Narrow, unpaved roads and paths are a ubiquitous feature of managed forest landscapes worldwide, with the potential to influence bird communities. However, compared to large roads with confirmed negative impacts like fragmentation and noise on birds, the effects of linear canopy openings and other structural changes caused by unpaved small forest roads and paths are less understood. In this study, we investigate the influence of narrow linear openings in the forest canopy caused by forest roads and paths on bird communities in the southern Black Forest, Germany. We aim to understand how these linear canopy openings affect species richness, community composition, and functional traits in forest birds. We surveyed bird communities in four distinct plot types, representing a gradient of ‘canopy openness’: forest interior, forest paths, forest roads, and forest edges. Forest roads and paths represent intermediate conditions between undisturbed interior forests (closed canopy) and a forest edge (open canopy). Our results show that while bird species richness remains relatively similar among those four plot types, the community composition at forest edges differs from the other plot types. In addition, functional traits like body mass and wing shape showed a weak response to the linear canopy openings. These results suggest that although unpaved forest roads and paths potentially introduce resources and structural modifications in the canopy, the effect on the birds seems limited compared to more pronounced habitat transitions, like those on forest edges. Our findings contribute to a better understanding of birds’ responses to linear and small-scale fragmentation introduced by unpaved forest roads and paths. However, there is a need for more research to distinguish the ecological impact on bird communities in temperate managed forests along a gradient of canopy openness.

IntroductionFrom fortified borders to decorative garden walls, fences form ubiquitous widespread networks that sprawl across the global terrestrial landscape. Fences and walls are one of the oldest tools used by people to manage other people and wildlife, e.g., by marking territorial boundaries, separating livestock from wild animals, or monitoring the movement of people through border controls. There is no reliable measure of extent of the global fence network, however it is estimated to be at least 10 times that of the global road network (Jakes et al. 2018), which is currently more than 64 million km (Dulac 2013), and expected to reach 90 million km by 2050 (Laurance et al. 2014). Despite the enormous extent of the fence network, fences are rarely subjected to Environmental Impact Assessments, and the ecological impacts of fencing are severely underestimated and understudied (Jakes et al. 2018, McInturff et al. 2020, Buton et al. 2024). The main purposes of a fence – to fragment, isolate and/or reduce interactions – often align with negative ecological impacts, while meeting the goals to protect livestock, prevent access of invasive species, and control the spread of diseases (Terborgh et al. 2001, Woodroffe et al. 2014, Jakes et al. 2018, Mysterud and Rolandsen 2019, McInturff et al. 2020). Fences and walls are erected through all landscapes, including those that are ecologically important and protected. One example that has received a lot of media and political attention, is the recently constructed border fence between Poland and Belarus which cuts through Białowieża Forest, the best preserved lowland temperate forest in Europe and transboundary World Heritage Site (Jaroszewicz et al. 2021). Such barriers have the goal to hinder the flow of people, and consequently also block the movement of wildlife and challenge landscape connectivity (Olson and van der Ree 2015, Linnell et al. 2016), which can trigger a cascade of direct and indirect ecological impacts (e.g. Nowak et al, submitted). Border fences are also used to control the spread of diseases, such as African Swine Fever (ASF). With the aim to reduce the movement of infected wild boar, and contain ASF, fences have been built along numerous country borders in Europe, e.g., the 70-km long ASF fence at the German-Danish border which runs directly through Natura 2000 areas (Eilenberg and Harrisson 2023, Klein et al. 2024). Similar border fences to control ASF exist at the German-Polish border, and are planned to be installed at the Norwegian-Swedish border. The fragmentation effect of border fences is clear and apparent, however the impacts caused by fences that are for example, along roads and railways, denote property borders, or enclose protected areas are often underestimated. Without proper research of the downstream or indirect impacts to non-target species and the functioning of ecosystems, negative impacts of fences will continue to be overlooked and unaccounted for. This special issue provides a collection of articles in which authors present different perspectives on fences and how we, as humans, use fences to communicate with the natural world and manage wildlife. Finally, the issue provides guidelines to improve fence design and implementation, and discusses research steps to move forward in the field.