When John Altringham witnessed hundreds of bats swarming around the entrance of a disused mine in Canada, he was filled with questions as to why these bats were exhibiting this behaviour at such an unusual time of year. John returned to the UK and studied bat swarming on the Yorkshire Dales to answer these questions: here he shares his story of over 20 years of research.
In August 1996, I visited eastern Canada to assist Brock Fenton (then at York University, Toronto) in running a bat-ecology field course. We visited a small, disused mine in the Ontario backwoods and in just a few hours, with a simple home-made harp trap across its door-sized entrance, we caught several hundred bats of at least seven species.