Could bats be an issue for offshore wind farms?

There is increasing evidence that wind energy developments have the potential to have significant negative impacts on bat populations (Voigta et al, 2012). Both Bat Conservation Ireland (Bat Conservation Ireland, 2012) and the Bat Conservation Trust consider that that bat mortality due to wind turbines is a serious potential issue in Ireland and Britain, and it is clear that bats and their habitats need to be considered carefully during the planning, construction and operation of onshore wind farms. But could offshore wind farms also pose a risk to bats?

The amazing finding that a tiny Nathusius’ pipistrelle Pipistrellus nathusii completed a 596km  journey from Bristol to the Netherlands is proof that bats do successfully migrate across the North Sea

Confirmed migration from UK to the Netherlands

The first record of a bat crossing the sea from the UK to continental Europe has been reported recently in the media (BBC, GuardianIndependentPhs,org, 2014). This discovery is the first direct evidence that a British bat species migrates over the sea between the UK and the European mainland, and is a significant finding. This discovery may have consequences for proposed offshore wind energy developments, according to some researchers

Could bats be an issue for offshore wind farms?
Could bats be an issue for offshore wind farms?

In the past offshore wind energy developers could question the theory that bats migrate across the sea, due a lack of scientific evidence. But ecologists have long suspected that bats do indeed fly across the sea between Britain and continental Europe. The amazing finding that a tiny Nathusius’ pipistrelle Pipistrellus nathusii completed a 596km  journey from Bristol to the Netherlands is proof that bats do successfully migrate across the North Sea. It has been reported that the subject Nathusius’ pipistrelle bat was first tagged near Bristol, and was found nearly 600km away in the Netherlands. Unfortunately it was dead, but its identification ring has provided definite evidence of a bat migration that many ecologists previously speculated about. 

it’s not just migrations that need to be considered as there is also evidence that non-migratory bats can fly offshore to forage on insects blown out to sea

Nathusius’ pipistrelle is similar in appearance to, but is slightly larger and has longer fur than, the much more commonly found Common Pipistrelle P. pipistrellus and Soprano Pipistrelle P. pygmaeus. It has been widely recorded throughout Britain and Ireland but records are sparse. However, this is probably mainly due to under recording. According to the Bat Conservation Trust an increase in records of this species in Britain and Ireland in recent years may reflect sampling effort, although the possibility that the species range is expanding cannot be discounted. 

It was already known that Nathusius’ pipistrelles travel significant distances, and there were existing documented records that confirmed that this species can travel more than 1,500 km over land and as much as 80 km in a single night in mainland Europe (Russ et al, 2000). The subject bat was originally ringed in the UK by the Bat Conservation Trust in 2012, at a lake in the south west of England.  Previous records from offshore oil platforms have been reported (e.g.Boshamer & Bekker, 2008), and another recent study also confirmed the presence of bats at Dutch offshore wind farms (Ahlén, 2007). However, it is clear that finding a tagged bat from the UK in the Netherlands is a very significant and exciting finding.

Implications for the siting of offshore wind turbines

According to the recent media reports, Dr. Fiona Mathews of University of Exeter (who leads the National Bats and Wind Turbines Project) said “the finding [of this Nathusius’ pipistrelle] may have implications for the siting of offshore wind turbines. “Nathusius’ pipistrelle is one of the species most at risk from land-based wind turbines throughout Europe. We now urgently need to identify the migration routes they use to cross the sea between the UK and continental Europe. Offshore windfarms in the wrong place could be very bad news.” 

Could bats be an issue for offshore wind farms?

Could similar migrations also occur to and from Ireland?

Nathusius’ pipistrelle also occurs in Ireland, having been first discovered in Northern Ireland in 1996. Due to confusion with the more widespread pipistrelle species, it is easily overlooked and is probably far more widespread than the present records would indicate, according to the County Cork Bat Group. In addition to the Nathusius’ pipistrelle, there are records of other bat species (which occur in these islands) undertaking significant migrations. For example, some Leisler’s bat Nyctalus leisleri populations migrate long distances in continental Europe. They often travel several hundreds of kilometres between summer and winter roosts and the greatest known distance travelled is 1,950 km (Russ, 2009). There is however currently no evidence that this species (or any other bat species) undertakes migrations like this within, or to and from Ireland. It has been suggested that the relatively mild weather in Ireland might have caused Nathusius’ pipistrelle to relinquish its migratory behaviour in favour of a more sedentary lifestyle. It is also possible that Britain and Ireland lie in a transitional region, with migratory individuals returning from the north-east of the species’ range supplementing the resident bats during the winter (Russ, 2000). 

the relatively mild weather in Ireland might have caused Nathusius’ pipistrelle to relinquish its migratory behaviour in favour of a more sedentary lifestyle

Recently there was also an unusual discovery of a Greater Horseshoe bat Rhinolophus ferrumequinum in Ireland by Bat Conservation Ireland (The Journal, 2013). At the time it was suggested that this bat came from Wales across the Irish sea. This may be a once off and this is the only record of this species in Ireland. More dramatic evidence of occasional displacement of bats includes a record of a Hoary bat Lasiurus cinereus – a North American species – in Orkney in 1847. This bat had apparently been blown across the Atlantic! There is of course a significant difference in the implications between accidental displacements, and a directional migration.

And what about non-migratory bats foraging offshore?

