Saturday, 25 February 2012

Winter Bats

Bats build up their fat reserves in autumn by foraging at locations where insect biomass is readily available in shed loads (such as at riparian sites, standing water, sewage works). They need to sustain themselves for torpor/hibernation over the winter months, when there is little or no food available. Insects generally do not fly (or are less available) when temperatures fall below 10 degrees centigrade (or during and after periods of heavy rain).

Optimal winter  roosting conditions are the opposite of those sought in the summer. Bats require cold and humid conditions. They hibernate as singles. They are not seeking warmth, but require cold conditions as they do not want to wake up to no food  as this would be high cost to their limited energy budget. Bats are very vulnerable in torpor and can take >30mins to wake up and fly.

Very little is known as to the whereabouts of our London hibernation sites: some bat species are known to migrate to European caves; or use green belt mines, dene holes and disused railway tunnels (Crystal Palace, Cavalry Tunnel, Highgate); or turn up during winter tree felling (behind loose bark or in cavities) and sometimes are found during renovation of windows or in door reveals. What is known is that Kingston only has one known winter hibernation site: Seething Wells.

This makes  the site very important indeed for overwintering bats as it has the correct levels of humidity, winter foraging opportunities during mild periods; lack of disturbance etc. If you want to see bats at their hibernation site it is possible but the correct conditions are necessary:

  • a cold spell should be followed by an exceptionally mild night;
  • watch a high potential site from sunset (in this case the small pumping station on the wharf);
  • wait patiently for up to thirty minutes.
Such an opportunity arose this week (23.2.12) when the  temperature at sunset was an unusual 16 degrees centigrade. Three observers watched nine common pipistrelles (one soprano pipistrelle) emerge to forage around the Filter Beds from sunset + eight minutes. This assigns an importance to the site unacknowledged and therefore unmitigated by the new development proposals. 

The status of the Filter Beds as a hibernaculum was made clear to the owners, from a data set supplied to them from the London Bat Group. The site has been known to be important as a winter roost since 2001, when pipistrelle bats were recorded emerging from the pumping station on a regular basis. 
Hibernating Daubenton's bat- note big feet! Photo Miranda


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