When I finished making my bat detector – frequency division type, circuit by Tony Messina – I needed to test it. Unfortunately, our bats had all gone into hibernation, so first I built a pulse unit to give me a 40khz noise – that worked. Then I tried it out on running water from a tap – that worked as well. However, it all seemed rather unsatisfactory, and I wanted to know how well it worked on real bats, what it sounded like with real bats, how far away it could detect them.

Then I thought of Chester Zoo and its Twilight Zone! They have a big building in which they keep the lighting on at night, and off during the day. As you walk through the main room, making your way by means of very dim lighting, you can just make out the bats hanging around (literally!) the place, and occasionally fluttering from one roost to another.

When I got to the Zoo at opening time (10.00 am), I was disappointed to find that the Twilight Zone – now renamed the Fruit Bat Forest – was shut and wouldn’t open until 12.30 because the time shift of the lighting had been altered to enable work on lots of changes in the place. I didn’t mind too much – I always enjoy spending time at Chester Zoo.

I looked for my favourite animals – the Asian Short Clawed Otters – in their new enclosure, but they were sensibly keeping warm and out of sight. Most Zoo animals do not seem to see the public outside their enclosures or cages – I suppose that it must appear to them like wrap-around TV with the same boring programme on all the time. The otters often seemed to be interested in their audience, sometimes standing on their hind legs to inspect us and chattering to each other, no doubt about the peculiarities of the visitors of the day.

The tropical house held some inmates I had not seen before – poison dart frogs. I spent some time photographing them, trying to capture their amazing colours. Time passed quickly, and it did not seem very long before I was making my way into the Fruit Bat Forest. I switched on the bat detector before entering the main room, and held it carefully – the slightest movement of my hand on the box caused a loud series of clicks – then walked through the double set of plastic sheets into the bat’s room.

As I stood waiting for my eyes to become accustomed to the gloom, the detector produced small noises now and then, and at first, I wasn’t sure if it was caused my my hand moving on the box. The clicks did not seem to correlate with a bat flying past me, but I slowly realised that the noises were indeed caused by the bats, and that they did not need to be close by for their ultrasonic squeaks to be detected.

Then I noticed a member of staff filling feeders on some trees in the central area, so I pointed the detector towards them. When she finished and left, I was deafened by the racket that the detector produced – the bats which descended on the feeders were obviously screeching at each other ultrasonically as they competed for the food.

So, it works. Summer starts for me in two weeks time, when we leave for New Zealand, and I shall take the bat detector with me. We shall be staying with daughter Sue and her family in Dunedin, South Island, for six weeks. Long enough to find out whether any of New Zealand’s only native mammals are in the woods around Dunedin.