and develop conservation measures, building on the earlier lake‐use study. At key scoter lakes managed for hydropower, the partnership negotiated a revised water level management regime, favouring shallow water availability in the breeding season. This regime was informed by additional research, showing the value of certain islands for nesting scoters, and water level requirements to maintain these as islands. In a different key scoter area — an extensive near‐natural peatland where recreational angling takes place — the partnership established a trial of increased brown trout angling, with increased fish removal, to determine whether this would increase invertebrate prey abundance and benefit breeding scoters. Both conservation interventions are currently being evaluated by detailed monitoring. Overall, the earlier study published in AQC, was important in strengthening and informing our researcher–practitioner partnership, as we collaborated within an adaptive management framework to reverse scoter declines. Long‐term interdisciplinary partnerships are likely to have an important role to play in general, for the successful conservation of aquatic biodiversity.

Weak migratory connectivity, loop migration and multiple non‐breeding site use in British breeding Whinchats Saxicola rubetra

Determining the links between breeding populations and the pressures, threats and conditions they experience presents a challenge for the conservation of migratory birds which can use multiple sites separated by hundreds to thousands of kilometres. Furthermore, migratory connectivity – the connections made by migrating individuals between networks of breeding and non‐breeding sites – has important implications for population dynamics. The Whinchat Saxicola rubetra is declining across its range, and tracking data from a single African non‐breeding site implies high migratory spread. We used geolocators to describe the migration routes and non‐breeding areas of 20 Whinchats from three British breeding populations. As expected, migratory spread was high, with birds from the three populations overlapping across a wide area of West Africa. On average, in non‐breeding areas, British breeding Whinchats were located 652 km apart from one another, with some likely to share non‐breeding areas with individuals from breeding populations as far east as Russia. Four males made a direct non‐breeding season movement to a second, more westerly, non‐breeding location in January. Autumn migration was through Iberia and around the western edge of the Sahara Desert, whereas spring migration was more direct, indicating an anticlockwise loop migration. Weak migratory connectivity implies that Whinchat populations are somewhat buffered against local changes in non‐breeding conditions. If non‐breeding season processes have played a role in the species’ decline, then large‐scale drivers are likely to be the cause, although processes operating on migration, or interactions between breeding and non‐breeding processes, cannot be ruled out.

Breeding density, fine-scale tracking, and large-scale modeling reveal the regional distribution of four seabird species.

Population-level estimates of species' distributions can reveal fundamental ecological processes and facilitate conservation. However, these may be difficult to obtain for mobile species, especially colonial central-place foragers (CCPFs; e.g., bats, corvids, social insects), because it is often impractical to determine the provenance of individuals observed beyond breeding sites. Moreover, some CCPFs, especially in the marine realm (e.g., pinnipeds, turtles, and seabirds) are difficult to observe because they range tens to ten thousands of kilometers from their colonies. It is hypothesized that the distribution of CCPFs depends largely on habitat availability and intraspecific competition. Modeling these effects may therefore allow distributions to be estimated from samples of individual spatial usage. Such data can be obtained for an increasing number of species using tracking technology. However, techniques for estimating population-level distributions using the telemetry data are poorly developed. This is of concern because many marine CCPFs, such as seabirds, are threatened by anthropogenic activities. Here, we aim to estimate the distribution at sea of four seabird species, foraging from approximately 5,500 breeding sites in Britain and Ireland. To do so, we GPS-tracked a sample of 230 European Shags Phalacrocorax aristotelis, 464 Black-legged Kittiwakes Rissa tridactyla, 178 Common Murres Uria aalge, and 281 Razorbills Alca torda from 13, 20, 12, and 14 colonies, respectively. Using Poisson point process habitat use models, we show that distribution at sea is dependent on (1) density-dependent competition among sympatric conspecifics (all species) and parapatric conspecifics (Kittiwakes and Murres); (2) habitat accessibility and coastal geometry, such that birds travel further from colonies with limited access to the sea; and (3) regional habitat availability. Using these models, we predict space use by birds from unobserved colonies and thereby map the distribution at sea of each species at both the colony and regional level. Space use by all four species' British breeding populations is concentrated in the coastal waters of Scotland, highlighting the need for robust conservation measures in this area. The techniques we present are applicable to any CCPF.

Colonization and extinction dynamics of a declining migratory bird are influenced by climate and habitat degradation

Uncovering the mechanisms involved in the decline of long‐distance migrants remains one of the most pressing issues inEuropean conservation. Since the 1980s, theBritish breeding population ofGardenWarblerSylvia borinhas declined by more than 25%. Here we use data from repeated bird surveys of woodland sites in the 1980s and in 2003–2004 to show that, although the overall population declined between the two periods, the probability of occupancy for this species increased at high latitudes and decreased at low latitudes. Range shifts such as this arise from a change in the ratio of colonizations to extinctions at the range margins, and we therefore related colonization and local extinction at the patch level to concurrent changes in temperature and habitat. The probability of patch colonization by this species was significantly lower where the percentage cover of vegetation in the understorey had declined, reducing habitat quality for this species. The probability of local extinction was significantly correlated with increasing mean May temperature, which may reflect a change in phenology, making breeding conditions less suitable. Changed regimes of grazing and woodland management could be used to increase habitat suitability and thereby increase colonization probability at the local scale, which may in turn increase the probability of patch occupancy despite future climatic unsuitability.

