Introduction

Navigation is the process of determining and maintaining a course or trajectory from one place to another. An animal navigates when it finds, learns and returns to specific places (Gallistel 1990; Trullier et al. 1997). All microchiropteran bats use echolocation for navigation, and most of them also use it to find food. Bats perform navigational and foraging tasks by emitting ultrasonic signals and analysing the information contained in returning echoes. Echolocation behaviour is often discussed in the context of the detection, localisation and characterisation of prey. In these perceptual tasks, background structures (such as trees or hedges) are classified as unwanted clutter, which masks the prey and affects the likelihood of a successful capture (e.g., Schnitzler and Kalko 2001). However, when bats perform spatial navigation tasks, background structures no longer constitute unwanted clutter; instead, they provide relevant information for navigation (Schnitzler et al. 2003).

After leaving the roosts, many bats use specific routes to reach their hunting area. Routes are characterised by a sequence of places connected by segments. Each place is indicated by a constellation of landmarks and a segment rule that leads the bat to the next place (Denzinger and Schnitzler 2004). If many bats from the same roost use similar routes, the bundle of individual routes is referred to as a flyway (see also Bateman and Vaughan 1974). Rahmel and Dense (1997, 1998), Rieger et al. (1990), Rieger and Alder (1993), and Rieger (1997) showed that bats consistently use the same flyways over many years. The flyways of bats are often parallel to linear landscape elements such as tree lines, hedge groves or canyon walls (Myotis daubentonii: Rieger et al. 1990, M. emarginatus: Krull et al. 1991; Keil et al. 2005, Rhinolophid bats: Zahner 1984; Beck et al. 1994, Mormoopid bats: Bateman and Vaughan 1974). Limpens and Kapteyn (1991) found a positive correlation between the presence of linear landscapes and the abundance of bats. Verboom and Huitema (1997) observed that small species such as Pipistrellus pipistrellus fly closer to linear landscape elements than larger species such as Eptesicus serotinus. The earliest reports of bats using landscape elements as guidelines for flight routes were provided by Eisentraut (1952) and Nyholm (1965). Nyholm (1965) used the observed stereotyped flight behaviour of commuting Myotis mystacinus and M. daubentonii on routes from roosts to foraging habitats to catch them. Since then, many researchers have relied on the route-following behaviour to catch bats on their flyways (e.g., Bateman and Vaughan 1974; Racey and Swift 1985). Linear landscape elements are mainly used for commuting and are rarely used for hunting, with species-dependent differences: pipistrelle bats are more often observed foraging on flyways than M. daubentonii (Racey and Swift 1985; Verboom and Spoelstra 1999).

Many authors also discuss the special ecological functions of linear landscapes with respect to wind shelter, protection from predators, and foraging habitat. In addition to these benefits, highly consistent commuting along flyways helps to strengthen knowledge of the route that the bat frequently takes. Knowing the way may result in higher flight speeds on flyways than during search and foraging. Britton et al. (1997) reported flight speeds of 7–9 m/s in commuting M. dasycneme and Jones and Rayner (1989) of 7.45 m/s for commuting P. pipistrellus, which is higher than in foraging flight.

It has been suggested that landscape elements provide orientation cues (Limpens and Kapteyn 1991; Verboom and Huitema 1997; Verboom et al. 1999). Limpens et al. (1989) hypothesised that the short range of the echolocation system is the reason bats use linear landscapes for orientation. In commuting pond bats (M. dasycneme), Verboom et al. (1999) found that signal structure changes with respect to the bats’ distance to canal banks indicating, that bats are in more or less constant contact with the background structure (see also Britton et al. 1997).

Denzinger et al. (2001) suggest that horseshoe bats use guidance as navigation mechanism when flying parallel to background contours and the ground. Guidance is the ability to maintain a certain egocentric relationship with respect to a particular landmark or object (O’Keefe and Nadel 1978; Trullier et al. 1997). A typical example for guidance is wall following behaviour where the distance to the wall is kept constant. We assume that bats flying in parallel to landscape elements use echolocation to keep a certain distance to background contours such as vegetation edges or walls, which should result in contour following behaviour. The aim of this study is to compare the contour following behaviour of three species of vespertilionid bats and its dependence from environmental conditions.

