Long term monitoring of bat boxes in Thetford Forest Park

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LONG TERM MONITORING OF BAT BOXES IN THETFORD FOREST PARK ALISON COLLINS, ARTHUR RIVETT AND SUE HOOTON Introduction In 1975, a bat box project was started by Dr Robert Stebbings to look at population dynamics of Brown long-eared bats Plecotus auritus in the Thetford Warren and Downham Highlodge Warren sections of Thetford Forest, Suffolk (Boyd & Stebbings, 1989). Further boxes were added in 1984 and 1985. This paper presents the findings of a subsequent bat monitoring programme which began in 2000 by Suffolk Bat Group, recording all bats in boxes placed in some of those original locations.

The aims and objectives of the analysis of the monitoring data were as follows:  To determine relative species abundance in bat boxes  To determine how species/sexes use boxes seasonally  To investigate site faithfulness  To examine the relationship between bat abundance/diversity with surrounding habitat  To provide initial advice on siting of bat boxes in coniferous forest.

Thetford Forest Park comprises about 10,000 ha of commercial plantation, mainly Corsican Pine and Scot’s Pine, and is the largest lowland forest in England. The area was designated as Breckland Forest Site of Special Scientific Interest in 2000 for its biodiversity and geodiversity interest, including ground-nesting birds, rare flora and invertebrate fauna. It is also a component part of Breckland Special Protection Area, notified for breeding woodlark and nightjar.

Methodology In 2000, a total of 14 locations were selected for monitoring in the area of Thetford Forest Park around High Lodge, including Brandon Country Park (Fig. 1).

At each location, six trees had previously been selected to have three bat boxes per tree facing north, south west and south east, erected at about 5m height. The bat boxes are a traditional wooden design with a removable lid, single chamber and an entrance gap with a crawling board at the base of the box (Plate 2). Boxes which were damaged by squirrels, woodpeckers or by weather were replaced. Occasionally over the period 2000 to the present day, trees which supported the boxes had to be felled and boxes were then moved to the nearest remaining trees.

For each box in each location in spring (April) and autumn (October) from April 2000 to October 2014, all bats were carefully removed from the boxes and their details recorded, i.e. species, gender, breeding or non-breeding, adult or juvenile.

Note that pipistrelles were not split into Common Pipistrelle Pipistrellus pipistrellus or Soprano Pipistrelle Pipistrellus pygmaeus as this split into separate species occurred after the start of the project. If a ring was present on a captured bat, the ring number was noted (Plate 3).

The broad surrounding habitat features were recorded at each location in February 2015.

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Figure 1. Location of bat boxes in Thetford Forest Park showing relative abundance of each species. Pie charts are scaled relative to the total number of bats recorded at each site. Largest N = 341, Smallest O = 58.

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MONITORING BAT BOXES IN THETFORD FOREST

Results Leisler’s Natterer’s Percentages of all bats in boxes over 15 years: Noctule The total number of all bats recorded was 2455.

Over half of all bats were Brown long-eared bats Plecotus auritus (61%), just over a quarter were Pipistrelles Pipistrellus spp. (28%) with Noctule bat Pipistrelle Nyctalus noctula comprising 8% and Leisler’s bat Nyctalus leislerii only 2% of all records. Natterer’s Brown bat Myotis nattereri were found very rarely in the Long-eared boxes comprising less than 1% of all bats recorded.

Figure 2. Percentage of all bats in boxes over 15 years

Seasonal usage of boxes by adult bats:

Brown long-eared bat The total number of adult Brown long-eared bats recorded over the period was 923.

Of these, females were three times more abundant than males, notably in the spring.

It is considered that boxes in the Forest are primarily used for breeding by Brown long -eared bats. Females gather in the boxes in large clusters in spring to have their babies in summer. The largest number of Brown long-eared bats found in any box is 30, comprising 22 adult females, 4 adult males and 4 immatures in Area W in April 2012 (Plate 4).

Brown Long-eared Bat Total 923

%

Pipistrelle bat species Total 621

% Leisler’s Total 44

Noctule Total 163

%

%

Figure 3. Seasonal variation in box occupancy for the four main species.

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Pipistrelle bat species The total number of adult Pipistrelles recorded over the period was 621. About twice as many females were recorded compared to males. However, the majority of pipistrelles, both male and female, were found in autumn. Although the boxes are used for breeding, it is thought that the main use of the boxes by Pipistrelles is as a mating roost in the autumn, where males gather together ‘harems’ of females.