The size and migratory origins of the population of Hen Harriers Circus cyaneus wintering in England

Capsule The majority of Hen Harriers Circus cyaneus that spend the winter in England are derived from the British breeding population.Aims To investigate the popular hypothesis that Hen Harriers which over-winter in England are derived principally from breeding populations in mainland Europe.Methods Demographic data were used to estimate numbers of Hen Harriers in Britain at the end of the breeding season. This figure was used, together with data from winter (October–March inclusive) recoveries of Hen Harriers ringed in Britain during the breeding season, to estimate the number of birds from the British breeding population wintering in England and in other parts of Europe.Results It was estimated that Hen Harriers wintering in England are derived primarily from British breeding populations. Published winter population figures for British Hen Harriers may be significant underestimates.Conclusion The link between breeding and wintering populations of Hen Harriers in Britain is closer than previously thought. Conservation action should be spread more equally between the environmental factors which limit populations in different seasons. The winter population in Britain, as monitored by the Hen Harrier Winter Roost Survey, may provide a good indication of the status of the breeding population.

Declines in British-breeding populations of Afro-Palaearctic migrant birds are linked to bioclimatic wintering zone in Africa, possibly via constraints on arrival time advancement

Capsule The bioclimatic zone and habitat type within which birds winter are the most important determinants of population trends.Aims To investigate whether regional factors on wintering grounds, phenological mismatch, or habitat on breeding or wintering grounds show relationships with population changes of Afro-Palaearctic migrant birds.Methods We modelled breeding bird survey trends of 26 species of Afro-Palaearctic migrant birds that breed in Britain, and assessed the most important variables. We also investigated spatial variation in population trends within the UK.Results Bioclimatic zone and wintering habitat type were the most important determinants of population trend. After accounting for the effects of the other variables, species that spend the winter in southern Africa or the humid tropics of central and western Africa showed more negative population trends than those that winter in the arid zone. Species occupying open and woodland habitats declined and generalist species increased, on average. We also suggest that the effect of bioclimatic zone on population trends may be through constraints on the timing of migration.Conclusion Correlations between population change and both wintering area and winter habitat suggest regional changes in climate or land-use in the humid tropics are driving declines in many long-distance migrant species, possibly partly through migratory constraints.

Status of Red‐throated Divers Gavia stellata in Britain in 2006

Capsule The population of Red‐throated Divers in Britain increased between 1994 and 2006. Aim To determine the population size and distribution of Red‐throated Divers in Britain for comparison with the previous national survey in 1994. Methods All freshwater bodies in Shetland, Orkney and two Special Protection Areas (SPAs) in the Hebrides were censused for Red‐throated Divers. A stratified sample of 5‐km squares was surveyed throughout the rest of the range. Two visits were made to all suitable habitats, at least 2 weeks apart, between 15 May and 31 July. Results The British breeding population of Red‐throated Divers was estimated to be 1143 breeding pairs, although around 12% of adults recorded as non‐breeding may actually have bred. Thus, a corrected estimate was calculated giving 1255 (95% CI 1014–1551) pairs. The total population of adults was estimated to be 4146 (95% CI 3430–4992). Shetland held 33% and the Outer Hebrides 26% of the breeding population. The estimated breeding population and total population of adults increased significantly, by 34% and 38% respectively, since the first national survey in 1994. The population remained stable in Shetland and Orkney, but there was a significant increase across the remainder of the range. Conclusion The breeding population of Red‐throated Divers in Scotland has increased significantly since the last survey in 1994, as has the total number of adults. While the population in the Northern Isles remained stable, there was a significant increase in the breeding and total population away from the Northern Isles.

Breeding status of the Red Kite Milvus milvus in Britain in 2000

Capsule The first co-ordinated Red Kite survey across Britain since the reintroduction programme began in 1989, yields 430 breeding pairs. Aims To estimate the current size and extent of the British breeding population. Methods A complete census of the populations in the East Midlands, Yorkshire, central and north Scotland was undertaken, while in Wales and the Chilterns, populations were surveyed using a stratified sample of tetrads. Breeding and territorial pairs were identified. Results The survey indicated that there were 430 breeding pairs in Britain (95% CIs, 372–490). There were 259 breeding pairs in Wales (95% CIs, 200–318) and 109 (95% CIs, 96–124) in the Chilterns. Elsewhere, 16 breeding pairs were located in the East Midlands, three in Yorkshire, seven in central Scotland and 33 in north Scotland. An additional three pairs were recorded in southern England, away from the main population centres. Conclusion The 2000 survey provides a baseline against which to measure future changes in Red Kite populations in Britain, using standard, repeatable methods.