Until now, the commuting behaviour of bats on flyways along landscape elements has mainly been discussed with respect to ecological aspects. The spatial extent of flyways, their spatial positions, and their geometrical dimensions in relation to the background have not yet been determined. It is unclear how a gap situation (background contours that are present on both sides of the bat) or an edge situation (background contour that is only present on one side of the bat) influences the commuting behaviour. An inter-specific comparison of flight behaviour on flyways is also missing from the literature.

In this study, we compared the flight and echolocation behaviour of three vespertilionid bat species while they commuted along flyways. Spatial position relative to vertical background contours and to the ground in edge and gap situations was measured and echolocation behaviour described. The first aim of the study was to describe the effect of available space (which we also refer to as spatial context) on the echolocation behaviour and on flyway dimensions of the bat M. daubentonii. In two comparable situations, we also evaluated the influence of different background structures on the flight behaviour of M. daubentonii. The second goal of the study was to compare the flight and echolocation behaviour of three bat species while they commuted along the same background structures. We wanted to test whether different species flying in an identical spatial context exhibited similar flight behaviour. Finally, we measured flight speed on flyways in order to investigate how flight speed is influenced by the spatial context and determine the flight speeds selected by different species under the same conditions.

Methods

Study sites and animals

Sound and video recordings of commuting bats were made in the summers of 2001–2003 at three study sites. One site was situated near Bremen (a lowland area), Northern Germany, and the two others were located near Schaffhausen, Switzerland. At all three sites, bats commuted from well-known roosts (two in trees and one in the roof of a farm house) to their foraging areas via flyways, which ran along background contours. At each location, we recorded the echolocation signals of bats after they left the roost at dusk. For further analysis, we only used data from the night with the highest commuting activity of the observed species on the flyway. We observed three species of vespertilionid bats: P. pipistrellus, M. daubentonii and M. brandtii. All species are medium-sized insectivorous bats that are often observed commuting on flyways after leaving the roost at dusk (Rahmel and Dense 1997, 1998; Rieger et al. 1990; Rieger and Alder 1993; Rieger 1997).

Near Bremen, we recorded all three species commuting along a road (from east to west). Trees on both sides formed a closed canopy above the road (referred to as “gap situation”; Fig. 1c). The resulting vegetation tunnel was 6 m wide and 11 m high. Our recording site was at a distance of 2 km from the roost (Rahmel and Dense 1997, 1998). At both sites near Schaffhausen, we recorded M. daubentonii in edge situations while bats commuted next to a house (6 m high, referred to as “edge situation 1”; Fig. 1a) and along the edge of a forest (25 m high, referred to as “edge situation 2”; Fig. 1b). In both edge situations, the contour was only present on one side of the bat (Fig. 1). At both sites, the bats roosted in the forest at a distance of about 3 km from our recording site. All flyways were well established, used by the bats over the whole summer, and have been observed over several years (Bremen: Rahmel and Dense 1997, 1998; Switzerland: Rieger et al. 1990; Rieger and Alder 1993; Rieger 1997).

Fig. 1
figure 1

Recording situations

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Species identification

The three observed species were identified by their echolocation signals. Identification of P. pipistrellus was straightforward since signal structure differs clearly from the Myotis species and P. pipistrellus was the only pipistrelle bat in this area. M. daubentonii and M. brandtii were identified by differences in start frequency (SF) and end frequency (EF). In our recordings, signals with the highest signal-to-noise ratio ranged from a maximal SF of 95 kHz and an EF around 29 kHz for M. daubentonii, whereas for M. brandtii, we recorded signals with SFs of above 105 kHz and an EF around 32 kHz. We confirmed our identification method once by netting bats of both species on the flyway and analysing the echolocation calls which were recorded just before they flew into the net. Recordings, which could not be classified beyond doubt, were excluded from the analysis.

Recording of flight and echolocation behaviour

Flights were recorded by infrared (IR) video. Two IR video cameras were fixed on a bar 2 m apart from each other. The bar was mounted on a tripod and positioned parallel to the flyway at a distance of 8–10 m for the background. Both cameras were angled such that the maximum overlap of the video images occurred at a distance of 6 m from the cameras. We observed a section of the bats’ flyway, which was about 10 m long. Two pulsed IR strobe lights (50 synchronized flashes/sec) mounted on top of the cameras illuminated the scene. Video recordings were stored on Mini DV tapes and synchronized with the sound recordings by adding the video time code to the sound files.

