Where do bats go in winter? Tim Reed, chair of The Friends of Holt Island, explains what happens on the Holt Island Nature Reserve in St Ives.
Bats are very fickle beasts. In some ways they are like people: they don't like going out in the cold and wet, and for the most part don't like windy days either.
The number of bats around on cool windy days is far lower than on balmy summer evenings when their insect food is about in abundance. The midges and mosquitoes that bother us are food and drink to a bat.
On Holt Island we have our fair share of bats: half of the list of British species occur there every year.
Seen or detected from early April through to the end of October - coincidently the main opening dates of the island - they are there most nights.
Rarely picked up by sight, most of the bats that are found after the light wanes are ‘detected’.
A detector is an electronic box of tricks that converts the ‘squeaks’ of bats - way beyond our normal hearing - into pulses.
These are presented as rises and falls on an oscilloscope (like the instrument that shows heart beats in hospital), or converted into noises through a microphone.
This allows us to differentiate between species that may look incredibly similar, as well as pick up bats that fly unseen in the dark. The bounce-back from the squeaks aids the bat’s navigation and hunting technique.
Species appear at different times of the evening. The Noctule - a chunky bat bigger than a Starling - is one of the earliest to emerge, and normally overflies the island on the way to feed.
Occasionally it cruises down the walkway rides catching insects, and met head-on, its 40-cm wing span commands respect.
Most of the bats recorded on the island are Pipistrelles: Common and Soprano. Both come in over the backwater that forms the northern boundary. Their timing as the light goes suggests nearby summer/maternal roosts, but we still need to find these.
On site, we have bat boxes, host over the summer to a range of bats, including the occasional rare Nathusius Pipistrelle. By now, all bats will be hibernating offsite until spring in cool, stable conditions, either locally or elsewhere. We will have to wait until then for their next island visit.
Holt Island is an award-winning nature reserve in St Ives owned by Huntingdonshire District Council. It is open from Easter to October. For more information visit: www.holtisland.org
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