Woodland ecosystems of Europe have undergone major transitions in the last centuries. Changes in land use and the loss of natural forest dynamics have often led to structurally poor, uniform and dense stands. Not surprisingly, open forest species relying on a heterogeneous stand structure have suffered dramatic population declines. The European Nightjar Caprimulguseuropaeus, a nocturnal insectivorous bird, has undergone such a decline in its main Swiss stronghold in Valais. Despite the species’ potential to colonize new sites and habitat restoration measures implemented since 2001, recolonisation of restored sites has not taken place, suggesting problems with the current habitat restoration strategy. In order to refine management recommendations, we compared habitat structure and moth abundance, a key Nightjar food source, at sites that are still occupied and at sites that had been abandoned but have recently been restored. Vegetation structure was more heterogeneous and moth abundance greater at occupied than at abandoned sites. More specifically, occupied sites harboured a greater coverage of bare ground, while abandoned sites exhibited a higher amount of regeneration and intermediate shrub layer. The occurrence of natural perches was also higher in occupied sites. Abandoned sites are thus characterised by lower prey abundance and denser vegetation cover, the combination of which is likely to lead to lower prey availability for hunting Nightjars. Restoration action would benefit from maintaining snags and dead branches and by targeting unproductive habitats characterised by mineral soils, thereby slowing down regeneration and shrub regrowth. For future successful management of Nightjar habitats, it seems thereby essential trying to find the balance between actions that allow opening the lower and mid-strata of the forest while sustaining high moth populations.

European Nightjar, forest, habitat management, moths, vegetation structure

European forest ecosystems have undergone substantial changes over the last few centuries (Nilsson 1997; McGrath et al. 2015). After strong declines following the expansion of agriculture, the forested area has started to recover since the beginning of the 20^th century. The changes in timber harvesting practices have led to a homogenisation of forest structures, which can have negative effects on biodiversity (Spiecker 2003). Awareness of the importance of sustainable forest management for biodiversity and ecosystem services only emerged within the last 20 years (e.g. Lähde et al. 1999; Bengtsson et al. 2000; Führer 2000; Seymour et al. 2002). Natural hazards and disturbances are important drivers of natural forest ecosystems (Angelstam 1998; Kuuluvainen 2009; Kulakowski et al. 2011): they create pioneer habitats whose natural successional stages harbour locally adapted faunal and floral communities (Angelstam 1998). Although some forest species might benefit from stand homogenisation (Paillet et al. 2009; Fonderflick et al. 2010), pioneer species will suffer, notably those depending on the early stages of the vegetation succession.

The European Nightjar, Caprimulguseuropaeus, is a nocturnal bird species and is mostly relying on open habitats in wooded areas. With the loss of heathlands in Great Britain, previously the main habitat for Nightjars (Conway et al. 2007), other wooded habitats became more important such as young coniferous forest plantations (Sharps et al. 2015), clear fells up to 10 years after tree plantation (Cross et al. 2005) or forested sites after the occurrence of a major disturbance (Scott et al. 1998). This selection for younger growth reflects a preference for semi-open habitats, where a subsequent tree regrowth of up to 2–3 meters results in an unsuitable habitat (Ravenscroft 1989). A study in an inner-alpine valley in Switzerland similarly revealed a positive effect of semi-open landscapes (mainly oak-pine scrublands) where Nightjars preferred habitats with 20–50% tree coverage (Sierro et al. 2001). The requirement of a fairly open habitat is associated with the hunting strategy of the Nightjar. Nightjars spot their flying prey, mostly moths (Glutz von Blotzheim and Bauer 1994), from the ground or a perch, discriminating them from the sky background (Jackson 2003). They prefer catching their prey from below (Glutz von Blotzheim and Bauer 1994) and hence need open space to be able to follow their prey. Recent studies indicate that Nightjars show a certain degree of plasticity in spatial foraging behaviour (Evens et al. 2017, Sharps et al. 2015). In order to reach optimal feeding grounds birds can travel long distances, which is however physiologically costly (Evens et al. 2018).