Echolocation signals were picked up with a custom-made ultrasonic microphone mounted on a tripod 30 cm above the ground and directed upwards (45°–60°) towards the bats. The microphone was placed against the direction of flight at approximately 3 m behind the observation area. A custom-made A/D converter digitized the signals with a sampling rate of 480 kHz and 16 bits. Custom-made software (PCTape) stored the signal sequences as .wav files on a laptop together with a video chunk that allowed the video to be synchronized with the sound recordings. A combined heterodyne unit made the signals audible. Voice notes about the observed flight behaviour were also stored on the computer.

Data analysis

Reconstruction of flyways

We used Simi motion® software to reconstruct the 3D positions of bats and background structures. We determined flight routes in relation to background structures with an accuracy of 1.7% (x-axis), 2.7% (y-axis) and 1% (z-axis). From the 10 m of flyway we observed, we reconstructed the section along which the video images overlapped, which was 6–9 m long. The length of the reconstructed flyway also depended on the distance from the flyway to the cameras.

To describe the flyways, we calculated the mean and standard deviation of the trajectories of all bats of the same species in y and z coordinates at every 25 cm along the flight path (Figs. 3a, b, 4a, b). The y–z values of each species were combined into one grand mean and standard deviation (Figs. 3c, 4c) to determine the average cross-section of a flyway.

Sound analysis

For sound analysis, we used a custom-made colour spectrograph (Selena, University of Tübingen). A Fast Fourier Transformation (256 points, Hann window, auto padding) delivered spectra, which were displayed as colour sonagrams with a frequency range of 240 kHz and an overlap of 94.14%. This resulted in a measurement accuracy of Δt = 0.02 ms and Δf = 469 Hz. The amplitude was displayed with a dynamic range of −90 dB. Only the dominant first harmonic of every signal was analyzed. The frequency-time course of every signal was tracked automatically in the spectrogram using the pixel with the highest amplitude for every FFT. Start, peak and end frequencies were extracted from these time courses (for further explanation, see Siemers and Kerth 2006). The peak frequency was determined as the pixel with the highest amplitude. Start and end frequencies were measured at 25 dB below the respective peak amplitude. From these values, signal duration, bandwidth and pulse interval were calculated. For statistical analyses, all weak signals with peak amplitudes of −25 dB relative to the maximal recording level were excluded. All foraging flights were also omitted from the analysis.

Statistics

Signal parameters were not normally distributed (Kolmogorov–Smirnov test). Accordingly, we used nonparametric statistics and compared signal parameters using the Kruskal–Wallis test. Data on flyway geometry were normally distributed (Kolmogorov–Smirnov test). T tests were used to compare the centre positions of flyways. In both tests, a Bonferroni correction was performed. Analyses of variance (ANOVAs) and Tukey post-hoc tests were used to determine the differences in flyway geometry (width) (Systat version 11.0 for Windows (© 2004 SPSS Inc.)). Summary statistics are given by means ± standard deviations.

Results

General behaviour

Common behavioural patterns of all species while commuting on flyways

At all sites and in all species, the bundle of individual routes formed a flyway which followed the linear background structure (house or tree line in Schaffhausen and tree line in Bremen). The bats used flyways only when leaving the roost at dusk but not when returning at dawn. Within the part of the flyway which was reconstructed in 3D, the bats used individual flight trajectories varied little in height or distance to background (<0.5 m) (Figs. 3a, b, 4a, b). Normally bats did not change their flight speed within the section of flyway under observation.

In pipistrelle bats, patrolling behaviour (flying back and forth) and feeding buzzes, indicating foraging activity, were observed several times. Only in these situations the bats varied their flight speed, height and distance to background. After capture attempts, bats returned to the flight behaviour they exhibited before. In Myotis bats, we only recorded foraging behaviour three times on all flyways over 3 years of observation. In all other cases, bats flew in straight trajectories and did not react to the presence of insects or to the foraging activity of pipistrelle bats.

