Lowton Bird Observatory, Day 46

Photo: Common pipistrelle spectrogram.

Our resident bat has been flying around for a few weeks now. We see it most evenings while we are watching the telly shortly after sunset flying in front of the French doors and around the garden. To be honest it's usually a good deal more entertaining than the telly and sometimes it's joined by a second bat and we often have them swirling around together for 30 minutes or so. Tonight I decided to get the bat detector out and have a go at identifying it to species and see what else is flying around in the garden after dark.

There are around 17 species of bat in the UK and they all occupy different niches in the ecosystem. Some bats like open spaces, others like woodland and a few like lakes and marshes. Different species also fly at different times, some at sunset and some well after dark.

It's well known that bats use echolocation to find their prey which basically means that they send out sound waves which bounce back to them when they hit their target and this guides the bat to its prey. Species echolocate at different frequencies and these can be picked up by bat detectors which then use this information to identify the species in the form of a spectrogram.

Photo: Classic "hockey stick" shape of common pipistrelle.

The spectrogram above shows the classic "hockey stick" shape of the pipistrelle species. It is the base of the hockey stick which gives you the frequency of the call, in this case just under 50khz, which identfies the species as common pipistrelle. This is our resident bat and the species which we see most frequently in the garden,

Bat detectors work better with some species than others. For example, some prey items such as moths have evolved their own type of bat detector and can recognise the approach of a bat that is echolocating and this helps them to avoid the bat. To counter this, Brown long-eared bat, which is sometimes also known as the whispering bat, has evolved a very quiet echolocation to make it more difficult for prey items to recognise them, but this makes it necessary for them to have large ears to receive the bounce back from the prey. It also explains why they are very rarely picked up on bat detectors despite being one of the commonest bats in the UK.

Other very similar species broadcast on frequencies too similar to be identified for certain by the detector and in this case they are usually just identified to family level.

Photo: Soprano pipistrelle spectrogram.

Also tonight we had a single flypast of soprano pipistrelle. This species broadcasts on a higher frequency than it's commoner cousin and if you look at the base of the hockey sticks you can see that it is in the region of 58khz.

Photo: Noctule spectrogram.

The third species of bat from the night was noctule. This is a larger species of bat which broadcasts on a very low frequency. You can see in the spectrogram above that this bat broadcast on about 25khz.