Noctule bat The total number of adult Noctules was 163. Overall, more males than females were found in the boxes. Males numbers were similar between spring and autumn with the highest occurrence of females in the autumn. This suggests that Noctules are primarily using the boxes for mating.

Leisler’s bat The total number of adult Leisler’s bats was 44. Similarly to Noctule bats, more males were found than females. Numbers of males were similar between spring and autumn with the highest occurrence of females in the autumn. This suggests that Leisler’s bats are primarily using the boxes for mating although definite conclusions are not able to be drawn as numbers are low.

Ringed recaptures The results from ringed individual bats that were recaptured on at least four occasions are presented in Table 1. This shows that, in general, bats are highly faithful to their roost location. Females of Brown long-eared and male and female Pipistrelle bats were recaptured in the same location on several occasions. However, a male Leisler’s bat was caught twice in location B and twice in location F which are within 1 km of each other.

Table 1. Ringed individuals that were recaptured on at least four occasions Ring Code

Species

Location

Years

T9999

Brown long-eared female

W x 5, U

2001, 2002, 2005, 2007, 2008, 2009

U5758

Brown long-eared female

Nx5

2001, 2006, 2009, 2010, 2011

T4404

Pipistrelle female

Sx4

2004, 2006, 2007, 2008

T9239

Pipistrelle male

Fx5

2001, 2004 x 2, 2006, 2010

T2907

Pipistrelle male

Ux5

2001, 2002 x 2, 2004, 2005

E9431

Leisler’s male

B x 2, F x 2

2001, 2006, 2007, 2008

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Distribution and abundance of bats Bats are widely distributed throughout the Forest with all four species being found at over half of all locations (see Fig. 1). Bats were the most abundant at location N (a total number of 341) and least abundant at nearby location O (58 bats). Using Simpson’s Diversity Index, the most diverse location was location U which had good numbers of all four species found in the study area and the least diverse was location R which only supported Pipistrelles.

Habitat features at bat box locations Comparing the Simpson’s Diversity Index with the habitat features noted at each site, there is an apparent relationship between the type of forest ride adjacent to the block of forestry supporting the bat boxes; this could not be tested statistically however.

Wide grassy rides and mixed (broadleaved and conifer) canopy species are associated with more diverse/abundant bat box usage in comparison to surfaced fire routes and single-species coniferous plantation. The most diverse location, U, was next to a Conservation Ride ( a wide grassy ride managed by Forestry Commission to benefit biodiversity) and the least diverse location, R, was between two wide fire routes (Plates 5 & 6).

Discussion It has long been thought that boxes can provide extra roosting and breeding sites for bats especially in coniferous plantations where there may be few suitable refugia (Boyd & Stebbings, 1989). However, not all bat species known to be using the Forest are found in wooden boxes. For instance, Barbastelle, Natterer’s and Daubenton’s bats have been regularly recorded from hibernation sites in the Forest (Gibbons, 2013), but have not been found in wooden boxes to date (apart from Natterer’s bats on very rare occasions). The use of different types of box, e.g. Schwegler, may encourage usage by additional species (Bilson, 2014).

Boyd and Stebbings (1989) found that Brown long-eared bats were the most numerous bat in boxes in Thetford Forest in the period 1975–1985 and the current monitoring project has confirmed this finding; Brown long-eared bats were easily the most numerous species over the period 2000–2014. In central Poland, boxes have greater importance for bats in late summer and early autumn with Noctule bats and Nathusius’ Pipistrelle showing evidence of mating in boxes (Lesinski et al., 2009). This current study suggests that boxes in Thetford Forest are similarly more important in autumn as mating roosts for Pipistrelles, Noctule and Leisler’s bats. In the Polish study, Brown long-eared bats were the first species to be found in boxes after winter.

Noctules and Leisler’s bats may be breeding in natural tree cavities in the Forest, indeed a few natural roost sites are known (Plate 7) with male bats using boxes as ‘suboptimal’ roosts in spring. Brown long-eared bats released after bat box checks in Area P were seen to enter a section of an occluded trunk in a Pine tree which was subsequently noted and protected as a natural tree roost. Pipistrelle species have been ‘lumped’ together in this study but the individual species may have different

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abundances and behaviours. As these species can be distinguished in the hand, further work on their local ecology is worthy of further consideration.