Densities and population estimates of breeding Skylarks Alauda arvensis in Britain in 1997

We present the results of a national survey of breeding Skylarks Alauda arvensis in Britain in 1997 carried out by the British Trust for Ornithology (BTO). Numbers of Skylarks and land-use types were recorded by volunteers in 608 1-km squares, selected using random stratification based on the Institute of Terrestrial Ecology's (ITE) Landscape Classification to avoid over- or under-sampling particular habitat types. The results suggest a maximum national population of around 1000 000 pairs. This agrees extremely well with the national population estimate of around 1046 000 pairs derived from the Breeding Bird Survey (BBS) of the BTO/Joint Nature Conservation Committee/Royal Society for the Protection of Birds, and provides further evidence for the robustness of the BBS as a national monitoring scheme for common and widespread species. The figure is approximately half that of the most recent published estimate of 2000 000 in 1988–91. However, the scale of this discrepancy is likely to reflect a bias in Atlas field methodology and site selection since there has been a real decline of approximately 9% since 1990. Arable squares supported the highest densities of Skylarks; 4.6–6.0 pairs per km2, and 46–49% of the British breeding population was associated with arable areas. Marginal upland and upland areas supported lower densities but still accounted for approximately 34% of the estimated national breeding population. Differences in density at broad habitat scales were reflected in geographical differences across Britain, with southern and eastern arable regions supporting much higher densities than western and northern ones. At a finer scale, Skylarks occurred at highest densities on, and showed highest habitat preferences for, set-aside and various types of ungrazed grassland. Winter cereal, improved grassland and set-aside held the highest proportion of the Skylark population on farmland in England and Wales; grazed pasture, winter cereals and spring cereals held the highest proportion in Scotland.

Habitat selection and population size of Corn BuntingsMiliaria calandrabreeding in Britain in 1993

This paper presents the results of a survey of breeding Corn Buntings in Britain in 1993. Numbers of Corn Buntings and land–use types and field boundaries present were recorded by volunteers from over 1300 tetrads selected across the range of the species in Britain. The British breeding population was estimated at around 20 000 territories. Bootstrapped 95% confidence limits differed from the calculated estimates by approximately 20%. Population sizes in different regions of Britain were positively correlated with the proportion of total farmland under tillage and cereals and with the total areas of tillage and cereals, but not with the proportion of tillage made up by cereals nor with the overall area of farmland. Densities in occupied 10-km squares were lower in regions which experienced the greatest declines in distribution between 1968–72 and 1988–91. Areas abandoned by breeding birds between 1968–72 and 1988–91 were higher in altitude and held more grassland and less tilled land than areas where bird...

Distribution and status of wintering ShovelersAnas clypeatain Great Britain

The distribution and status of Shovelers using British wetlands in autumn and winter was examined using ringing recoveries and National Waterfowl Count data. Maximum counts have been around 8500 and suggest a total population of c. 10 000. Some of these birds originate from north-west continental Europe, eastern Scandinavia and the Western former USSR, whilst others originate from British breeding populations. Numbers peaked in October, after which there was a steady emigration from Britain as birds moved to France, the Iberian Peninsula and Mediterranean for the winter. The population fell sharply during the early 1960s and then increased up until 1971/72. Since then there has been a gradual, though slight, decline. The number counted decreased nationally during periods of cold weather, suggesting cold weather emigrations. Wintering Shovelers have increased their usage of reservoirs and mineral workings, and slightly decreased their usage of natural lakes. Patterns of seasonal occurrence varied between regions and habitats. Numbers peaked early in northern Britain and progressively later in regions to the south. An early season peak was evident on natural lakes and reservoirs, but occurred progressively later on gravel pits, on the coast and on floodlands. Shovelers were recorded at 10–20% of the c. 1800 sites visited during the period 1985/86 to 1989/90 and, of these, 84% held fewer than 50 birds. A total of 51 sites supported nationally important numbers (>90) and these held 46–¸70% of the population. Whilst protection of these sites is a high priority for the conservation of this species, wider countryside policies are needed to maintain the population throughout its range in Britain.

Rapid microevolution of migratory behaviour in a wild bird species

THE Blackcap, Sylvia atricapilla, a widespread Palearctic migratory bird, rarely wintered in Britain until the 1950s. The winter population has since increased to several thousand birds1,2. Ringing indicates that these are not British Blackcaps forestalling migration, but birds breeding in Continental Europe reaching Britain on a novel westerly migration route3,4. The proportion of north-western migrants among Blackcaps ringed in parts of Germany and Austria has increased from 0% before 1960 to currently 7–10%5–7. We bred British wintering Blackcaps in captivity and determined the migratory direction of their offspring. Here we report that these birds migrate west-northwest in autumn, a direction genetically distinct from the British breeding population and the predominantly southwestern migratory population of west–central Europe. The novel route must have evolved within the past 30 years with selection favouring birds wintering some 1,500 km further north than most of their conspecifics. To our knowledge, this is the first case in any vertebrate in which a drastic and recent evolutionary change of behaviour has been documented and its genetic basis established.

Breeding Populations of Ringed Plovers and Dunlins in the Uists and Benbecula, Outer Hebrides

Some 780–950 pairs of Ringed Plovers and 310–375 pairs of Dunlins breed in these three Outer Hebridean islands, representing significant proportions of their British breeding populations. Habitat preferences are discussed.