Abstract
Observations of tagged common noctules up to 12.7 km offshore in August 2019, provided the first evidence that at least some local, non-migrating bats extend their foraging trips to the airspace above the North Sea. These observations triggered Rijkswaterstaat to commission the current study, under the umbrella of the Offshore Wind Ecological Programme (WOZEP), to establish the importance of the maritime habitat for local common noctules living in coastal colonies.
The aim of this study is to assess the home range of both male and female common noctules in two small forests (‘t Wildrijk and Ananas) near the coast in the province of North Holland, Dutch mainland. Within the home range, the habitat use and the relative importance of the airspace above the sea are assessed.
To determine home range and habitat use of common noctules, we equipped 56 individuals with GPS tags: 31 females (8 young of the year [YOY] and 23 adults [AD]) and 25 males (13 YOY and 12 AD). Of these, four females (AD) and two males (1 YOY and 1 AD) were tagged twice. Of the resulting 62 ‘deployments’ (i.c. deployed devices) 40 GPS tags with data could be retrieved. On average each recovered deployment yielded six monitored nights (range 1–11), whereas the number of GPS fixes per deployment was on average 29 fixes (range 3–75).
The GPS fixes (n=1145) were used to determine flight paths per tagged individual and per group (YOY and AD females, YOY and AD males). Of these GPS fixes 17 fixes were obtained above sea, of which three occurred close to the Wadden Sea coast and 14 above the North Sea up to 2.7 km from shore.
Home range sizes were estimated using the AKDEc method for 31 individuals. Home range estimates of adult females only were used for the habitat use analysis, since the number of GPS fixes and the number of individuals in the other groups was too small for an analysis of habitat use.
From our current study we conclude:
- that the AKDEc home range of adult female common noctules is on average 45 km2 and extends not further than a few kilometres onto the North Sea.
- that adult females are almost completely ‘terrestrial’ and show the least preference for the maritime habitat. When they occur at sea, this is probably driven by specific weather conditions.
- that adult females face low risks of potential negative impacts from offshore wind farms outside the 12 mile zone.
- that home ranges could not reliably be assessed for adult male, young male and young female common noctules due to a lack of data. In comparison to adult females, however, the home range of adult males seem to be larger, while the home ranges of young males and young females are even larger than those of adult males.
- that habitat use could not reliably be assessed for adult males, young males and young females, due to small sample sizes.
- that adult male common noctules face possibly a slightly higher risk in comparison to adult females from offshore wind turbines.
- that young male and young female common noctules face uncertain risk of potential negative impacts from offshore wind farms outside the 12 mile zone
- that our results are valid for Ananas and ‘t Wildrijk, and extrapolation of these conclusions to home ranges and habitat use of common noctules in other areas in the Netherlands is not necessarily straightforward.
Though outside the scope of this study, our previous bat detector work concluded:
- that common noctules are detected in the offshore wind farms outside the 12 mile zone
To answer remaining questions we recommend:
- to obtain more data from the study area in late summer, put more emphasis on young males, young females, and adult males
- to tag common noctules in other areas, in particular close to the current offshore wind farms.