While the European Nightjar is categorized as least concern by the IUCN (BirdLife International 2012), the species is of conservation concern in Switzerland (Keller et al. 2010), with roughly 80% of the Swiss population occurring in the southwest (Valais). Over the past five decades this Nightjar population has steadily declined, with the abandonment of several sites and the colonisation of only a few major new sites created by forest fires (Sierro 2013). This decline might partly be associated with habitat degradation by changes in land use and forest management (Langston et al. 2007) that may affect the optimal vegetation structure and/or the abundance of prey, i.e. large moth species (Jackson 2003, Sierro et al. 2001). In effect, the majority of Valais forests were progressively transformed into denser stands (Bollmann et al. 2009), which represent unsuitable habitats for an open habitat species such as the Nightjar. Several targeted habitat restoration measures, based on evidence from the UK (Morris, Burges and Fuller 1994; Cross et al. 2005) and central Europe (Sierro et al. 2001; Wichmann 2004) have been implemented at 18 abandoned sites in Valais since 2001. Measures included varying sizes of clear-cuts (up to 2 ha), selective logging and habitat maintenance via grazing by highland cattle and goats (Sierro 2013). Despite a long term Nightjar monitoring scheme (Sierro 2013), only scarce observations of displaying males in seemingly restored habitats could be gathered. This, despite the fact that Nightjars in Valais are able to recolonize new sites as shown in a burnt woodland site where several individuals colonized the area a few years after the fire. Overall, this suggests that the locally implemented Nightjar restoration programme was not properly targeting the species-specific ecological requirements of the Nightjar.

In this study, we analyse variation in vegetation structure in the southwestern part of Switzerland, as well as the abundance, biomass, species richness and diversity of moths in abandoned and restored sites and compare them to sites occupied by the European Nightjar. Vegetation structure was investigated to identify fine-scale habitat preferences of the Nightjar while data on moth abundance and diversity should highlight whether local prey could be a limiting factor in abandoned sites. Based on these findings, we shall provide new, refined management recommendations to hopefully improve restoration actions.

This study establishes that Nightjar occurrence in the upper Rhône valley (SW Switzerland) can be associated with an open vegetation matrix that offers abundant prey (moth) resources. Although our findings cannot disentangle the relative importance of these two factors in explaining the occurrence of the Nightjar, they can help to refine habitat restoration guidelines for this endangered nocturnal bird species.

Vegetation structure differed between abandoned and occupied sites at different height levels. At the ground level, an open habitat seems to be especially important, as evidenced by the high percentage of bare ground coverage found in occupied sites. Our results contrast with the study by Wichmann (2004) who did not find any such relationship, most likely because the study area consisted of nutrient poor soils across the whole range (Wichmann 2004). The importance of habitat openness at ground level was emphasized by several studies and may be important for nest building (Cross et al. 2005; Verstraeten et al. 2011), egg and chick concealment (Jackson 2003) and to escape approaching predators (Camacho 2014). Alternatively, an open habitat at low levels facilitates the hunting strategy of the Nightjar, which often sits on the ground and spots flying moths that are more easily discriminated in front of a lighter sky background (Jackson 2003). The cause for the open ground in occupied sites may ultimately lie in the stone- and gravel-rich substrate that hinders vegetation regrowth, as typically observed on unproductive soils of floodplains or rocky steppes. The fact that several abandoned, and subsequently rehabilitated sites in our study area were on rather productive soils led to a rapid regrowth after habitat restoration measures, probably rendering the habitat unsuitable for Nightjar already in a short term.

