A study carried out in Belgium (Walloon Region) in 2023 investigated the flight altitude of great bats (Myotis myotis). This question was posed in the context of growing demand for the installation of wind turbines in forests. 24 suckling females from 2 breeding colonies were fitted with miniaturized GPS receivers. Of the 13 beacons recovered, 9 provided a dataset (n=672 fix) relevant to assessing the hunting grounds occupied and the flight altitudes of the individuals. Hunting grounds are located an average of 9.8 km from the colony (min 1.7 km, max 15.4 km). These are deciduous, mixed or coniferous forest stands. Flight altitudes measured on filtered data give the following results: in the transit phase, great murins fly at a median height of around 15m above ground level; this value could oscillate between 4m and 30m, representing a flight altitude of 5 to 6m above vegetation.

The majority of movements take place less than 20m above the vegetation, and less than 10% of measuring points reach an altitude of more than 36m above the vegetation, although it is not possible to determine with any certainty the degree of inaccuracy in GPS measurements. In the active hunting phase, they generally fly 4m lower than in transit, in the vegetation, at a median altitude of 3 to 5m below the treetops. The implications of these results for the protective measures required when installing wind turbines in forests suggest that, for the great murin, a distance of 35 m between the blade tip and the canopy would be sufficient to significantly reduce the risk of collision with individuals in transit (i.e. when individuals are flying highest).

This study was carried out by Ecofirst in collaboration with CSDingénieurs, with funding from the Service Public de Wallonie (DEMNA).