Echolocation behaviour

The two Myotis species emitted signals typical for this genus. Myotis signals typically begin with an initial steep frequency modulated (FM) part, followed by a shallower middle element and a steeper terminal part (Fig. 2b, c). In short signals, the middle part is reduced in length. The signals of M. daubentonii and M. brandtii are often hard to distinguish. In our recordings of M. brandtii, the shallow part of the signal was often absent and the terminal FM part was reduced to a very short hook, whereas in M. daubentonii, the shallower middle part was generally present and the steeper terminal part was always present. Signal structure in pipistrelle bats is very characteristic. It begins with a very steep initial FM part, followed by a longer, shallow modulated component (Fig. 2a) (Kalko and Schnitzler 1993; Boonman and Schnitzler 2005).

Fig. 2
figure 2

Examples for echolocation signals emitted by the three investigated species while commuting in the gap situation: a P. pipistrellus; b M. daubentonii; c M. brandtii

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At each recording site, bats did not change signal structure within a sequence. The only exception occurred in foraging situations, in which the observed changes in signal structure were characteristic of approach behaviour.

Myotis daubenonii at three different sites

Flight behaviour

Along the forest edge (tree height 25 m), the average cross-section of the flyway (indicated by the standard deviation) used by M. daubentonii (n = 17) ranged from 2.6 to 3.8 m in height. The flyway was located at a horizontal distance of 3.2–5.8 m to the background. At the edge of the house (height 6 m) (n = 14), the average cross-section ranged from 2.6 to 4.3 m in height. The flyway was located at a horizontal distance of 2.1–4.5 m to the wall (Fig. 3c). In the gap situation (road), the cross-section of the flyway (n = 15) was narrower, ranging from 2.3 to 3.1 m in height at a distance of 1.9–2.8 m to background (Fig. 3d).

Fig. 3
figure 3

Flyways of M. daubentonii in two edge situations and one gap situation. Flyways are given as means (centre positions) and standard deviations. a side view; b top view. Dark grey shading indicates the position of background structures in the three situations and light grey shading indicates the position of the second tree line in the gap situation. c, d front view. (gap: n = 12, edge 1: n = 14, edge 2: n = 17)

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The centre positions of the flyways in both edge situations did not differ significantly from each other (t tests: horizontal: t 21 = 2.211, P = 0.115; vertical: t 21 = 0.486, P = 1) but both differed from the centre positions in the gap situation (all t tests: t df > 2.51, df ≥ 21, P < 0.05).

The horizontal width of the flyways at the edge of the forest (2.6 ± 0.4 m) and of the house (2.4 ± 0.5 m) did not differ. They were both significantly wider (F 2,81 = 149.2, P < 0.0001) than the flyway along the road (0.9 ± 0.2 m). In all three situations, the vertical width of the flyways differed significantly (F 2,81 = 184.3, P < 0.0001). The vertical width was smallest in the gap situation (0.8 ± 0.2 m), greater in the forest edge situation (1.2 ± 0.2 m) and greatest in the house edge situation (1.7 ± 0.1 m). Flight speed did not differ in the two edge situations (house 7.0 ± 0.7 m/s and forest 6.4 ± 0.6 m/s) but was significantly higher (F 2,41 = 18.2, P < 0.0001) than in the gap situation (5.6 ± 0.5 m/s).

Echolocation behaviour

M. daubentonii used broadband, steep, frequency-modulated (FM) sweeps with a prominent first harmonic. In all three situations, we observed similar pulse duration, pulse interval, and end frequencies. Pulse durations were slightly (but not significantly) longer at the edge of the house than in the other situations. Start frequency and, consequently, bandwidth varied considerably. In the gap situation (road), start frequencies and, consequently, bandwidth were significantly higher as well (both Kruskal–Wallis test; P < 0.0001) (Table 1).

Table 1 Parameters of signals emitted by M. daubentonii while commuting in different situations
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Three species at the same site

To compare the flight and echolocation behaviour of different species under similar conditions at one site we recorded flyways and echolocation behaviour of commuting M. brandtii, M. daubentonii and P. pipistrellus in the gap situation on the same night (Fig. 1c).

Flight behaviour

The centre position of the flyways used by M. daubentonii and M. brandtii were similar (t tests horizontal: t 25 = 0.083, P = 1; vertical: t 25 = 0.202, P = 1). M. brandtii (n = 15) used a flyway positioned at a height of 2.0–2.8 m and at a horizontal distance of 1.9–2.8 m to the background. The flyway used by M. daubentonii (n = 12) was positioned at a height of 2.3–3.1 m and was also at a horizontal distance of 1.9–2.8 m to the background. In the Myotis species, we found no significant difference in the horizontal flyway width (M. bra. 0.8 ± 0.2 m; M. daub. 0.9 ± 0.2 m) or in the vertical flyway width (M. bra. and M. daub. 0.8 ± 0.2 m) whereas the flyway used by P. pipistrellus was significantly narrower (width 0.6 ± 0.2 m) (F 2,81 = 33.50; P < 0.0001).

