Worcestershire Record No. 6 April 1999 p. 26


Mike Averill

During the summer of 1996, droppings from a Leisler's bat colony were collected from a roost in Hartlebury in order to examine the composition of the bats diet. This was part of a study to see how the feeding habits of Leisler's bats in England and Germany differed to those in Ireland where there is no competition from larger bats like Noctules. Both Leisler's and Noctule bats are fast flyers and because they have reduced manoeuvreability they tend to forage in flight and do not glean like smaller bats. The diet therefore predominantly comprises flying insects.

The study gives us a chance to see what the diet is like in Worcestershire where Leisler's Bat is far from common. Leisler's are normally known as forest bats but in the case of Hartlebury the surroundings are predominantly arable. The roost is 6 kms from the River Severn, quite close to heathland and there are also some large pools nearby. There is some woodland locally in the form of the Wyre Forest which is within the bats flying distance of 14 kms. Droppings were collected at the end of each month in order to identify any seasonal change of diet. Insect parts were identified to family and further where possible. As can be seen from the first pie chart Midges (Chironomidae and Ceratopogonidae) were the most important part of the diet (25%) followed by Moths (Lepidoptera)(18%), Window midges (Anisopodidae) (14%), Green Lacewing (Chrysopidae) (10%), Lesser Dung-fly (Sphaeroceridae) (8%) and Yellow Dung-fly (Scathophagidae) (7%).

As Leisler's bat would appear to be imperfectly adapted to the pursuit of individual midges because of its poor manoeuvreability, there seems to be some evidence that it catches the smaller insects by flying through swarms, filter feeding. One researcher noted how the bats would change this habit to one of flying with shallow dives when in pursuit of larger prey like the Yellow Dung-fly. Many of the taxa making up the diet are from swarming insects and so they are obviously important to the bats. This is reinforced by the fact that Leisler's emerge to feed very early, even before dusk when these swarms are most active. The Yellow Dung-fly for instance tends to stay around cow dung for about 1-1.5 hr after sunset before flying to roost.

Aquatic insects like midges figured quite strongly in the diet and as has already been noted, there are large pools near the roost.

As regards seasonal variation only four food categories showed any evidence of a consistent trend. They are displayed in the pie charts below and expressed as the percentage of those four groups. Most noticeable is the way midges reduce through the months from May. Yellow Dung-fly are important and peak in June. It is not clear why the Green Lacewing is so predominant in the August sample. It was rarely found at all in the other Leisler's roosts in the survey.

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