In relation to offshore wind farms it’s not just migrations that need to be considered as there is also evidence that non-migratory bats fly offshore to forage on insects blown out to sea. One relevant study here is ‘Bats and offshore wind turbines studied in southern Scandinavia’ which was produced by the Swedish Environmental Protection Agency (Ahlén et al, 2009). This study found that bats can be attracted by offshore turbines when insects are gathered there. The study states that “It became quite obvious that the great amounts of prey organisms, flying, drifting and in the water surface, are an important food source for migrating bats and also for non-migratory species“. They also found that both migrating and resident species search and hunt insects close to the turbines. The authors found nothing indicating that bats avoid the turbines like some birds do, and indeed suggest that the opposite is the case.

Offshore wind turbines should be surveyed in the same manner as land-based turbines

This study was in the Baltic sea and many of the insects the bats were feeding on are produced in the sea itself (e.g. chironomids). However, the potential for bats to fly out to sea to forage needs to be considered for any proposed offshore wind farm. Activity of bats over the sea is likely to be strongly influenced by the weather, and conditions suitable for offshore bat foraging will not be as frequent in less sheltered marine areas (i.e. the west coast of Ireland). Weather also affects migrating bats, with strong winds stopping the activity based on the results from the Swedish study. They said that “bats migrating over land that reach coastal points at bad weather usually accumulate until a change in weather occurs“.

Any marine area where insects are blown offshore could well be used by bats following their prey out to sea. Estuaries in particular are likely to be used extensively by bats for foraging, and many insects with aerial phases to their life cycle are produced in estuaries. ECOFACT staff have a record of a Lesser Horseshoe Bat Rhinolophus hipposideros roosting in an old derelict ship (the Matrisha at Cahiracon Quay) in the Shannon estuary. This derelict ship was located over 100m away from any vegetation corridors, and is an interesting and unexpected observation. We also have records of Soprano Pipistrelles foraging along the shoreline of the Shannon estuary in the same area, and they could easily have been travelling out over the estuary if food items were being blow out there by an offshore breeze. It is clear that if the food is there that the hunter will follow, although there are probably only very certain weather conditions during which this would occur in relation to bats. During moderate to high wind conditions when wind turbines are turning fastest, it is of course unlikely that bats would be feeding.

The derelict Matrisha, Cahiracon Quay. A Lesser Horseshoe bat was found roosting here.

In the study ‘Presence of bats confirmed in Dutch offshore wind farms’ by Poerink et al (2013), evidence of the presence of bats was reported for two offshore wind farms on the Dutch Continental Shelf in the North Sea. Of the sequences recorded on ultrasonic recorders, 98% consisted of Nathusius’ pipistrelle and 2% were Common Noctule Nyctalus noctula. Most of the bat activity occurred during early September. The authors of this study noted that “both species are known to cover long distances during migration, so these observations could refer to migrants . However, they could also be local populations flying back and forth from the mainland to forage“.

Further work is needed

This is all a very interesting area, and it is clear that further research is needed. It is understood that the University of Exeter have recently installed bat detectors on ferries crossing to and from South-West England, and they are analysing the data to find out when and where bats are recorded at sea. It is clear that when the results of these and other studies are available we will have a better understanding of the potential impact of offshore wind farms on bats.

A new interesting technology, that has the potential to answer questions in relation to bat migrations and movements over the sea, is the DeTECT MERLIN radar system. Radar has been used previously by Ahlén et al (2007)  to look at larger bat species foraging over the sea. The MERLIN radar system has been used to monitor migrating pink-footed geese and their behavioural responses to an offshore wind farm development (Plonczkier and Simms, 2012). Indeed, radar has even been used to monitor insect movements over the sea (Drake, 2013). ECOFACT staff have recently received detailed operational and technical training on use of the MERLIN radar system, and are very excited on the implications of this new technology for monitoring both bats and birds at proposed sites for both onshore and offshore wind farms. Ultrasonic recorders were used in the study at Dutch offshore wind farms (Poerink et al, 2013), and this study demonstrated the feasibility and value of using this method for assessing bats at offshore wind farm sites. Radar can ‘see’ further than ultrasonic units, but may miss smaller species. Ultrasonic recorders have a more limited range, but a much higher resolution and species will be able to be identified from the data collected. 

The bat survey guidelines for assessing wind turbine developments in Ireland (Bat Conservation Ireland, 2012) make no reference to offshore wind farms, and bats do not seem to have been considered an issue at offshore wind farms in Ireland up to now. There is no evidence in Ireland that there are significant bat migrations, and also no evidence that bats in Ireland use offshore marine areas for foraging. However, there is increasing awareness that the issue needs to be at least considered. The EUROBATS manual ‘Guidelines for consideration of bats in wind farm projects’ by Rodrigues et al (2008) states that “offshore wind turbines should be surveyed in the same manner as land-based turbines“. This manual gives a recommended approach for offshore bat surveys, and suggests combining them with nocturnal bird surveys for example.

Overall, it is clear that further work is needed in this area and the gap in current knowledge of bat migrations from the British Isles to mainland Europe should be addressed through strategic studies. The potential for bat migrations and bats foraging offshore also needs to be investigated further, and considered in relation to any proposed offshore wind energy developments. The recent findings discussed above may have implications for the siting of offshore wind turbines. However, it is also clear that at present bats remain a more important issue for onshore wind farms in Ireland.

For further information on bat surveys in general, and the ecological impact assessment of proposed offshore wind farms, please visit our main website

Media references


Nathusius’ pipistrelle – datasheets and information

Bat groups and organisations mentioned

Also if you have any questions regarding the above article, or require any additional information on bats in relation to wind farms or indeed any other development, please do not hesitate to contact the author Dr. William O’Connor. Also extensive information on bats, bat surveys and the assessment of the ecological impact of wind energy developments can be found on our extensive main website

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