Along the vertical dimension, the centre position of the flyway used by P. pipistrellus (n = 12) differed significantly only from M. brandtii (t test: t 25 = 3.333, P = 0.0081). It was positioned at a height of 2.4–3.6 m, a slightly higher range than that of the Myotis species, but was significantly wider (1.2 ± 0.2 m) (F 2,81 = 23.1; P < 0.0001) (Fig. 3c). Along the horizontal dimension, the flyway used by P. pipistrellus was 2.4–2.9 m away from background structures and its centre position was not significantly different from the others (all t tests: t df < 2.40, df ≥ 22, P > 0.0729). In the gap situation, flight speed was about 5.5 m/s (Table 3) and did not differ significantly amongst species (F 2,33 = 0.026; P = 0.975).

Echolocation behaviour

M. brandtii and M. daubentonii signals had similar shapes, but differed in start frequency, end frequency and bandwidth. Start frequencies of M. brandtii (92.2 ± 2.8 kHz) differed highly significantly from M. daubentonii (87.3 ± 3.0 kHz) (Kruskal–Wallis test; P = 0.003). End frequencies were also significantly different (Kruskal–Wallis test; P = 0.02). Because of these differences in start and end frequencies, bandwidth was also higher in M. brandtii (58.9 ± 2.9 kHz) than in M. daubentonii (55.5 ± 3.2 kHz) (Kruskal–Wallis test; P = 0.045). Signals emitted by P. pipistrellus had narrower bandwidth (Kruskal–Wallis test; P < 0.0001) and longer pulse duration. Pulse intervals did not differ significantly amongst species (Kruskal–Wallis test; all: P = 1) (Table 2).

Table 2 Parameters of search signals emitted by different species while commuting in the same situation
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Constancy of flyways over 3 years

To determine the long-term stability of flyways, we compared the width and position of the flyways used by M. brandtii on one night in three successive years. We found no differences in the flyways’ depth (Y) and height (Z) between the years (Fig. 5) (ANOVA: Y dimension: F 2,27 = 1.236 P = 0.306; Z dimension: F 2,27 = 0.473).

Discussion

Influence of spatial context on flight and echolocation behaviour

A flyway is a bundle of individual routes used regularly by many bats, e.g., for direct flights to their hunting area (Schnitzler et al. 2003). To understand how the type of background structures and the available flight space affect flight and echolocation behaviour, we determined the spatial position of flyways and analyzed the echolocation signals of M. daubentonii commuting in two different edge situations and one gap situation.

In the edge situations, the flyways were wider and their centres were further out than in the gap situation (Fig. 3). However, the lower and closer parts of the cross-sections of the flyways were located at about the same position in edge and gap situations (Fig. 3c, d). This partial overlap may indicate that similar sensory processes are used to maintain a species-specific minimal flight height and horizontal distance to the vertical background contour.

In the gap situation, the bats did not choose to fly in the middle of the gap, which indicates that they prefer to use one side as a guideline. One explanation for the smaller diameter of the flyways in the gap situation may be that bats are compressed by the other side of the gap whereas in the more open edge situations, the flyways are extended towards the open space with no potential obstacles.

The type of background structure had no significant effect on the spatial position of the flyway. The flyways’ centre positions and widths were similar regardless of whether the edge consisted of a wall or vegetation. Bats commuting along the flat wall of a house flew only slightly closer to the background than when they flew along the more structured forest edge.

Echolocation behaviour depended solely on whether bats flew in an edge or in a gap situation. In the gap situation, signals had higher start frequencies, resulting in broader bandwidth. These results support the findings of Schaub and Schnitzler (2007) who showed that the structure of the signals emitted by commuting Vespertilio murinus varied systematically with the horizontal distances between bat and background. Similar tendencies have been described for P. pipistrellus (Kalko and Schnitzler 1993) and for M. dasycneme (Britton et al. 1997; Verboom et al. 1999).

