Light levels.

It seems to have been wet for ever. Not the extreme weather of last year. Just continuously wet and damp.

Bat surveys can’t be done in the wet and bats can’t forage. I did one bat survey, arriving on site fifteen minutes before sunset, and there were Common Pipistrelles already feeding amongst the trees at the end of the garden. It had rained for a couple of days before so the bats were hungry. The evening was overcast, so the light level was low. Hungry bats had therefore come out earlier than usual. Braving the possibility of being picked off by birds of prey, in order to get fed!

The books say that Pipistrelles will come out about fifteen to twenty minutes after sunset. But, should we rather being saying that bats will come out at certain light levels?

Bats don’t watch the clock. They don’t read the books about what they are supposed to be doing. They must have some sort of sense about when the light level is right for them to go out, and not be liable to be someone else’s dinner. So, should we be measuring the light levels when the first bats emerge?

A talk with another bat worker who understands technology better than I do led to an idea that you could use the old style light meters used with 35mm cameras, and available second hand. The light levels on the meter should be able to be correlated to light levels measured in Lux from a light meter. This would allow light levels to be measured in a standard unit that could be used any where. Measurements could then be compared between sites and at different times of the year to see if there are any patterns about emergence.