Experimental evidence of light disturbance along the commuting routes of pond bats ( Myotis dasycneme )

A population of pond bats (Myotis dasycneme) inhabits a network of foraging areas and separate male, female-breeding and temporary colonies. These are interconnected by commuting routes which are also used for foraging. The functioning of these networks is crucially important for the conservation of the species. Observations and anecdotal evidence suggest that light may be an important source of disturbance along commuting routes and potentially affect the connectivity of the networks. The disturbing effects of light on pond bats were experimentally studied by placing a strong lamp (1000 W) along existing pond bat commuting routes. Each experimental site had specific characteristics which allowed us to explore the interacting effects of light disturbance and the environment. The number of passing bats, the percentage of feeding buzzes relative to total commuting calls and flight patterns were compared between dark control nights and experimentally illuminated nights. There were no clear effects of experimental light on the number of passing bats nor did more bats use an alternative commuting route when just one of two possible routes was lit. However, light did reduce the percentage of feeding buzzes by more than 60%, although the abundance of insect food tended to increase. It was observed that light disturbs the flight patterns of pond bats. When approaching the beam of light, between 28% and 42% of pond bats turned before continuing on their normal commuting route. Virtually all pond bats (96%) turned when the light was erected on an existing barrier and they had to fly straight into the beam of light. These disturbing effects also seemed to occur at low levels of light intensity. This study is the first known experimental evidence on the disturbing effects of light on pond bat behaviour along commuting routes and raises many questions, especially as to whether these disturbing effects will have fitness consequences.