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We tested the hypotheses that the distance bats fly from tree lines depend on food abundance and protection from wind. We monitored the activity of pipistrelle bats (Pipistrellus pipistrellus) and measured insect abundance and wind speed and direction at fixed distances up to 50 m from tree lines. We compared bat behaviour in different situations: with and without wind and with low and high insect abundances in adjacent open areas. In all situations, pipistrelle bats' activity decreased with increasing distance from the tree line. Within nights, we found no effect of wind speed on bat activity (sound recorded per 5 min) on the leeward side of the tree lines. Between nights, however, bats concentrated their activities closer to the tree lines at high wind speeds or angles of incidence of wind from 45° to 90°. A significant relationship between bat and insect abundances was found only when the tree line was bordered by insect-rich grassland. Since wind and insect abundance only partly explained the distances bats flew from tree lines, two alternative explanations, namely predator avoidance and the use of tree lines as acoustic landmarks, are discussed. Pipistrelle bats using a double row of trees as a commuting route at dusk flew mainly between the tree lines, regardless of insect abundance or wind speed. It is argued that predator avoidance explains this behaviour, being a constraint on movements of bats at relatively high light levels. At high wind speeds and angles of incidence greater than 45°, the proportion of pipistrelle bats commuting on the leeward side of the tree lines increased.

The effects of woodlot size and isolation, in relation to habitat fragmentation, on the distribution of the red squirrel were studied. In The Netherlands, 50 woodlots (0.55-13.78 ha) were surveyed in an agricultural landscape for the presence of red squirrel. In 26 woodlots squirrel dreys (nests) were found. Logit regression analysis showed that woodlot size and the area per woodlot covered with coniferous trees were the best predictors of squirrel presence. Addition of isolation variables by means of a stepwise forward regression method showed significant effects of the distance to a large, permanently inhabited wood and the amount of surrounding wood. No effect was found for the distance to the nearest woodlot (> 0.5 ha). The model could be further improved by adding a measure of the amount of hedgerows surrounding a woodlot.

With 6 figures in the text)Flight performance and echolocation behaviour of the pond bat. Myoris dasycnerize (Boie, 18251, were investigated over canals and a lake in the Netherlands. Multi-flash stereophotogrammetry, linked with synchronous recording of echolocation calls, was used to reconstruct the three-dimensional flight paths of bats and to correlate flight behaviour with changes in pulse emission during echolocation. Echolocation calls during commuting flight at the canal were sigmoidal in structure. with an initial steep frequency modulated (FM) sweep followed by a more shallow sweep, ending with a second, rapid FM sweep. Similar calls were produced during the search phase of foraging, though longer duration quasi-CF (constant-frequency) calls, with longer interpulse intervals and much reduced initial and final FM sweeps, were produced by bats hunting over a large lake. Overall, calls were generaliy lower in frequency, with significantly longer pulse durations and interpulse intervals. compared with those of smaller trawling Myotis bats. Flight speeds were compared with predicted optima from aerodynamic models. Commuting bats exceeded predicted maximum range speed. which was considerably higher than that noted for other smaller species of trawling Myotis. Flight speed was significantly higher closer to the roost, at a narrower canal site. A negative correlation between wingbeat frequency and flight speed, and a faculvative I:1 link between pulse emission and wingbeat during search phase were established. During commuting, the bats flew at a height above that which would normally confer substantial power savings through ground effect. It is argued that, by flying at a lower height, bats would compromise commuting speed. Echolocation calls produced during attempted prey capture were remarkably similar in design to those produced by those other Myotis species that forage in a similar fashion. Prey was detected at 2 m, and reaction distance was 1.8 m.Evidence of a physiological limit on pulse duration and interpulse interval during prey capture is presented. Overlap of emitted pulse and received echo may degrade the performance of echolocation in many bats, and overlap is generally avoided in M. dasycneme except perhaps when the bat is very close to the prey item.

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We tested the hypothesis that echolocating bats use landscape elements as acoustic landmarks. We predicted that bats should be able to perceive distant landscape elements using echolocation and hence change their echolocation behaviour in relation to these elements. Echolocation parameters of commuting pond bats Myotis dasycneme (Boie, 1825) were related to the bats' distance from banks of different sized canals. Pulse durations emitted while bats¯ew over canals of 13, 19, and 25 m wide increased successively. Interpulse intervals were equally long at 13, 19, and 25 m wide canals, but signi®cantly longer at a 30 m wide canal. The mean interpulse interval used by bats¯ying along the midline of the 30 m wide canal was just suf®ciently long to prevent overlap of an echo from the canal bank and a new outgoing pulse. The results indicate that pond bats perceive the canal banks by gradually adapting their pulse emissions to the distance to the banks. This suggests a role for the banks as acoustic landmarks.

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