Five seabird species are added to the Birds of Conservation Concern Red List in this addendum to the 2021 update, bringing the total number of Red-listed seabird species to 10, up from six since seabirds were last assessed. The Amber List of seabirds moves from 19 to 14 species, and the Green List increases from one to two species.

Accumulation of plastic and waste contamination in the natural environment is a growing problem, but little is known of the role animals play in transporting plastics (a process known as ‘biovectoring’) particularly to inland waters and habitats. This study used diet sampling, population monitoring and GPS tracking data of Lesser Black-backed Gulls to monitor the transport of plastics and anthropogenic debris from landfills to an inland reservoir in Spain. The study, led by the Institut de Ciències del Mar and Estación Biológica de Doñana, CSIC, Spain, used diet samples collected in January 2022 at the Fuente de Piedra – an inland lake in Spain that is internationally important for biodiversity. Monthly counts of Lesser Black-backed Gulls were carried out to assess the numbers of birds present at the lake. GPS tracking data collected from studies across Northern Europe, including some led by BTO, were used over 12 ‘winters’, between January to March 2010–22, to understand the spatial movements of individual birds in the local region, coinciding with when birds are either on wintering grounds or on migration. The authors found that a high proportion (94%) of material regurgitated by gulls contained human-related material such as glass and textiles and 86% contained plastics, whereas only 1% plastic mass was present in faeces. Numbers of Lesser Black-backed Gulls at the lake were highest in January, peaking at a maximum count of 33,000 in January 2020. Agricultural habitats were most frequently used by gulls (88% time) as well as three landfill sites in the local area, with some landfills visited more than others, linked to use of bird deterrence measures. A total of 77% of the tracked birds visited at least one of the three landfills on a given day. The authors estimated that, on average, the annual deposition of plastic and other debris by the wintering gull population into the lake was 1,888 g per day, meaning that 400 kg of plastic were estimated to be deposited into the lake each winter, though only ca. 5 kg were from faeces. This study sheds new light on the importance of gulls for biovectoring within ecosystems, and the leakage of plastic and debris from landfills into inland waters. The plastics can accumulate in sediments and become incorporated into food webs, presenting a global contamination issue that needs addressing. The study further highlights the importance of adopting initiatives to tackle the problem, such as waste separation and plastic reduction measures, and bird-deterrence to reduce ingestion by birds at source.

In an effort to cut carbon emissions, many countries (including the UK) are constructing ever more offshore wind farms. Seabirds are at risk of colliding with the blades of offshore wind turbines, posing the potential for injury or death. However, the extent of this risk depends on several factors including how high the seabirds fly in relation to the area swept by turbine blades, the seabirds’ flight speeds, and any avoidance behaviour they undertake in response to either individual turbines or entire wind farms. Accurate understanding of these behavioural aspects of seabirds is important when assessing the potential population impact an offshore wind farm will have through so-called Collision Risk Models (CRMs). As previous BTO research has shown, bird-borne tracking devices are essential for shedding light on seabird flight behaviour. Different methods for gathering flight height information within tracking devices can influence the altitudes they produce, with implications for the collision estimates ultimately produced. This study compared the flight height altitudes produced from GPS and barometric altimeter data from tagged Lesser Black-backed Gulls to investigate this matter. Novel methods for calibrating GPS and altimeters were trialled to reduce error within the altitudes calculated by these methods as much as possible. The data were collected by attaching tracking devices to adult Lesser Black-backed Gulls captured while they were incubating their eggs at two breeding colonies (the Isle of May in Scotland and Havergate Island in England) in 2019 and 2020. Tags contained both GPS, which estimates a bird’s position using a network of satellites, and altimeters, which use air pressure measurements to estimate flight height. These two systems enabled flight heights obtained from either method to be compared. The results showed that altimeter data, when combined across years and colonies, produced higher flight heights than GPS data, and that this difference was more marked at higher sampling frequencies (e.g. one data point every 10 seconds) than at lower ones (e.g. one data point every five minutes). This in turn led to a greater number of collisions being predicted by CRMs for altimeter data than for GPS data. However, when GPS-altimeter data was split by sampling rate, year, and colony, flight heights from altimeters were not consistently higher, indicating local environmental conditions play a role influencing the accuracy of altitudes recorded. Tracking devices can provide information on flight height across a range of behaviours and environmental conditions. Improvement of calibration steps to account for the influence of the environment on tracking devices will help improve the accuracy of flight height information collected in future. This will increase confidence in the use of tracking devices in future assessments of collision risk and help to better predict mortality associated with wind farms. Forming a better understanding of the impacts of wind farms on seabirds will help ensure their appropriate development in future throughout the North Atlantic.