In a comparison across species, we investigated how two Myotis species (M. daubentonii, M. brandtii) and one pipistrelle bat (P. pipistrellus) arranged their flyways in a gap situation. We compared their flight and echolocation behaviour while they commuted along the same part of a road. The flyways used by the two Myotis species (M. daubentonii, M. brandtii) were similar in cross-section and position, whereas the flyway of P. pipistrellus was located slightly higher (0.5 m), narrower in the horizontal dimension (0.3 m) and wider in the vertical dimension (0.4 m). Myotis bats flew at a height of about 2–3 m above the road, while P. pipistrellus flew at heights of 2.4–3.6 m. All three species kept away from the middle of the road and used one side of the gap as their guideline. The similar flight behaviour of the three species may indicate that the bats arrange their flight routes by maintaining a specific or even species-specific egocentric relationship to the ground and the vertical contour lines alone. This kind of guidance behaviour could be the result of a low-level processing of the sensory input data which delivers specific distance values.

Another possibility is that bats react mainly to ecological features of the environment, such as light, wind and predators when choosing a specific route. It is also possible that the position of the flyways is the result of a combination of guidance and ecological conditions. The literature focuses mainly on ecological constraints governing flight route positions. In P. pipistrellus, Verboom and Spoelstra (1999) also observed flight heights of 2.5–3.5 m above a road bordered by tree lines. They proposed that this tendency may be explained by higher wind speeds above 3.5 m and an increased ability to avoid echo overlap, which occurs when bats are too close to the ground. In our bats, signal duration was always shorter than 4.3 ms, leading to an overlap zone up to a height of 0.73 m. Therefore we assume that overlap avoidance does not explain the preferred height of 2.5 m. There are other possibilities which might have influenced the flyways of the bats. Maybe the bats preferred to fly in the shade and avoid the lighter parts of the road. Bats on flyways are highly predictable prey, so it may be an advantage to fly in darker areas to avoid predators that rely on optical cues. The avoidance of lighter areas is supported by the fact that the centres of the flyways were not in the middle of the road but slightly shifted towards the somewhat darker southern side below the canopy (Fig. 4c). It is also possible that bats had some experience with cars when crossing streets or following tree lines. Thus they may have chosen a height that enables them to avoid collisions with cars as well as with branches of the canopy. But even if the ecological conditions have some influence on the position of flyways the bats still have to use a navigational mechanism such as guidance to keep the favourable distance to the background contour.

Fig. 4
figure 4

Flyways used by the three bat species M. daubentonii (n = 12), M. brandtii (n = 15), P. pipistrellus (n = 12) flying in the gap situation. Flyways are given as means (centre positions) and standard deviations. a side view; b top view; c front view. Dark grey shading indicates the position of background structures

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The analysis of echolocation behaviour revealed species-dependent differences in signal design in the gap situation which may indicate species-specific differences in the perceptual abilities of the three species according to their adaptation to different niches (see also Boonman and Schnitzler 2005). In all species, echolocation signals had equal pulse intervals, resulting in identical repetition rates. Therefore, the different species acquired the same information update from their surroundings.

We conclude that the position and dimensions of flyways were mainly determined by the available space. This may explain that there are only small differences in the width and positions of the flyways in the three species, which are all about the same size, fly at similar speed, and therefore have similar manoeuvrability. This conclusion is further supported by the fact that in M. brandtii, the width and the position of flyways did not vary over 3 years. We therefore assume that our three species have a similar perceptual mechanism which may lead them to maintain a specific distance to horizontal and vertical background structures. We also assume that other factors such as avoiding wind, echo overlap, or predators, affect the flyways to a lesser degree, if at all.

Influence of spatial context on flight speed

In commuting M. daubentonii, we observed an increase in flight speed from the narrow situation to the more open situations (gap situation to forest edge and house edge). Britton et al. (1997) report higher flight speeds of M. dasycneme commuting along a narrow canal (13 m; ∼9.13 m/s) than along a wide canal (25 m; ∼7.35 m/s). They assume that the bats flew faster at the narrow canal because it was closer to the nursery colony and predation risk was higher whereas further off at the wider canal, the bats could not maintain the high flight speed. In contrast to these data, we observed the lowest flight speed in the gap situation which was nearest to the nursery colony. We assume that the width of available space has some influence on the flight speed. As width decreases, the bats have to be more careful to avoid collisions with potential obstacles and therefore fly more slowly.

