A bat in the hand is worth two in the bush: using capture data to assess how our commonest bat species are using urban areas

At the last count there were 1,240 known bat species around the world, 18 British species, and even nine Scottish bat species so the odds that I would get to study a wide diversity of bats during my five year PhD were high. However what I naively failed to account for was the extreme pressures that the built environment places upon wildlife.

In 2011 we spent an enjoyable summer surveying urban woodlands throughout Central Scotland in an attempt to determine how the vegetation characteristics (e.g. tree species richness), woodland size and shape, and the surrounding landscape influence the distribution of Scottish bat species. On our first night of surveying, equipped with a detailed guide outlining the key features required to distinguish between species, I nervously approached on of our mist nets bearing the distinctive outline of a bat. Extracting it from the net, the size and its small dog-like face with its flat broad head, made it easily identifiable as one of the pipistrelle species. Although a rather stark invasion of its privacy, its orange penis identified it as a male (!) and a soprano pipistrelle (Pipistrellus pygmaeus). The following nine bats that we caught that night (7 soprano pipistrelles and 2 common pipistrelles) hinted that a pattern was emerging.

IMGP0056 (2)

It wasn’t until the end of August, after 27 long and tiring nights of surveying which included chasing after youths who had stolen our equipment, arriving at woodland only to find it had been deforested and frequent visits from the police that we finally caught a non-pipistrelle bat. Again, my identification skills weren’t required given that its ears were nearly as long as its body; it was a brown long-eared bat (Plecotus auritus). We gleefully sent photos of us holding the bat to Rebekah Mayhew, my long suffering field assistant who had been there every morning emptying the invertebrate traps, every wet afternoon identifying tree species, and every night bat surveying with the exception of this one night… By our last survey of the season in early September (fittingly having caught a further 16 bats that night – all pipistrelles), it soon became apparent that whilst we were lacking species diversity we had a really interesting large dataset which we could use to investigate the behaviour of our commonest species.

As we were trapping bats (alongside using a bat detector to record their echolocation calls) it was possible to identify the sex, age (adult or juvenile), and reproductive state of the females. This gave us the opportunity to examine how the different demographics of a population respond to urbanisation. As we inspected the data what was most striking was that we were only catching females in particular woodlands whereas males appeared to be widespread. After the normal head scratching, numerous cups of tea, and despair that is run of the mill when dealing with complex statistics we were able to confirm our suspicions. We found that females favour high quality woodland which is well connected to other woodland patches. Males, on the other hand, seem to be less particular and are just as likely to be found in poorer-quality woodland patches surrounded by built-up areas.

P9010026 Soprano pipistrelle

The findings, published in the Royal Society journal Open Science (http://rsos.royalsocietypublishing.org/content/1/3/140200) suggest that the demands of pregnancy and raising offspring are driving females to select woodlands which provide good feeding opportunities and a safe route to fly between feeding grounds and roosting sites. As breeding females are of key importance in ensuring the survival of future generations of bat populations within the city environment, it is therefore important that we manage our urban green space for their benefit. The adaptability of many bat species to urban areas is frequently assessed by recording their echolocation calls which gives you a good indication of which habitats bats are frequently using or avoiding, however it fails to provide any information on sex differences in habitat use. By using bat call data it may therefore be relatively easy to mistake species presence across the city as adaptability (and therefore assume that a species requires less conservation effort) without first considering if there are differences in where males or females are foraging.

As I near the end of my PhD, although I haven’t studied an array of species or devised any solutions to save those species which are critically endangered, by focusing on our commoner species I have gained a greater understanding of how the pressures of urban living are shaping their behaviour and distributions. Whilst pipistrelles are relatively common throughout Britain, these are the species that people will encounter whilst on bat walks or watch flying around their back gardens and therefore have a fundamental role to play in engaging the public with science and conservation. Monitoring and conserving our commonest species will therefore not only give an indication of how our rarer (and so harder to survey) species may be responding to the urban landscape but also ensure that future generations are able to appreciate the wonder of watching bats forage at dusk from their own doorstep.

Wildlife watching in Central Park, New York. Copyright ©Robert DeCandido

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