A collaboration between BTO, Natural England and University of Exeter researchers has used GPS-tracking technology to compare the movements and habitat use of Lesser Black-backed Gulls breeding at neighbouring coastal and urban colonies in Cumbria, northern England. The study found that the distance birds covered making foraging trips and the size of their 'home ranges' - the area that an individual bird uses on a daily basis during foraging, breeding and roosting activities - differed between the urban and coastal colonies and revealed a marked segregation in habitat choice.

GPS tracking is a valuable tool for increasing our understanding of bird behaviour. In this study, researchers used tracking technology to investigate movements of Amber-listed Lesser Black-backed Gulls around offshore wind farms. Their results reveal a detailed picture of both avoidance of and attraction towards turbines in this species, which may be used to inform future collision risk assessments.

Offshore wind farm developments form a major part of the UK government’s commitment to obtain 15% of the UK’s energy needs from renewable sources by 2020. However, there is concern over the potential detrimental effects that offshore developments may have on bird populations.

Initial findings suggest that Lesser Black-backed Gulls in north-west England fly within wind farms, but may avoid wind turbines once there. Offshore wind farms may affect birds in many ways, such as stopping them moving between places, or restricting access to areas where they feed. Collision risk is a key concern for seabirds, yet there has been little study to date of the use of wind farms by birds from known breeding colonies. This study, led by BTO, provides a first look at the movements of Lesser Black-backed Gulls at a colony in north-west England and their interaction with five operational offshore wind farms. The birds were tracked with GPS tags which recorded location, speed and height. Out of the 24 birds tracked, 15 visited wind farms, and in total spent 1.3% of their time there. A detailed look at the movements of two birds that visited wind farms most often, showed that their flight heights were greater within wind farms than outside, particularly during the day, potentially placing them at greater risk of collision. However, within wind farms, their use of the zone swept by the rotor blades was significantly less than expected. Although preliminary, these results suggest that Lesser Black-backed Gulls at this site may not avoid wind farms, but could avoid individual wind turbines once inside the wind farms. Further work is now being conducted to verify these patterns, which will be important in improving our understanding of the likely collision risk for seabirds.

This annex to BTO Research Report 694 (Clewley et al. 2016) presents a summary update for 2017 on the results of a GPS tracking study of Lesser Black-backed Gulls Larus fuscus breeding in the Bowland Fells Special Protection Area (SPA).

Within an avian breeding population, there can be considerable variation in egg and nest site characteristics that have implications for individual reproductive success. Here we present a detailed case study of Lesser Black-backed Gulls Larus fuscus nesting on Flat Holm island, Wales, at a time when the colony was growing. This species is ground-nesting, with a modal clutch size of three. We surveyed 714 nests across the island during two consecutive years and recorded data on nest and egg characteristics, along with hatching success. We modelled how hatching success was associated with clutch size, egg volume, egg laying order and local habitat features, i.e. the amount of vegetation surrounding the nest and each nest's proximity to neighbouring nests. Eggs were most likely to hatch when they were laid in the middle of the season, were large in size, part of big clutches and in nests with a substantial amount of surrounding vegetation. Lesser Black-backed Gull productivity is currently low in many protected rural and coastal colonies throughout this species' range. Detailed information on factors influencing reproductive success could therefore indicate ways in which this species could be better managed to help maintain and conserve breeding populations. Within an avian breeding population, there can be considerable variation in egg and nest site characteristics that have implications for individual reproductive success. Here we present a detailed case study of Lesser Black-backed Gulls Larus fuscus nesting on Flat Holm island, Wales, at a time when the colony was growing. This species is ground-nesting, with a modal clutch size of three. We surveyed 714 nests across the island during two consecutive years and recorded data on nest and egg characteristics, along with hatching success. We modelled how hatching success was associated with clutch size, egg volume, egg laying order and local habitat features, i.e. the amount of vegetation surrounding the nest and each nest's proximity to neighbouring nests. Eggs were most likely to hatch when they were laid in the middle of the season, were large in size, part of big clutches and in nests with a substantial amount of surrounding vegetation. Lesser Black-backed Gull productivity is currently low in many protected rural and coastal colonies throughout this species' range. Detailed information on factors influencing reproductive success could therefore indicate ways in which this species could be better managed to help maintain and conserve breeding populations.