All three species commuting in an identical gap situation had similar flight speeds (∼5.5 m/s). Based on a model of Rayner (1987) and the morphological data of Norberg and Rayner (1987), we calculated optimal flight speeds for the three species of interest (Table 3). The predicted flight speeds for the three species are similar despite the fact that only M. brandtii and P. pipistrellus are similar in weight and wing morphology, whereas M. daubentonii is larger and has a larger wingspan. Field studies conducted by other authors (Baagoe 1987; Jones and Rayner 1988, 1989) have confirmed that the three species have similar flight speeds.

Table 3 Flight speeds of M. daubentonii, M. brandtii and P. pipistrellus reported in this study compared to values reported by other authors and in different flight situations
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The comparison of our measured data with other field data from literature indicates that in all species flight speed in commuting flight was higher than in search flight (Jones and Rayner 1989, Britton et al. 1997). Norberg and Rayner (1987) used morphological measurements to calculate optimal flight speeds. They predict maximum range speeds (V mr) that are close to the observed search flight speeds for M. daubentonii (Baagoe 1987; Jones and Rayner 1988), P. pipistrellus (Baagoe 1987; Jones and Rayner 1989) and M. brandtii (Baagoe 1987) but lower than our measurements for commuting bats (Table 3). According to the model of Norberg and Rayner (1987) commuting flight will result in higher energy costs. Ekman and de Jong (1996) assume that commuting bats can compensate for these higher costs by reaching the foraging areas faster and feeding earlier. This is a benefit because insect density declines rapidly after dusk and bats’ food intake should be highest within the first few hours after dusk (Racey and Swift 1985).

Advantages of using stable flyways along linear landscape elements

Many bats commute on flyways which are arranged along linear landscape elements (M. brandtii: Rahmel and Dense 1997, 1998, M. daubentonii: Limpens and Kapteyn 1991; Rieger et al. 1990; Rieger and Alder 1993; Rieger 1997, M. emarginatus: Krull et al. 1991, Mormoopid bats: Bateman and Vaughan 1974; Denzinger et al. 2001, Rhinolophid bats: Beck et al. 1994; Denzinger et al. 2001; Zahner 1984). We observed our bats over 3 years and found a high spatial consistency on flyways (Fig. 5) and a stable pattern of activity along the flyways. Rahmel and Dense (1997, 1998) and Rieger et al. (1990), Rieger and Alder (1993), Rieger (1997) also found that emerging bats used flyways over long periods and Bateman and Vaughan (1974) described highly stable flyways in emerging tropical bats.

Fig. 5
figure 5

Flyways used by M. brandtii flying in the gap situation at 3 days in three different years. Flyways are given as means (centre positions) and standard deviations (front view). Dark grey shading indicates the position of background structures

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The usage of flyways along linear landscape elements must provide valuable advantages for bats since it often results in longer routes than the shortest possible connecting path (Krull et al. 1991). While navigating in the dark, bats require landmarks to recognize places and contours to perform guidance behaviours such as contour following (Denzinger and Schnitzler 2004). Linear landscape elements provide both good landmarks and contours which can be used to build up a cognitive map. Moreover, by using flyways over a long period of time, the bats gain familiarity with the situation and the route. The high spatial consistency on a familiar flyway over years reduces the danger of sudden collisions, enables the bats to fly faster and reach the foraging areas earlier in spite of detours caused by the flyway.

Additionally, flying in the shadow of tree lines may reduce predation pressure because the bats are less visible for predators relying on optical cues. Rodriguez and Lewis (1985) observed two flyways used by thousands of bats (Pteronotus fulginosus) from one cave. One was located in a corridor of two tree lines and the other was above open pasture. They found that hunting success of foraging falcons was higher over the pasture than within the corridor. On the other hand, while flying on flyways bats can be highly predictable prey, especially for some predators such as owls or hobbies (Verboom and Spoelstra 1999; pers. comm. Christian Dietz).

We conclude that regularly used flyways along linear landscape elements are advantageous for bats because they provide landmarks and contours for orientation, thus enabling bats to fly faster in a familiar situation, and reduce predation pressure. The advantages conferred by using flyways outweigh the costs of having to make detours and of being predictable prey.