A preliminary analysis of ringed recaptured bats revealed that bats in the Forest are faithful to their roost sites. This behaviour is well known for bats using roosts in domestic properties and is the reason that roosts receive legal protection.

The influence of surrounding habitat on bat diversity and abundance in the Forest needs further investigation. Entwistle et al. (1997) found that Brown long-eared bats forage close to roosts and select sites within or very close to good foraging habitat.

This study has initially indicated that proximity to good feeding habitat is important for a diverse bat population and it seems likely that bats are selecting open, sheltered grassy rides which have the most diverse and abundant invertebrate populations for feeding and commuting to good feeding sites. Further bat activity surveys using static bat detectors to evaluate the use of rides by foraging and commuting bats are under way and will be reported on in future.

Preliminary advice from this study is to site new boxes in coniferous plantation forestry near to open, wide grassy rides, preferably where there is a mix of broadleaves in canopy. Ideally, trees selected to have boxes should remain outside the felling rotation.

Figure 4. Training is an important element of the Thetford Forest project. Arthur Rivett (left) demonstrates handling to volunteers.

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Finally, this monitoring exercise is also a successful educational and training facility, showing bats at close quarters to people of all ages and allowing members of local Bat Groups who are training for a Natural England Volunteer Bat Roost Visitor licence to handle bats under supervision (note that anyone wishing to handle bats must be rabies-vaccinated and use protective gloves) (Fig. 4).

Conclusions Bat boxes in coniferous plantation with few natural roosts provide additional roosting sites. In Thetford Forest Park, boxes are used mainly for breeding by Brown longeared bats and for mating by Pipistrelle, Noctule and Leisler’s bats. Bats in the Forest are faithful to their roost locations. Locations within the Forest are variable in terms of the diversity and abundance of bats found in roost sites; the most diverse location was adjacent to a Conservation Ride. Open grassy wide rides next to forest with a mix of broadleaved and coniferous trees may provide the optimum conditions for location of new bat boxes.

Acknowledgements The authors would like to thank Neal Armour-Chelu, Forestry Commission, for allowing access to the bat boxes and for his helpful advice, Suffolk Bat Group volunteers, John Goldsmith and members of the Norfolk Bat Group for their help with recording bats and Clare Collins for data processing.

References Bilston, H. (2014). Maximising occupation of bat boxes in an ancient woodland in Buckinghamshire: a summary of recent research. BSG Ecology.

Boyd, I.R. & Stebbings, R. E. (1989). Population changes of Brown Long-Eared bats (Plecotus auritus) in bat boxes at Thetford Forest. Journal of Applied Ecology 26: 101–112.

Entwistle, A. C., Racey, P. A. & Speakman, J. R. (1997). Roost selection by the Brown Long-Eared Bat (Plecotus auritus). Journal of Applied Ecology 34: 399–408.

Gibbons, N. (2013). Two Mile Bottom Bat Hibernaculum - from Folly to Fantasy. Trans.

Suffolk Nat. Soc. 49: 1–11.

Lesinski, G., Skrzypiec-Nowak, P., Janiak, P. & Jagnieszczak, Z. (2009). Phenology of bat occurrence in boxes in central Poland. Mammalia 73: 33–37.

Alison Collins suffolkbatgroup@suffolkwildlifetrust.org.uk Arthur Rivett 1 Place Farm Cottage , Old Bury Road, Stuston, Diss, Norfolk IP21 4AD Sue Hooton Dove Cottage, Dove Lane, Eye, Suffolk IP23 7BA

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A. Rivett Plate 2: The bat boxes are a traditional wooden design with a removable lid, single chamber and an entrance gap with a crawling board at the base of the box. (p. 19).

A. Rivett A. Rivett

Plate 3: Leisler’s Bat Nyctalus leisleri with ring. (p. 19).

Plate 4: Thetford Forest Area W. (p. 21).

A. Rivett A. Rivett

Plate 5: Thetford Forest Location U. (p. 23).

Plate 6: Thetford Forest Location R. (p. 23).

A. Rivett Plate 7: Natural cavities in trees can provide roosting sites for Noctules and Leisler’s Bats. (p. 23).