Native and non-native deer populations are increasing in temperate woodlands across Europe. Previous work by the BTO found that these changes are associated with declines in some breeding bird populations, and suggested this is due to modification of the woodland understorey through browsing (consumption of vegetation by herbivorous deer). Now a new study by the BTO and the University of East Anglia has demonstrated effects of deer activity on birds outside the breeding season. Birds were monitored in several plots of woodland, half of which were fenced to exclude deer. Common species such as Robin, Wren and Blackbird were less likely to be found in woodland browsed by deer than at unbrowsed sites in the winter, although this effect was not seen in the spring. The detrimental impact of deer browsing in the winter could result from both reduced shelter from the cold, and a loss of protection from predators afforded by dense understorey vegetation. There could also be changes in the feeding opportunities available to birds once the understorey has been browsed. This study illustrates the importance of understanding the complex interactions between deer and birds at a time when many woodland specialists are in decline, and the need for more research to be undertaken in woodland outside the typically-studied breeding season.

Mammals are an important part of the UK’s biodiversity but can be difficult to monitor, requiring special surveys to determine their distribution or how their populations are changing over time. However, some widespread and easily detected species, such as deer, rabbits and squirrels, can be monitored as part of bird surveys. Volunteer birdwatchers taking part in the Breeding Bird Survey have been recording mammals alongside birds since 1995. Now BTO scientists have used these data to investigate national and regional population changes of nine common species. Populations of four deer species (Roe Deer, Reeves’ Muntjac, Fallow Deer and Red Deer) all increased between 1995 and 2012. This is of concern given that increased deer browsing in woodland can reduce the vegetation understorey and impact negatively on other wildlife. In contrast, Rabbit numbers declined by 48% over the same period, following a large increase prior to 1995 (as revealed by other studies). There are fascinating regional differences in the trends for some species; Brown Hare populations declined by 50% in Northwest England but increased by 70% in the East Midlands. A significant increase in non-native Grey Squirrel numbers in Northwest England may be of concern because of their potential impacts on native Red Squirrels that are now largely confined to Scotland. The study highlights the value of the BTO’s network of volunteers for monitoring groups other than birds. The species covered include some rapidly spreading invasive non-native species, and others that are of conservation concern. Many important conservation questions could be investigated in future using this valuable long-term dataset.

New research led by the BTO shows that the UK’s internationally important seabird populations are being affected by fishing activities in the North Sea. Levels of seabird breeding failure were higher in years when a greater proportion of the North Sea’s sandeels, important prey for seabirds, was commercially fished. The UK’s seabirds are under pressure from human activities, such as resource extraction and fishing, as well as climate change. Under the European Marine Strategy Framework Directive, the UK is legally bound to make sure human activities are kept at levels consistent with “clean, healthy and productive” seas, and as top predators, monitoring seabirds can give insights into the state of the wider marine environment. In many species, counts of breeding individuals reflect population-level impacts of environmental pressures, but this is not necessarily the case with seabirds. This is because seabirds are long-lived and can delay breeding for several years after they reach maturity, or skip breeding seasons when conditions are poor. Scientists at the BTO and JNCC have now shown monitoring seabird breeding performance to be the way forward. The study, using long-term datasets from the JNCC’s Seabird Monitoring Programme for nine seabird species, showed the knock-on effects of fishing activities in the North Sea on seabird breeding at colonies on the east coast of England and Scotland. Sandeels are typically fished for use in animal feed and fertilizer. There is a large fishery on Dogger Bank, which is within the foraging range of many seabirds. In years when a greater proportion of the North Sea’s sandeels was fished, rates of seabird breeding failure rose. The study also found that seabirds breeding on the UK’s western colonies are faring better than those on the North Sea coast. Population declines and elevated breeding failures were found for eight out of nine species at North Sea colonies (with Kittiwakes particularly badly affected), compared to three out of nine on the west coast. The results demonstrate that seabird breeding can show how these key species are responding to environmental pressures before such changes become evident at the population level. Detecting such impacts as early as possible is a priority, as the management of the marine environment is changing, with expansion of offshore developments, the introduction of marine protected areas, and modification of fishing discards policy.

New research by the BTO reveals that most seabirds fly near the sea surface, avoiding collision with wind turbines by flying under the blades. Those birds that fly higher above the sea are at greater risk of collision. Building offshore turbines higher above the sea surface, or installing fewer large turbines instead of several smaller turbines, could reduce the number of collisions. In a project funded by The Crown Estate via the Strategic Ornithological Support Services (SOSS) work programme, BTO scientists examined the importance of flight heights in determining the risk posed to seabirds from collision with offshore wind turbines. Innovative statistical techniques were used to combine data from over 30 sites and a detailed description of the proportion of birds that fly at different altitudes were produced for each species. Results show that many species, including Puffins and Arctic Terns, spend most of their time within 5 m of the sea surface, while gulls flew more regularly at 20 m above the sea surface. These findings are particularly critical, in the light of the high number of offshore wind farms which are currently proposed in UK waters. Although wind farms are a key part of the government’s strategy to meet its renewable energy targets, they may potentially affect local birds, including protected and declining species. In severe cases, birds can collide with turbine blades and die. Estimates of the proportion of birds flying at different heights are fed into an assessment of the potential impact on wildlife for each wind farm, and can influence the likelihood that proposed renewable developments will go ahead. The results presented in this study show that if turbines are located higher above the sea surface, more birds will naturally fly underneath the turbine blades. The authors also suggest that seabird collision risk could be reduced by installing fewer but larger turbines, which produce the same energy output as a greater number of smaller turbines.