It's a short while after sunset and I am standing in my garden staring at the house roof. Midges nip my face as the light drops. Suddenly from just above the gutter, there's a flurry and a bat darts out and flies above my head. It's the first of the night to emerge and a burst of noise from the bat detector registers its inaudible sound.
Ruth Hadden, from my local bat group, tunes a knob on a black box to align it with the frequency of this particular bat's call. She is here to find out what species of bats dwell under our roof. They live under the stone roof slabs that are a feature of older houses in this area, and we can hear their scrabbling, flapping noises at dawn as they come back to roost.
This first bat is a common pipistrelle, the clicking sounds it produces registering at 45khz, and we count them out as they loop over the garden and head off towards the fields. A faster set of clicks signals a different frequency on the detector, and the next group are Natterer's bats that always appear some minutes after the pipistrelles. Now there's a different set of clicks, slower and louder than the Natterer's, and Ruth says we have a whiskered bat too, confirmed by its different behaviour. As the dusk deepens we watch it fly systematically up and down in front of the house, sweeping up all the insects as it passes.
Of the three species we count some 40 bats and it is likely we have a fourth as well. Ruth explains the earthy smell often noticed in an upstairs room as evidence of soprano pipistrelles. Given off by their fur, it is described variously as curry, bacon or coffee, but it is not unpleasant. These have now dispersed from the roost. Each bat can eat 3,000 midges a night, making them the gardener's friend. The air is now heady with night-scented stock as we head indoors leaving the bats to a night's foraging.