Aarhus University Seal / Aarhus Universitets segl

No. 133: First year post-construction monitoring of bats and birds at Wind Turbine Test Centre Østerild

Therkildsen, O.R. & Elmeros, M. (eds.) 2015. First year post-construction monitoring of bats and birds at Wind Turbine Test Centre Østerild. Aarhus University, DCE – Danish Centre for Environment and Energy, 126 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 133 http://dce2.au.dk/pub/SR133.pdf



Bats were recorded at the National Test Centre for Large Wind Turbines and in its vicinity to assess the potential effects of the wind turbines and to study selected aspects of the potential conflicts between wind turbines and bat conservation interests.

Bats were detected using state-of-the-art bat ultra sound detectors to record the bats’ echolocation calls. The surveys and studies were carried out during August, September and October 2013. 

A total of nine species was recorded during the 2013 survey and studies: pond bat, Daubenton’s bat, Nathusius’ pipistrelle, soprano pipistrelle, serotine, parti-coloured bat, noctule, Leisler’s bat and brown long-eared bat.

Pond bat, Daubenton’s bat and Nathusius’ pipistrelle were the most common species in the test centre area.

The overall species composition and occurrences of bats at the monitoring sites in the test centre area and its vicinity in 2013 were similar to the baseline survey in 2011.

Bat activity levels at the monitoring sites along forest roads in 2011 were similar to the levels recorded in 2013 at the wind turbine sites.

At the ponds in the test centre area and its vicinity the bat activity levels were significantly higher in 2013 than in 2011. The ponds are probably highly important as foraging sites for the local pond bat and Daubenton’s bat populations throughout the summer and autumn.

Bat activity was higher around the wind turbine towers than along nearby forest edges and around the open-structured meteoro­logi­cal masts. These differences suggest that bats are attracted to the turbine towers. Most likely the bats forage on the large insect assemblies that some nights congregate on the turbine towers.


In June 2010, the Danish Parliament passed a Public Works Act to establish a national test centre for wind turbines near Østerild in Thy, Denmark. This legislation requires that a bird, bat and vegetation monitoring programme should be implemented.

Since 2011, the technical facilities at the test centre have gradually been developed and various structures erected on site. The test centre comprises a total of seven test sites for wind turbines of up to a maximum height of 250 m. Each test site consists of a single wind turbine, each with a mast for meteorological measuring equipment (up to 150 m in height) located immediately to the west of each turbine. These masts are secured with guy-wires.

The test centre also comprises two masts supporting meteorological equipment at heights up to 250 m secured with guy-wires. These masts also support aviation safety lighting.

The Department of Bioscience at Aarhus University was commissioned by the Danish Nature Agency to undertake a monitoring programme of birds in the test area. The monitoring programme comprises one baseline (2011/12) and two post-construction study periods (2013/14 and 2015/16).

The test centre is located near several Special Protection Areas (SPAs), which are sites designated for their particular importance for birds. These SPAs have been classified for rare and vulnerable breeding birds (as listed on Annex I of the Directive) as well as for regularly occurring migratory species according to Article 4.2 of the EC Birds Directive and generally following the criteria for designation of wetlands of international importance. As a result of their high conservation interest the monitoring programme has focused on this group of species in both the baseline and post-construction studies.

In 2012 we presented the results from the baseline monitoring programme. On the basis of a preliminary assessment, we considered the potential impacts of the combined structures on the bird species occurring in the study area unlikely to be significant.

Here we present the results from the first year of the post-construction bird studies, which were carried out from August 2013 to October 2014, together with an intermediate assessment of the potential impacts of the test centre on the bird populations occurring in the study area.

The test centre has not yet been fully developed. Therefore this intermediate assessment was carried out on a total of four operational turbines in operation together with their associated structures, e.g. meteorological measurement and aviation safety lighting masts, which had all been established at the beginning of the study period.

