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Bats - Aug 2020

 

At Portswood Rec we are lucky enough to regularly have bats hunting over the garden and among the trees. As dusk starts to move to earlier times, we are more likely to catch a glimpse of these amazing nocturnal creatures. There are 18 species of bat found in the UK.

 

The common pipistrelle is the most numerous, with a population of 2.4 million, while the least common is the greater mouse eared bat, with just one individual currently hibernating in the UK.

 

During the day, bats roost in a variety of different places, including crevices in buildings, holes in trees, and caves. Then as the day draws to a close, they leave the roost to hunt. The first to emerge is the noctule bat, the biggest of the UK bat species, which can sometimes start flying before the sun has set. Around 20 minutes after sunset, the common pipistrelle takes flight, ready to begin its daily meal of up to 3000 insects, and about 10 minutes later the Daubenton’s bat starts flying low over bodies of water to fish insects from the surface. The diet of bats is made up of all kinds of flying invertebrates, including midges, flies, moths, and beetles.

 

The primary method bats use for detecting and closing in on their prey is echolocation. The bat emits a short ultrasonic call, and listens for its echo from any unfortunate passing insects. The frequency of this call depends on the species of bat, and ranges from 20-25 kHz for the noctule bat, to 108 kHz for the lesser horseshoe bat. In fact, the frequency of the call can help to identify species, and it is the easiest way to distinguish between the common and soprano pipistrelles, only recognised as different species in the 1990s. Some bat calls are audible to children and some adults, but a bat detector can be used to convert the ultrasonic frequencies into lower pitched sounds, or into a visual format which makes certain characteristics of the calls more apparent. For instance, as a bat closes in on its prey, its calls become more rapid and it also drops the pitch of the sound, giving the call a “chirped” quality. Interestingly, this technique has been developed independently by humans as well – for use in car proximity sensors!

 

Because bats need to detect the echo from tiny insects some distance away, their calls have to be extremely loud. Their ears have special muscles that temporarily dislocate the ear bones while they call, to avoid deafening themselves, and then relocate them within a few milliseconds in time to hear the returning echo. The loud volume also means that echolocation uses up a lot of energy for the bat, as does flying, and they therefore want to know if it is worth leaving the roost on a particular night to hunt. They are able to detect lower barometric pressures, which indicate rain, and prefer to hunt in these conditions as they provide the most insects – although they avoid heavy rain, which impedes their echolocation. 

 

Unfortunately, bats have seen a large decrease in numbers in the last few decades, partly driven by losses in suitable roosting sites like old buildings, woodland, and ancient trees. However, there are many things that can be done in our own gardens and parks to attract and help bats. Planting trees, hedges and shrubs, leaving old trees in place where possible, and putting up bat boxes in sunny locations can all provide places to roost. Attracting insects for bats to feed on is also important, by having ponds and log piles, avoiding pesticides, and planting flowers to attract insects. Particularly good for this in the late summer and autumn are michaelmas daisies and single flowering dahlias - and outside the flowerbed, ivy is also excellent for insects. For bats, evening-scented flowers such as honeysuckle and evening primrose are also especially important. Additionally, the Bat Conservation Trust is running a sunrise / sunset survey until the end of September, for people across the country to record sightings of bats in their area - with identification of the specific species welcome but not required! 

 

Interesting and useful links:

 

A video of bats at Portswood Rec, with audio of their calls from a home-made bat detector:

https://www.facebook.com/watch/?v=603965260558622&extid=x3uPLcemhYZUozhd

 

Some wonderful videos of bats and other wildlife, taken around Southampton by Ian Baker:

https://vimeo.com/ianmbaker 

 

Information on building your own bat detector from Bertrik Sikken; the sound in the video above is from the Enhanced TCA440 detector from this page:

http://bertrik.sikken.nl/bat/index.html  

 

The Hampshire Bat Group website:

http://www.hampshirebatgroup.org.uk/ 

 

The Bat Conservation Trust website:

https://www.bats.org.uk

 

And their page about the sunrise/sunset survey:

https://www.bats.org.uk/our-work/national-bat-monitoring-programme/surveys/sunset-sunrise-survey

 

 

 

 

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