Although several studies have indicated the importance of shrubs for Nightjar (Bartolommei et al. 2013; Sharps et al. 2015), our study refines these findings by demonstrating a quadratic relationship with an optimum at around 50% cover, in line with Sierro et al. (2001). The presence of a dense regrowth in abandoned sites might hinder the bird’s hunting activities and again be a direct consequence of fast succession on fertile soils in combination with the on-going changes in land use. The progressive decrease of grazing pressure in loose forest stands and semi-open wooded habitats since World War II has promoted woody stands mainly dominated by deciduous shrubs (Rigling et al. 2006). However, very low levels of shrubs seemed unattractive for Nightjars too, most likely because they provide not enough shelter for roosting (Camacho 2014) and constitute unsuitable habitats for many moth species (Spitzer and Lepg 2012).

Additionally, our results point out the importance of perches (standing and lying dead wood) for Nightjar, corroborating the findings by Wichmann (2004). These structures are used as vantage hunting and singing posts during nighttime, and as daytime roosting sites, respectively. Camouflage among dead bark and branches seems particularly important for daytime roosting sites, again to avoid predation (Jackson 2003; Wichmann 2004; Langston et al. 2007). Improving this habitat feature is easily manageable by either refraining from cutting dead snags or by felling large trees and retaining them in the restored areas.

Moth abundance and biomass were higher in occupied than abandoned sites, with the strongest difference late in the season in July. We observed a general increase in both moth richness and diversity with the advancement of the season, reaching the highest values during the nestling provisioning period (Alexander and Cresswell 1989; Cross et al. 2005). While it is known that Nightjars can forage far away from the nesting site (up to 4km, Evens et al. 2017), enhanced food availability in close proximity to the nesting site most likely reflects the optimal foraging habitat for food-provisioning parents (Evens et al 2018). A potential explanation for reduced prey abundance in abandoned and restored Nightjar sites may result from the restoration management itself. Moth abundance usually peaks in old and undisturbed forests with high amounts of native tree species (Summerville and Crist 2004; Spitzer and Lepg 2012; Sharps et al. 2015) where clear-cuts are known to negatively impact moths (Summerville and Crist 2002; Mönkkönen and Mutanen 2003; Summerville 2013; Fedrowitz et al. 2014).

Drawing on these results, we suggest that the long-term persistence of the Swiss Nightjar population relies on open and semi-open forests with a high abundance of moth, this result being in line with previous studies (eg. Sierro et al. 2001; Sharps et al. 2015; English et al. 2017). Most importantly, Nightjar habitat restoration should focus on creating semi-open forest stands on poor, fairly mineral substrates with a poor ”succession potential”, i.e. with a low regeneration speed of shrubs. A wooded matrix that incorporates a network of smaller clear-cuts that retain heterogeneous vegetation layers would increase the amount of forest edges, which appear to be crucial for Nightjar preservation (Langston et al. 2007; Sharps et al. 2015). Keeping a few old dominating trees, some snags and lying dead wood will further guarantee sufficient availability of perches and daytime roosts. Such a heterogeneous matrix should provide a suitable habitat for the Nightjar and its insectivorous prey. The long-term persistence of an optimal habitat matrix, resembling a dynamic succession stage, is challenging but could be achieved by controlled grazing with highland cattle and goats, as proposed by Sharps et al. (2015) thereby repressing regrowth on lower vegetation strata and creating small-scale disturbances that should positively affect biodiversity in general. More studies investigating the fine-scaled foraging behaviour (Evens et al. 2017) are needed in order to fully understand the species habitat requirements in an inner-alpine habitat context.

Designed and wrote the manuscript: NW, PK, RA, AJ.

We would like to thank Pierrick Buri and Jean-Yves Humbert for statistical advice and Antoine Sierro and Hans-Peter Wymann for moth identification. We are very grateful for all people helping during fieldwork especially Laura Bosco, Laura Bruppacher, Reto Burn, Rico Hasler, Irina Hotz, Hans Jenni, Denise, Hene and Kaspar Keller, Beat Müller, Antoine Sierro, Simon Stricker, Eveline Tissot, Beni, Vreni and Christian Winiger. In addition, we appreciate the valuable inputs to the manuscript by David Plotkin and Jean-Nicolas Pradervand.