Hark the Herald...Moth

Visits to bat hibernacula sometimes produce sightings of other species, which also choose to hibernate in caves and mines. Mosquitoes and bees are occasionally found, but the ever-present companion of the hibernating bat is the Herald Moth (Scoliopteryx libatrix). I don't think I can remember every going into a hibernaculum without also finding Herald Moths: sometimes many dozens, often just one or two.



According to Butterfly Conservation, the organisation charged with conserving our diverse butterflies and moths, there are over 2,500 species of moth in the UK. So why the Herald should be the only one that seems to choose to overwinter in cavses and mines isn't clear. Many species spend the winters as eggs or as pupae, but quite a number apparently do overwinter as adults.

The Herald is a rather attractive animal, with wavy-edged wings coloured with reds and browns, which help it to blend in with dead leaves and avoid predators.

An interesting coincidence is that the Herald is a member of the Noctuidae family of night-flying moths. Something which sets them apart from other moth families is that they have developed a rudimentary hearing organ, which is used to detect the echolocation calls of approaching bats. On hearing an approaching bat the moths wings go into spasm, causing erratic flight, so that the moth is able to evade the bat.

The bats have the last laugh however, in the form of the Long-eared Bats (Plecotus spp.). which have evolved to get by with a very faint echolocation call. Their slow flying speed means that they have less need than other species for advanced warning of obstacles and their phenomenal hearing allows them to listen for prey. Their very faint call (they are known as the Whispering Bat) means that moths aren't able to hear them coming. Thus, a commonly-found sign of Brown Long-eared activity is a pile of discarded moth wings.

It is quite usual to see hibernating bats covered with droplets of condensation. What sparked off this foray into lepidoptery was the discovery today of a Herald in the same state, with huge droplets of water on it's antennae, creating the impression of some kind of miniature bog-eyed monster. Maybe this is the first sign of the moths evolving some form of revenge on the Long-eareds...

For more information on Butterfly Conservation go to http://www.butterfly-conservation.org

Another good source of information on moths: http://ukmoths.org.uk

My website: www.plecotus.co.uk

Bats, bombers and acoustic mirrors

One reason why I first started working with bats was that bat work combines my love of wildlife with my longstanding interest in electronics. Fortunately people who do surveys with me tend to tolerate my tendency to come up with strange new ideas or pieces of equipment!

Several years ago I found myself wondering whether parabolic dishes might have a place in bat work. These are used in the recording of bird calls and other wildlife sounds and look similar to TV satellite dishes. In essence, the dish reflects sounds into a focal point, where a microphone is placed. This has two effects:

1. Sounds are concentrated by the dish, making them louder.

2. Sounds are picked up from a narrow direction, filtering out background sounds from either side.

Incidentally, not just any dish will work: a parabola is a specific curve, calculated using a mathematical formula.

I experimented with an old Sky TV dish and found that when I put a bat detector at the focal point of the dish, bat calls became louder and so could be heard from further away. However, it was clear that this would have limited practical uses, as it only worked when the dish was pointed directly at a distant bat. People recording bird calls can see a bird and point a dish at it. Bats don't allow that luxury!

More recently I became aware of something called a parabolic plane. This is sometimes used in microwave radio applications. It is like a dish, but only curved in one plane, a bit like Rolf Harris's wobble board. Between the wars gigantic parabolic planes up to 200 feet long by 26 feet high were built at strategic sites around Britain's coast. These "acoustic mirrors" were used to listen for approaching bombers: operators could hear an aircraft from 20 miles away. Unfortunately they were never used for their intended purpose: by the time World War II broke out, radar had taken over.

Image (c) English Heritage

The beauty of a parabolic plane is that it only concentrates sound from the horizontal plane, so that a bat flying in front of one at any height should be heard. It could be possible to create a kind of listening curtain: in theory any bat passing through it would be detected. To test this I made up a prototype reflector, using foam board, card and sticky-backed plastic (eat your heart out, Blue Peter) and tried it, using a low-level ultrasound source as an artificial bat.



The prototype is only 60cm by 20cm, so I wasn't expecting miracles, but tests at varying ultrasound levels showed that, using the reflector a Duet bat detector could pick up the artificial bat between 30% and 70% further away than it could on it's own. Positioning of the detector microphone is critical, as it needs to be at the precise focal point of the reflector for best results. The half-brick will need to be replaced by something with a bit more finesse!

There is some interesting potential for parabolic planes, but there are some potential draw-backs too. In theory, the bigger the reflector, the further away you can hear bats. But a bigger reflector also means the bat would be audible for a shorter time, passing through a narrower curtain. (The acoustic mirrors could hear aircraft from 20 miles away and place them to within 1.5 degrees.) Ideally a bat needs to be audible for 1-2 seconds, to be sure enough of it's call is
recorded, to allow for identification, so this may limit possible size..

Another problem is the practicality of lugging a big reflector around. Even using it in a fixed position, more than 2-3 metres in width would probably be impractical, unless as a permanent fixture. Any fixed-location survey involves predicting where the bats will fly, and they don't alway read the same books as us!

More about acoustic mirrors: http://www.ajg41.clara.co.uk/mirrors/

My website: Plecotus.co.uk