First he tried the station car-park in Farnham, a favourite Surrey bat haunt three minutes' walk from his house. "Insects cluster around the streetlights, and the bats fly back and forth eating them," he explained.
He put on headphones and produced his "bat detector", a small black box which picks up the high-frequency waves bats emit for navigation and converts them into audible sound.
Waving the box as if it were a divining rod, he stepped towards the lights. Nothing. He frowned behind his spectacles. But not for long: as a Senior Field Officer in a county with at least 14 species of warmth-loving bats - like much of southern England - he was bound to find some this sticky night. He mopped his brow and drove deeper into the country.
He came to a large pond, popping with insects. "This is where you find the Daubenton's bat," he said, with a little tremor in his voice.
Just beyond the pond, lightning flickered over a ruined abbey. The air thickened.
"You should never hold a bat detector in a thunderstorm," he said, then abruptly ran forward, check shirt flapping.
"Brilliant! There's a Daubenton back here!"
He stopped at the edge of the water and switched on a torch. A small grey shape shot across its beam, skimming a few inches above the surface of the pond. Then it came back again, level and fast like a miniature Dambuster.
"Look at that! Worrr! Brilliant little animals," Mr Catto nearly shouted, high-pitched cheeps leaking from his headphones.
He settled for a minute. "This is a very specialised bat," he said. The Daubenton's bat apparently has large hairy feet that help it snag insects, and can fly in a perfect figure of eight. It can live for as long as 30 years, but during this year's cold spring many came close to starving for lack of prey.
Mr Catto looked up again. "It's coming back!" This time there were three, their ribbed wings wide, flashing forward in Star Wars close formation.
Then the rain thumped down. Bats do not like rain. Back in the car, Mr Catto explained his background (zoology followed by a PhD on the Serotine bat) and his reasons for living in Farnham ("It's hard to do good bat work in London").
The various species of bat, he went on, are named after famous naturalists; and under the umbrella of the Bat Conservation Trust there are no fewer than 90 British bat groups, much excited by their furry favourites' "unique" internal radar and apparent lack of shyness under observation.
Mr Catto's evening was not over. Turning off the main road into Farnham, he indicated an estate of neatly-kept Fifties bungalows. Parking opposite a streetlight, he shuffled forward again with his bat detector, hair plastered to his forehead. Hedges dripped. Then he pounded back to the car in his Doc Marten's. His headphones were bleeping like sonar: "It's a Noctule bat! I've never heard one like this before." Pursing his lips, he began to make a matching pinging sound.
"When it comes close to an insect, its pulses increase," he said. Noctule bats savour the kind of beetles particularly found in suburban lawns. They like to hang from the south-facing edges of roofs and prefer modern houses for their constant spillage of heat.
Mr Catto proffered the headphones. The bat was getting closer. There was nothing to see - Noctules prefer to flit in the shadows just above streetlights - but he was all lit up: "I'm glad I came out tonight. It's a perfect night for bats."
Wasn't he scared of being bitten? "Oh no. About four years ago I was handling a lot of bats and I just got a rabies vaccination." The bats' strain of the disease had affected European bats for "probably 1,000 years", he said, and had never spread to other animals.
Besides, "every year at least 15 people in Britain will die of bee stings ... I'm much more worried about bats being persecuted after what's happened."
But his bat-watching can have its perils. As he was talking, he noticed a man peering over one of the garden walls. In the past there have been stories in the Farnham press describing Mr Catto as strange, a potential burglar even, for hanging around under the streetlamps of this particular estate.
"We'd better get in the car, because I'm going to get hassled," said Mr Catto resignedly. "This is Surrey, home of the Neighbourhood Watch."Reuse content