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Bats And Dippers In Bridges


1.01 The purpose of this paper is to update the Bridge Maintenance Committee on discussions and trials carried cut to date to protect bats and dippers during bridge maintenance operations.
1.02 Discussions have been held with the DOE Conservation Branch, a seminar to present the problems that can be caused by wildlife in our bridges was held September 1988 and trials were held to repair a bridge and at the same time make provision for bats and dippers.
2.00 BATS
2.01 By far the bigger problem in bridge repairs is protection and provision for bats. Unless we can prove otherwise it has to be assumed that a masonry bridge which does not run full at any time and which has gaps between the stones could be a possible roost site for hibernation or breeding of bats.
2.02 Hibernation sites are used from September to April. In bridges observed prior to our maintenance works, signs were visible that bats were present in a few of our bridges and bats were located in a semi-dormant state in October. When the same bridges however were surveyed in December no sign of the bats were visible. This would suggest that as weather gets colder the bats go up further into the cracks and become much more difficult to see. Inspection for bats in Downpatrick and Belfast Divisions again in December failed to find the presence of bats as the weather was cold. This does not however eliminate these sites as possible hibernation roosts.

The breeding sites used from June and July. In the Omagh Division we are only aware of 2 confirmed sites where bats breed and colonies are present. One site was that used in the September Seminar near Florencecourt and the other is at Moneymore. Evidence of parasites which are associated with breeding females were observed at bridges over the River Derg although no colonies were observed.
2.04 On 23—25 September 1988 I attended a Bat Workers Training Weekend and met with approximately 20 bat enthusiasts in the Province. Trying to enlist assistance in surveying bridges for bats met with little success. There is keenness and keenness and most are not prepared to travel any distance from home to carry out studies. Certainly one can appreciate that female members of the group, who are in the majority, would not be happy about being on strange ground at dark usually away from habitation. Another problem is access to the bridges. Again at night time when the river bed is uneven and rivers are fast flowing it is not safe for one person to survey alone. For to survey a bridge properly we have found that where the structure is greater than 2½ m high the use of ladders or staging is required which is extremely hard for one person to manhandle and erect again with great safety after dark. The Department’s Topper Unit has been extremely useful in looking for signs of bats (droppings, scratch marks, parasites etc). The Topper Unit is best used during the day time for as well as avoiding overtime payments to drivers and staff and the problem of lightings and signing, the noise of the engine has caused bats to climb higher into the structure making them harder to detect.
2.05 During maintenance work involving the removal and rebuilding of a one metre segment of masonry arch near Ballyronan 6 bats were displaced, but apart from this one occasion no other bats were discovered during maintenance work in the Omagh Division during the past year.
2.06 The effect of the floods in October 1987. Due to the high water levels in many of the bridges in the west of the Province running full it is felt that as a consequence bats were drowned, and although no bats were found in these bridges in 1988, there is a possibility that they may be used this winter and in future winters for hibernation.
3.01 The protection of nest sites.
Dippers are much more resilient to repair work on bridges and will return in the evening, after the workforce has gone, to their usual roost in the structure. Their presence is more easily spotted than that of bats in the structure due to the nest being built in holes in the masonry joints or where stones have been eroded or are missing. As long as nesting sites are left on the bridge the dipper will adapt quickly to them.
3.02 Dipper nests are usually found in bridges approximately one metre above water level. As all nests with eggs are protected under the Wildlife Order (NI) 1985 they should not be disturbed until hatching has occurred. This only creates a delay of approximately one month until the birds are able to fly and will not prevent repairs being carried out during the summer months although it may affect the phasing of these repairs.
3.03 The effect of the floods on dippers.
Where it is obvious a large number of dipper nests were submerged by the flood waters in October 1987 the nests were rebuilt and the eggs observed the following Spring.
4.01 Bat roosts and dipper nest sites. Although no sign of either species are visible voids should be left to allow the structure to be used or to continue to be used by this wildlife.
4.02 Dippers require a void of approximately 100mm x 100 mm x 100 mm although they may make do with a ledge of lesser size.
4.03 Bats normally like cracks in the upper sector of the arch and some narrow voids between stones 20—25 mm wide should be left unsealed if practical.
5.01 A copy of the report on the repair of this bridge to the Chief Engineer, Dr W M C Stevenson is attached at the end of this report marked Appendix A together with photographs of the bridge showing the provision made for bats and dippers which forms Appendix B.
6.01 The Seminar in Killadeas 26—27 September 1988.
At this Seminar our guest speaker Dr Bob Stebbings of the Institute of Terrestrial Ecology suggested that a survey should be made of all bridges but masonry bridges in particular for signs of bat use. Those requiring maintenance work should be given the first priority. He suggested that this work would take a expert such as himself trained at looking not only for bats but for the ‘tell—tale’ signs that bats used or are using the structure. He discounting the effectiveness of the local bat group workers expertise in this matter hinting that he would be the best person for such a survey.
6.02 Discussions with Conservation Branch
On 7 November 1988 I attended a meeting at Calvert House with Mr J Furphy, Principal Scientific Officer and Mr John Milburn, of the Countryside and Wildlife Branch regarding the question of surveying bridges for the presence of bats along the line suggested by Dr Bob Stebbings. Neither the Roads Service or Conservation Branch could due to restrictions in Departmental running cost afford to employ an expert for this work and the value of such a wide ranging survey would be questionable. Bats it was agreed are more predominant in the west of the Province and the number of bridges being worked on per year in that area would be approximately 100. It was felt it would be preferable if an expert (local) could examine these bridges prior to work commencing and determine whether bats were present, or had used the structures over the Winter, and give advice regarding the provisions for bats in the future. The financing of this expert should be a matter for discussion at a higher level.
6.03 Discussion also ensued regarding a research project to determine the importance of bridges on the ecology of bats and to the necessity of bridges for breeding and roost sites. This would involve a research project over several years and although it was agreed this would be desirable it would not assist the immediate problem of determining the presence or otherwise of bats in the structures.
6.04 As previously stated the presence of dippers is much more easily detected and can be accommodated during repairs to structures as long as there are no eggs in the nest.
7.01 Without further survey or research it is not possible to determine the ecological effect repairs to bridges will have on the bat population, the percentage of bridges frequented with bats and the effect of expelling bats from all or some of our bridges.
7.02 It is considered that bridges due for repair should be checked for signs of bats prior to the work being ordered by a local bat expert. Each Division should prepare a work schedule, a copy of which should be sent to Mr J Milburne at Countryside and Wildlife Branch as long as possible before the work is to be carried out.
7.03 Work on masonry bridges should if possible be carried out between April and September inclusive unless there are signs of breeding colonies in the structure.
7.04 Where practical some crevices between stones should be left near the crest of the arch where this can be achieved without affecting a load carrying capacity or the integrity of the structure.
7.05 Two suitable ledges for dippers should be provided per bridge where practical approximately one metre above water level or where evidence of existing nests exist.
7.06 Discussion should take place at superintendent grade or above on the financing of the services of a bat expert to inspect the bridges on our maintenance programme and to advise on the use made if any of the structure by bats and the relevant importance of that structure to bat ecology.

Roads Service
Omagh Division
County Hall

8 February 1989