Initially, whooper swan, taiga bean goose, pink-footed goose and common crane, were included in the baseline investigations. However, on the basis of the results obtained during the baseline studies, white-tailed eagle and light-bellied brent goose were also subsequently included as focal species in the post-construction programme. 

Apart from minor modifications and special efforts targeted towards light-bellied brent goose and nightjar (see below), the design of the post-construction study was similar to the baseline study, which aimed at generating species-specific data, whenever this was technically possible. For this reason, although data was partly collated from comprehensive automated recording processes, the collection of high quality and high resolution data at the species level was given priority at all times in the investigations. We used visual transect counts, vertical radar and laser range finder data, which was combined to provide the basic information for the assessment. In addition, we conducted carcass searches using trained dogs under turbines and masts to quantify actual fatality rates.

In general, the post-construction study supported the conclusions from the baseline study. We confirmed that the test centre is not situated on a migration corridor, although seasonal migration took place to some extent, particularly during the night. During the day, flight activity in the study area was dominated by local birds moving between feeding areas and night roosts in northwest Jutland, some of which has been designated as SPAs for the species included in the study. As was the case during the baseline study, we demonstrated local movements to take place on a regular basis for a number of species.

From the results of the baseline study, the species for which we estimated that more than one annual collision with wind turbines would take place were cormorant (3 individuals per year), pink-footed goose (21-46), greylag goose (3-6) and golden plover (65). 

Based on the post-construction study, we estimated that the annual collision rate with wind turbines that exceeded one would be for cormorant (6-14), pink-footed goose (10-23), greylag goose (23-52), buzzard (0.8-1.6), golden plover (3-7), wood pigeon (0.5-1.2) and passerines (3-5).

For all of these species, a high proportion of individuals passing the study area did so at rotor height.  Nevertheless, this still only resulted in a relatively limited number of predicted collisions even for these species. It is also important to note that in contrast to the baseline study, the post-construction study period covered the whole annual cycle, except for June-July.

For the remainder of the species that regularly occur in the study area, including the focal species whooper swan, taiga bean goose, common crane, light-bellied brent goose and white-tailed eagle, we predicted that the annual number of collisions would be less than one. This was typically because, for these species, a high proportion of individuals and flocks migrating occurred at flight altitudes below the rotor height of the wind turbines.

Five territories of nightjars were registered on the basis of the spatially consistent presence of advertising males in July 2014. We were unable to estimate the collision risk between turbines and nightjars and therefore the assessment of potential adverse impacts on the local breeding population awaits further studies. This issue will therefore be addressed in the second post-construction study year.

On the basis of this intermediate assessment, which used more reliable estimates of collision risk than those obtained during the baseline study, we still consider the potential impacts of the combined structures on the bird species occurring in the study area unlikely to be significant. We stress that our crude estimates of the number of collisions should be interpreted with caution. We are therefore cautious when comparing the collision estimates between the two study periods. 

Since the test centre had only four turbines in operation during the post-construction study period, the assessment do not consider a fully developed test centre, which may have up to seven turbines in operation. The presence of more turbines may affect flight behaviour and migration pathways, which may potentially affect the risk of collisions between birds and turbines and other structures. Therefore the calculated number of collisions must be regarded as a crude estimate, which means that the potential impact of the test centre on bird species may be somewhat underestimated. However, this does not affect the conclusion that the overall impact of the test centre on bird species is considered unlikely to be significant.

Although no bird corpses were retrieved during the carcass searches, we consider the apparent complete absence of collisions between birds and the structures at the test centre to be highly unlikely. We therefore assume that either some fatalities were not detected because their remains were not available or missed by the dogs or they were removed by scavengers between searches.  Nevertheless, the results from the carcass searches indicate that the number of collisions is probably rather small. The results from the carcass searches therefore support our conclusion that although collisions between turbines and other structures at the test centre are to be expected, they will occur at a low rate.

It is important to keep in mind that the data collected during the baseline and the post-construction programmes only covers less than two years. We are therefore cautious when we 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, which may affect the risk of collisions.