Monday, 18 July 2011

Bat Trilogy!

Following on from a previous postings "Batman and Dobbin" and "Detection of Squatters" here is the last part of the bat detector trilogy.

We went for a walk around the grounds of our local stately home (Wentworth Woodhouse) late on Saturday and Sunday night. A mixture of meadow, farm land, woodland and large lakes and streams provides a wonderful habitat for a wide variety of bats. We took the bat detector with us. The full moon was just rising over the horizon on Saturday, it was bright enough to cast our shadows.  We had a good view of a Barn Owl quartering a field and heard Tawney owls calling.  Sunday was cooler and overcast.

Amongst the bats we identified were Noctule, Pipistrelle (common and soprano) as well as Daubentons.

With bats most of their feeding is done around dusk and dawn when the majority of night-flying insects are active.  If you are up and around just before dawn, this is when the best views are available as many bats seem reluctant to call it a night and return to their roost, staying out into the morning light especially when food is abundant.

Bats are most active on warm, humid, windless nights. If there is shelter provided by trees you can find them on the lee side of the sheltering trees. They will tolerate a little light rain. Bats may be less active on bright moonlit nights because they are more vulnerable to predation by owls. The moonlight can make it much easier to observe them. Even very powerful torches do not appear to disturb bats on the wing and used in conjunction with a detector, can facilitate good views.

For thousands of years people have marvelled at the ability of bats to navigate in complete darkness. Many nocturnal animals and birds such as owls have large, super sensitive eyes and acute hearing to aid foraging for food. However most bats eyes are a bit on the small side, but contrary to the saying, their eyes are not "as blind as a proverbial bat".

How could they possibly navigate through tangled forests, in the pitch darkness of a cave or anywhere else on a moonless night, if not for ultrasound?

In the 18th century Lazarro Spallanzani, Bishop of Padua, first experimented by blindfolding bats and then he even physically blinded them before setting them free to fly round in a room strung with wires. The bats flew confidently around the room, proving that sight was not necessary for their navigation in the dark.

It was not until the late 1930s that Donald Griffin discovered that bats produce ultrasonic calls and use of what he described as echolocation to navigate. Griffin did seminal research in animal behavior, animal navigation, acoustic orientation and sensory biophysics. While an undergraduate at Harvard University, he began studying the ultrasound navigational methods of bats.

Echolocation, or biosonar, is used by animals, such as bats, whales and dolphins. Echo locating animals emit high pitched calls out into the environment and then listen to the echoes of those calls that return from various objects. They use these echoes to locate, range, and identify the objects. Echolocation is also used for navigation and contact between groups.

Individual bat species use specific frequency ranges that suit their environment and prey types. This can sometimes be used to identify bats flying in an area using a "bat detector". However calls are not always species specific and some bats overlap in the type of calls they use so ultrasonic calls cannot be used to identify all bats. The frequency, pitch, number and type of pulses plus the habitat and the flight pattern used during hunting can give additional identification pointers.

When searching for prey bats produce sounds at a low sound pulse rate typically 10 to 20 a second. After detecting a potential prey item, bats increase the rate of pulses, ending with the terminal buzz, at rates as high as 200 pulses a second. During approach to a detected target, the duration of the sounds is gradually decreased, as is the strength of the sound.

It is believed that some insects can actually detect the ultrasound frequency used by bats and to sense the direction that the bat is approaching from. This gives the insect time to take avoiding maneuvers.

The time interval between sound pulses determines the maximum range that a bat can detect an object. This is because bats can only keep track of the echoes from one call at a time. As soon as they make another call they stop listening for echoes from the previously made call. A pulse interval of 100 ms (typical of a bat searching for insects) allows sound to travel in air roughly 34 meters so a bat can only detect objects as far away as 17 meters (the sound has to travel out from and back to the bat). With a pulse interval of 5 ms (typical of a bat at the final moments of a capture), the bat can only detect objects up to 85 cm away.

Wonderful, sophisticated animals are bats!


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