The movements of temperate, insectivorous bats (Chiroptera, Vespertilionidae and Rhinolophidae) have movements driven by a range of factors which manifest themselves in distributions and landscape use derived largely from calorific intake (i.e., a landscape of energy (Shepherd et al. 2017)) and perceived predation risk (i.e., a of fear (Gallagher et al. 2017).
Most insectivorous bats are central place foragers (Rainho and Palmeirim 2011), gathering food and returning to a chosen roosting site (or in the case of bats, often several roosting sites). Roosts are chosen based on a range of internal and external characteristics, and foraging areas are fundamentally selected based on how calorifically lucrative they are. Still, the proximity of these to each other, and the qualities of the connective routes between them play vital roles in site selection. The utilisation of these sites by bats is a balance between potential calories consumed and potential calories burned. Basic metabolism, commuting, foraging, mating and hibernating all contribute to a bat's calorific burden, which is also dependent on sex and season, as females experience an increased calorie burden during lactation driven by an increased need to drink (Adams and Hayes 2008), produce milk, and undertake return trips to the roost to nurse each evening (Henry et al. 2002). All things being equal, the energy landscape would result in an optimal network of roosts and foraging grounds, with direct and efficient commuting routes connecting them. But this network is profoundly affected by the landscape of fear.
The fear landscape directly and negatively impacts the energy landscape, through behavioural changes driven by perceived predation risk. Bats thus have the option to use commuting routes with higher perceived predation risk to keep the calorific burden of travelling to their favoured foraging grounds low, to forage instead at suboptimal grounds to which there are ‘safer’ routes, or to take a longer, yet ‘safer’ route with increased calorific burden where predation risk is deemed too high.
Nowhere is the juxtaposition of these two landscapes more dynamic than in the urban matrix, where artefacts of the urban environment can affect both the energy and fear landscapes profoundly. These artefacts may act as either positive or negative charges, sculpting bat commuting routes by deterring or attracting them. Physical artefacts (e.g., motorways and hardstanding) create barriers which are either impassable to many insectivorous bats (excluding high-flying, open space foragers such as Nyctalus spp.) or result in a bat either accepting an increase in calorific burden to avoid or instead visit suboptimal foraging areas (Altringham and Kerth 2015). Perceived predation risk can also be increased simply by a lack of cover, which creates a barrier for species that are reluctant to cross open ground (Altringham and Kerth 2015; Andrews 2018). Non-physical, operational artefacts such as noise and light can also have a direct impact on both energy and fear landscapes. Light pollution increases perceived predation for some species, while noise pollution can have a powerful impact on the energy landscape through acoustic masking (Siemers and Schaub 2021), reducing calorific intake, particularly for species in the gleaning feeding guild (Allen et al. 2021; Hughes et al. 2023a). Both noise and light can delay emergence, affect the health and development of pups, and ultimately cause roost abandonment (Boldogh and Samu 2007; Stone et al. 2015), resulting in suitable roosts being further from foraging grounds, and thus increasing daily calorific burden.
The additional complexities notwithstanding, not all urban artefacts are deleterious to bats, and many effects are species- and feeding guild-specific, impacting size and fitness (Hughes et. al, 2023a), creating advantages for synanthropic or urban-adapted species. Artificial light sources can create a food source (light-seeking insects) which is capitalised upon by some hardier bat species (Jung and Kalko 2010); railway verges can be used by some bat species for commuting (Vandevelde et al. 2013), and even offshore wind farms have been recorded as being used as roosting opportunities by migratory bats (Brabant et al. 2019). Bats will frequently use waterways to pass under large infrastructure barriers (Abbott et al. 2012), and will also readily use smaller, more artificial structures such as aqueducts as ‘blue bridges’ to do the same (Hughes 2023).
For many urban planning authorities in Europe, bats are a taxonomic group of conservation priority, with many taxon-specific NGOs lobbying for bat conservation (Racey 2013). However, little is known about how and by what routes bats traverse conurbations, and as such, mitigation and conservation schemes (however well intended) are rarely evidence-based in practice (McKie 2024), and are seldom effective, with gantries although often used rarely proving effective mitigation in urban development schemes (Berthinussen and Altringham 2012). Moreover, most landscape-scale data on bat movements are gathered to assess or monitor the predicted impact of large-scale infrastructure projects, and though often based on robust radio telemetry studies, such analyses rarely happen in the context of urban planning for conservation, as they are prohibitively expensive and provide data on only a limited number of individual bats.
In these case studies from the West Midlands, UK, we demonstrate the effective application of the Multi-Point Activity Surveys (MultiPAS) method (Hughes et al. 2023b) in two survey models: a Linear study and a study of canal Junctions. Based on the identification of key commuting routes, we propose candidate urban dark corridor sites, and present results which shed light on the complex palimpsest of energy, fear and anthropogenic landscapes which urban bats inhabit, providing key principles for informing urban planning with evidence-based urban sustainability in mind.