Aarhus Universitets segl

No. 28: Baseline investigations of bats and birds at Wind Turbine Test Centre Østerild - bird

Therkildsen, O.R., Elmeros, M., Kahlert, J. & Desholm, M. (eds.) 2012. Baseline investigations of bats and birds at Wind Turbine Test Centre Østerild. Aarhus University, DCE – Danish Centre for Environment and Energy, 128 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 28 http://www.dmu.dk/Pub/SR28.pdf


Summary - birds

In June 2010, the Danish Parliament passed a bill that regulates the establishment of a national test centre for wind turbines near Østerild in Thy, Denmark. This legislation required that an environmental monitoring programme should be implemented.

When fully developed, the test centre will comprise a total of seven test sites for wind turbines (up to 250 m in height). Each test site will consist of a single wind turbine, each with a tower for measuring equipment (up to 150 m in height) located immediately to the west of each turbine. The test centre will also require the construction of illuminated towers supporting meteorological equipment (up to 250 m in height) secured with guy-wires associated with each test site.

The Department of Bioscience, Aarhus University was commissioned by the Danish Nature Agency to undertake a bird monitoring programme in the test area.

Here we present the results of the baseline monitoring programme, which was undertaken in 2011/12 prior to turbine construction to establish a reference for the future analysis of the potential impacts on birds caused by the operation of the test centre and to provide a preliminary risk assessment for relevant species.

Throughout the annual cycle, large numbers of birds occur along the west coast of Jutland. In particular, the wetlands along the west coast are important breeding, staging and wintering areas for numerous species. During the migration periods in autumn and spring many species stage in the area for shorter or longer periods, whereas other species arrive in autumn and overwinter in the area until they return to the breeding areas in spring. Little is known about the seasonal migration and local movements that occur over land at the test centre. However, because of the lack of topographical features to concentrate avian migration, under normal circumstances, we would not expect high densities of migrating birds to be concentrated in or near the test area.

We assumed that daytime local movements accounts for more passages of birds at rotor height within the area than genuine seasonal migration. Therefore, we considered local movement to represent a greater source of potential collisions with wind turbines, although the design of the baseline programme allowed for an assessment of the collision risk associated with both types of migration/movement. Since collision risk may be elevated during periods of darkness, nocturnal migration in the area was included as a focal issue in the baseline programme.

On the basis of a preliminary assessment whooper swan, taiga bean goose, pink-footed goose and common crane were included as focal species in the ornithological investigations. Besides these four species, for which SPAs have been designated in the vicinity of the test centre, the study was designed to obtain species specific data on all bird species occurring regularly in the study area.

During spring 2011, visual transect counts, which have the advantage of providing a detailed quantitative species specific description of bird migration during daytime, were undertaken from a location west of the turbines and measurement towers. At the same time, a horizontally operated radar unit was used to obtain detailed temporal and spatial information on species specific bird movements, as well as information on flight direction. During autumn 2011 and winter 2012, visual counts were carried out from a central observation station, along transects running south and north between the turbine platforms and the measurement towers. Throughout the study period, we used a laser range finder to collect species specific data on flight altitudes of the visible migration during daytime. During night-time, a vertically operated radar unit was used to obtain measurements of flight altitudes and relative migration/movement intensity in the study area. This data collection was based on an automated minute-by-minute screen-dump system that archived the echoes detected by radar. An object-based image analysis was undertaken of the images on the screen and the bird echoes were automatically extracted from screen-dump images by a computer based track recognition procedure.

Modelling of the collision risk was undertaken for the selected species based on the data collected on the count transects and measurements of flight altitude. The avoidance rates incorporated in the collision models varied between 97.75 and 99.00% according to species based on the existing literature.

The baseline study confirmed that the test centre is not situated on a migration corridor, although seasonal migration took place to some extent both during night and day. During the day, flight activity in the study area was dominated by local birds moving between feeding areas and night roosts in Northwest Jutland.

The species for which we estimated that more than one annual collision with wind turbines would take place were cormorant (3), pink-footed goose (21-46), greylag goose (3-6) and golden plover (65). However, even though these species were all characterized by a high proportion of individuals passing the study area at rotor height, given the overall movement in the area, this only resulted in a relatively limited number of predicted collisions. For the remainder of the species that regularly occur in the study area, including the focal species whooper swan, taiga bean goose and common crane, we predicted that the annual number of collisions would be less than one. This was typically a result of a high proportion of individuals and flocks migrating at flight altitudes below the rotor height of the wind turbines.

In order to provide a crude estimate of the expected number of collisions at the towers, a simple equation was derived from the relationship between tower height and the number of casualties at other study sites. The heights of the towers at the test centre were inserted in the equation and the expected number of casualties at individual towers was multiplied by the number of towers. This resulted in an annual number of casualties of approximately 750 to 1,050 individuals.

On the basis of this preliminary assessment, which uses crude estimates of collision risk, we consider the potential impacts of the combined structures on the bird species occurring in the study area unlikely to be significant.

However, it is important to keep in mind that the data collected during the baseline programme only covers one year. Therefore we are unable to assess the extent to which there may be year-to-year variation in the occurrence of birds both during night and day. In particular, different weather conditions can affect flight behaviour and migration pathways on both small and large scales. Although the baseline programme covered most of the annual cycle, some important periods were not included in the study. Therefore the post-construction programme will fill out the gaps from the baseline programme by targeting those periods from which little or no current data are available.

By using the data compiled during the baseline programme, the post-construction programme will assess the possible effects and impacts on migrating birds, which may be caused by the operation of the test centre. Particularly, the baseline programme will be used as a reference to assess potential impacts at the population level (mortality caused by collisions) and effects related to behavioural questions (e.g. barrier effects, avoidance of and attraction to the combined structures at the test centre).