Saturday, 22 August 2009

Our passageway diner

The passageway that separates our end of terrace cottage from the neighbours is littered with the wings of moths. Most are from a common and familiar species around here, the Large Yellow Underwing. In addition to the moth wings there are tiny dark brown droppings, some on the floor and others caught on the rough plaster walls. It is evident that something has been perched on the walls feeding on moths caught above the garden; that ‘something’ is a Brown Long-eared Bat. Every now and then I catch sight of the bat, either hawking around the trees or perched on the wall, mid-meal.

You might imagine that bats have it all their own way when it comes to moths; that the bats simply fly around, detect and then catch the moths, thanks largely to their echolocation system. Well, the arms race that is evolution has equipped the moths with one or two tricks that can help them to evade capture.

Most bats use echolocation, through which a series of high frequency sounds are produced by the bat, which then listens for the returning echo as the pulse of sound bounces back off an object. Different bats use echolocation in different ways but some general patterns can be seen, largely centred on the habitat in which the bat is foraging. One group of bats that hunts out in the open uses a technique known as aerial-hawking (this group includes the Common Pipistrelle and the Noctule), while another group uses a technique known as ‘flutter-detecting’, which relies on the Doppler effect. A third group of bats, whose members tend to glean food from the surface of leaves, adopts a different approach.

Many moth species have specialized hearing organs which can detect the echolocation calls made by bats from up to 30m away. Since most bats can only detect moths present within 5m, this gives the moths time to turn away from the approaching bat. If, however, the bat is much closer to the moth when it is first detected, the moth will make a sudden and pronounced dive towards the ground. A few moths respond to the calls by making clicking sounds of their own which, when heard by the bat, cause it to turn away.

Such adaptations appear to give the moths an advantage so it is perhaps unsurprising that, despite what you might think, moths usually only form a small proportion of a typical bat’s diet (up to 40% in the case of the Brown Long-eared Bat but as low as 5% in the case of the pipistrelles). Some bats do, however, specialise on moths (e.g. Barbastelle) but these tend to take smaller moths, which lack the anti-bat defence systems possessed by most larger moths.

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