Action Synopsis: Bat Conservation About Actions

Maintain bat roosts in road/railway bridges and culverts

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Two studies evaluated the effects of maintaining bat roosts within road bridges on bat populations. One study was in Ireland and one in the USA.




  • Use (2 studies): One before-and-after study in Ireland found that a maternity colony of Daubenton’s bats continued to roost in a road bridge over a river in similar numbers after crevices were retained during repair work. One review in the USA found that when bat roosts were maintained during bridge replacement works, Yuma myotis and Mexican free-tailed bats recolonised most roosts in similar numbers to before the works, but pallid bats did not return.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A before-and-after study in 1988–2005 of a road bridge over a river in northwest Ireland (Marnell & Presetnik 2010) found that after crevices were retained during strengthening work and repairs to the bridge, a Daubenton’s bat Myotis daubentonii maternity colony continued to roost in the bridge in similar numbers as before the work. A maternity colony of approximately 25 Daubenton’s bats was first recorded roosting in the bridge in 1988 (no more recent data provided). After the repair work was complete, four bats were recorded in the original roost crevice in 2004, and 25 bats were recorded in 2005. Strengthening works (including laying cement, pointing, and grouting) were carried out on the five-arch masonry bridge in September–October 2003. Roosting crevices were marked and temporarily filled with polystyrene to prevent them from being filled. Bats were counted in the bridge in July 2004 and 2005.

    Study and other actions tested
  2. A review in 2017–2018 of case studies at five road bridges in California, USA (Harvey & Associates 2019) found that when bat roosts were maintained during bridge replacement works, Yuma myotis bats Myotis yumanensis recolonised two of three roosts and Mexican free-tailed bats Tadarida brasiliensis recolonised three of four roosts in similar numbers to before the works, but pallid bats Antrozous pallidus did not return to either of two roosts. Yuma myotis bats recolonised two roosts in similar numbers to before the works (before: 220, 100; after: 220, 100), whereas numbers declined at a third roost (before: 40; after: 20). Mexican free-tailed bats recolonised three roosts in similar numbers to before the works (before: 200, 3,000, 994; after: 200, 3,000, 1,010), whereas numbers declined at a fourth roost (before: 2,000; after: 600). Pallid bats did not return to either of two roosts used by 18–20 bats. At all of five sites, the original bat roost structures (abutments: one site; hinges and expansion joints: four sites) were retained during bridge replacement works (dates not reported; see original report for details). Bats were temporarily excluded from roosts in hinges and expansion joints. Counts of bats before and after the works were taken from questionnaires completed by the California Department of Transportation. Field surveys (including daytime inspections, colony and emergence counts) were conducted by the authors in spring and summer 2017 and 2018 after bridge works were complete.

    Study and other actions tested
Please cite as:

Berthinussen, A., Richardson O.C. and Altringham J.D. (2021) Bat Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.


Where has this evidence come from?

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Bat Conservation

This Action forms part of the Action Synopsis:

Bat Conservation
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