This report builds on Ross et al. (2016) using computer simulations to examine how survey coverage affects the precision of population estimates. This work will provide a first step towards improving the population estimates of urban gulls, assuming surveys go ahead in the future. Includes annex: Results for Northwest England. To support delivery of the latest census of the breeding seabird population of Britain and Ireland, a previous Natural England commissioned report (Ross et al. 2016) reviewed the survey methods in order to make recommendations for the most cost-effective survey design for quantifying (urban) gull abundance in UK and Ireland. Within that report, recommendations were made on the expected necessary coverage, with reference to previous analogous bird surveys across similar geographic scales. This report builds on Ross et al. (2016) using computer simulations to examine how survey coverage affects the precision of population estimates. This work will provide a first step towards improving the population estimates of urban gulls, assuming surveys go ahead in the future.

New research led by the BTO has used a combination of GPS-tracking and advanced statistics to provide new insights into seabird flight heights by night and day. This study gives important information on the risk of seabirds colliding with offshore wind turbines and at a time when governments worldwide are investing in offshore wind farms. Offshore wind farms are now operating or under construction in many locations, but while spinning turbine blades are crucial for generating renewable energy, they also represent a potential threat for flying animals, particularly seabirds, which can be injured and killed if they collide with them. In order to correctly characterise the extent of this threat, we need accurate measurements of the height at which seabirds fly. New research by the BTO has demonstrated a novel way to do this, using state-of-the-art GPS tags and cutting edge statistics. The work, led by Viola Ross-Smith and Ali Johnston, used data downloaded from long-life GPS tags fitted to Lesser Black-backed Gulls and Great Skuas breeding at protected colonies in Suffolk, Orkney and Shetland. GPS tags record flight height with error, which varies depending on the configuration of satellites in the sky at the time each measurement is taken. The study solved this error problem using a powerful statistical technique known as a Bayesian state-space model. The analysis also examined the association between a bird’s location and its flight height, as well as how flight behaviour varies between day and night. The results showed that Lesser Black-backed Gulls flew significantly lower at sea than over land, and significantly lower at night, when turbines might be harder to detect and avoid, than during the day. Great Skuas flew lower than Lesser Black-backed Gulls, but did not show significant relationships between flight height, location and time of day. Importantly, the study found that both species, but particularly Great Skuas, primarily flew beneath the height of offshore wind turbine blades. The results also support previous BTO research in suggesting that Lesser Black-backed Gulls and Great Skuas are safer when turbines are designed so that the lower edge of the blades is 30 m above sea level (as opposed to some current turbine designs, where the blade tips reach 22 m above sea level). This study describes an effective way of resolving the problem of error associated with GPS tracking data, which can be used to provide important information on seabirds’ risk of collision with offshore wind turbines. This approach could be applied more widely, to reliably model animal movement at a time when tracking is increasingly being used as a tool to inform management and conservation. For further information on the modelling used in this study, please see this blog for the Journal of Applied Ecology.

Report of work carried out by the British Trust for Ornithology in collaboration with the Environmental Research Institute on behalf of the Marine Scotland Science. Accurately estimating birds’ risk of collision with offshore wind turbines is a key part of the decision-making process for proposed renewable developments. However, the evidence base for quantifying the number of birds likely to avoid colliding with turbines is limited. Recent BTO-led work helping to fill this gap, improving the understanding of the impacts of offshore renewables on marine wildlife.