Caltrans bat mitigation: A guide to developing feasible and effective solutions

Published source details Harvey & Associates H.T. (2019) Caltrans bat mitigation: A guide to developing feasible and effective solutions. California Department of Transportation (Caltrans) report.
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This study is summarised as evidence for the following.

	Action	Category
	Provide alternative bat roosts during maintenance work at road/railway bridges and culverts Action Link
	Create spaces for roosting bats in road/railway bridges and culverts Action Link
	Maintain bat roosts in road/railway bridges and culverts Action Link

Provide alternative bat roosts during maintenance work at road/railway bridges and culverts

A review in 2017–2018 of case studies at four road bridges in California, USA (Harvey & Associates 2019) found that bat houses provided as alternative roosts during bridge replacement works were used by fewer bats than the original roost or were not used at all. At one site, seven bat houses built to replace a roost for four years during bridge replacement works were used by fewer Mexican free-tailed bats Tadarida brasiliensis (2,000 bats) than the original roost (40,000 bats). At three other sites, bat houses built to replace roosts used by pallid bats Antrozous pallidus (18 bats), Yuma myotis bats Myotis yumanensis (40–100 bats), and/or Mexican free-tailed bats (994 bats) during bridge replacement works were not used at all. At all four sites, bat houses (or ‘condominiums’) were built as temporary roosts while bats were excluded from their original roosts during bridge replacement works (dates not reported). Counts of bats before and after the works were taken from questionnaires completed by the California Department of Transportation.

(Summarised by: Anna Berthinussen)
Create spaces for roosting bats in road/railway bridges and culverts

A review in 2017–2018 of case studies at eight road bridges in California, USA (Harvey & Associates 2019) found that spaces created for roosting bats to replace those lost during bridge works were recolonized by bats in similar or greater numbers to the original roosts at half of the sites. At two sites, ‘add-on’ hanging roost boxes were used by Yuma myotis bats Myotis yumanensis in greater numbers (300 and 1,200 bats) than the original roosts (700 and 4,000 bats). At one site, recessed ‘cast-in place’ elongated roost boxes were used by Mexican free-tailed bats Tadarida brasiliensis in greater numbers (total 82,052 bats) than the original roost (40,000 bats). At another site, concrete ‘Oregon wedge’ panels were used by Mexican free-tailed bats in similar numbers (500 bats) to the original roost (400 bats). At four other sites, roosting spaces (including concrete slabs, concrete or plywood ‘Oregon wedge’ panels, and recessed ‘cast-in place’ elongated roost boxes) were used by 75–99% fewer Mexican free-tailed bats than the original roosts (see original report for details). At all eight sites, roosting spaces were created to replace those lost during bridge works. 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. Twenty-seven other case studies were reviewed in the report, but numbers of bats before and/or after bridge works were not reported.

(Summarised by: Anna Berthinussen)
Maintain bat roosts in road/railway bridges and culverts

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.

(Summarised by: Anna Berthinussen)