Pipistrelles use bat roost boxes on Kau Sai Chau
Two species of bats have been identified foraging over northern Kau Sai Chau, Pipistrellus sp. and Hipposideros armiger. The numbers of foraging bats are often large, particularly in spring, summer and autumn when flying insects are abundant (see Ades 1994). Although foraging bats are numerous, no bat roosts were discovered on the island prior to December 2000. This was in part because there were no buildings, abandoned or occupied, on the northern part of the island until 1994, when golf course construction began. Also, over much of the island the tree cover had been either felled or burned decades ago, leaving few mature trees on the island. Finally, there are no caves on the island where bats might roost. Ades (1994) observed that the availability of suitable roost sites could be a factor limiting bat numbers or species representation in Hong Kong, but availability of forage was probably not limiting for insectivorous bats. Based upon that observation I was interested to learn if increasing the availability of suitable bat roosts would lead to increased numbers of roosting bats and ultimately greater numbers of foraging bats. I was interested to increase the numbers of insectivorous bats foraging over the northern third of the island to reduce flying insect populations (particularly mosquitoes) to the extent possible through bat predation. Provision of roost boxes for bats has been undertaken on a continental scale by the North American Bat House Research Project coordinated by Bat Conservation International (Tuttle & Hensley 1993a). Many bat box designs and strategies for placement have been evaluated and reported on by BCI staff and associates over the 9+ years of the project. The results of that project proved promising so I chose it as a model for application at Kau Sai Chau. This manuscript summarises the results of the first two years of implementation of a bat roost project on Kau Sai Chau, an island of 6.7 km2 area in Port Shelter of northeast Hong Kong Special Administrative Region (HKSAR).
Methods & Materials
Bat boxes were constructed according to blueprints shown in Tuttle & Hensley (1993b). Rough-cut lumber for the exterior walls and roof of the boxes was sourced from a sawmill in Tuen Mun. Internal partitions were made of plywood. In each box one internal partition was covered with green plastic screen material to provide an alternate substrate in the event that roosting bats could not cling to the smooth internal plywood surfaces. Ten boxes were installed in early December 2000, six on trees in woodlands, one on an isolated tree, and three on buildings (Table 1). Boxes were monitored periodically thereafter using torches to illuminate the interior of the boxes.
Table 1. Locations of bat roost boxes installed at Kau Sai Chau in December 2000
Results & Discussion
No boxes were occupied by bats during the first 10 months after the boxes were installed. It is not uncommon for roost boxes to remain unoccupied for months or years, even when foraging bats are abundant in the vicinity of the roost boxes (Tuttle & Hensley 1993a). This can be due to several factors including duration of daily solar input or presence of flight path obstacles such as tree branches near roost boxes (ibid.). Either of these factors may have affected the boxes that were installed in relatively dense woodlands at Kau Sai Chau. Also, some of the roost boxes at Kau Sai Chau were colonised by ants. To avoid this problem one box was relocated from a streamside woodland to the wall of a building in late June 2001.
The first record of roosting bats was made on 20 October 2001 when box 1 in a small plantation of Sea Hibiscus was occupied by two Pipistrelles (Pipistrellus sp.) (Table 2). The plantation was at the golf course clubhouse in an area frequented by vehicles, golfers and staff.
Table 2. Occupancy of 10 roost boxes at Kau Sai Chau between December 2000 and December 2002.
On 27 October 2001 two additional boxes were shifted because they had been colonized by ants, one from a Camphor Tree in a dense woodland to a plantation of Sea Hibiscus at the clubhouse, and one from an isolated fig to a Bauhinia at the clubhouse building. On 29 November 2001 the remaining three boxes initially installed on trees in woodlands were relocated to a plantation near the clubhouse because of colonization by ants and because only the boxes near the clubhouse had attracted roosting bats. After shifting roost boxes from the woodlands the box locations were as follows: on the walls of a metal-sided maintenance shed (3 boxes); on the exterior masonry wall of the clubhouse (1 box); on a Bauhinia at the clubhouse (1 box); and in a Hibiscus plantation at the clubhouse (5 boxes).
Numbers of roosting bats ranged between 2-6, and box 1 was the only box occupied until 24 October 2002 when a second box was occupied on a Sea Hibiscus at the clubhouse. Just over one month later two additional boxes were occupied, one on a Sea Hibiscus at the clubhouse, and the second on a Bauhinia also at the clubhouse. On 29 November 2002 four boxes were occupied by a total of 11 Pipistrelles (Table 2). All four occupied boxes were shaded for most of the day, but received some mid-day sunlight through the tree canopy.
Roost box locations will be shifted in winter 2002 to increase bat occupancy. Some boxes will be moved from shaded to more sunny locations to increase absorption of solar radiation, thereby increasing the interior temperature of the box. Such a shift increased bat use of boxes at 31-32o N latitude in North America (Anon 2000). Three unoccupied boxes in shaded locations on exterior walls of buildings at Kau Sai Chau are also distant from freshwater bodies such as streams and ponds. Those boxes will be shifted to tree or free-standing locations <400 m from water because distance from water has also been found to affect bat use of roost boxes (Tuttle & Hensley 1993a).
Studies reported here were funded by The Jockey Club Kau Sai Chau Public Golf Course Ltd., whose support is gratefully acknowledged.
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