SLBG: Swarming Survey

Last night I and around 10 other members of SLBG took part in a swarming survey at an abandoned mine in Blackburn. It was undoubtedly my best conservation experience to date!

This was the entrance to the area of the mine harbouring the harp net. Inside the mine we witnessed some spectacular bat flight activity

The survey required bats to be caught and handled, nets to be checked and measurements to be completed on the specimens, offering a great range of activities and experiences. Arriving on site at around 8pm, we had a short group discussion to discuss survey expectations, health and safety issues and legal restrictions to activity. After this it was time to set the nets. Two types of net were employed for this fieldwork, 6m mist nets were erected outside 2 entrances to the mine in order to capture any bats during entry and emergence, and a harp net was set up inside the mine to capture bats active within the tunnel network.

View of part of the mine in which bats are known to swarm

Mist nets are simply large (in this case 6m long) finely meshed nets which, in still conditions, are generally not recognised by echolocating bat. They work like any other net, catching bats in flight and holding them, pretty successfully in a one of a row of pouches on the device. These nets can be fairly hazardous to bats who will become stressed and struggle as they remain tangled in the webbing and as such it is important to get individuals out as soon as possible. As such, once these nets are assembled they must be constantly monitored. The task of removing bats from these nets was the only one that I and other unlicensed volunteers were not allowed to complete due to the high risk of injury to trapped bats.

One of the 2 mist nets set around the mine entrances

One slightly disheveled Brown Long Eared bat waiting to be untangled.

Harp nets differ considerably from mist nets and present a much lower risk to bat safety. They consist of vertical lines of fine line which, if hit, stop bats in flight and cause them to drop downwards where they are kept safe and free to move in a fabric compartment. The harp net was where the majority of an incredible 63 bats where caught due to its prime location inside the mine.

Harp net set up inside mine

Daubentons bat at the base of the harp trap. Defining features of this bat include its dusky white underbelly and relatively large feet. 

With the workstation and nets assembled it was time for the surveying to begin. As the site in question represents the only confirmed swarming site in Manchester it was hoped that our work would not only improve scientific knowledge of swarming behaviour, but also build evidence to support the site gaining some legal, or at least physical protection.

Almost immediately the harp net began to produce bats, with a check every 20 minutes yielding an average of 3 individuals. My first chance to handle a bat came through removing one from the mist net and placing it in a cloth bag to be transported back to the workstation. It definitely takes a bit of getting used to, especially due to the need to wear gloves which limit dexterity and sensitivity when handling the animals.

Back at the work station we were tasked to remove our specimens from the bags and complete a range of measurements and observations to identify (amongst other factors) sex, species and physical condition of each individual. Again this involved a lot of practice in holding the bats steady in various different positions and two of mine escaped before I really started getting the hang of it. As well as identifying species, sex, age and reproductive status; measurements such as weight and forearm length were also noted to help assess the health of the populations.

Looking at the nipples of female individuals allows identification of an individuals maturity and reproductive status. This nipple seems darker suggesting that the female is parous. A bulbous or raised nipple also suggests that the female is or has recently, lactated.

Over the course of the night – we stayed out until around 3am – I processed 5 bats in detail, both daubentons (pic above )and BLE’s (brown long eared) species, in addition to regularly checking the traps and aiding others in their bat processing. As well as obtaining a lot of hands on practice in species handling I was also very keen to improve my identification skills and as such spent a lot of my time shadowing the license holders on the survey who were greatly informative. With their guidance I could soon identify key features allowing me  distinguish between species such as the rounder nose of the soprano pip relative to the common, the presence of a row of bristles on the tail membrane of the natterers or the relative length of the calcar distinguishing the daubentons (3/4 length from foot to tail) from other Myotis species.

By stretching out the wing, the individual can be checked for ectoparasites. Holding the wing up to the light also allows the ossification of wing bones to be assessed. Juvenile bats will not have fully ossified bones and lights will be visible through the joints whilst adult wings are fully ossified.

By the end of the night we had recorded a site record of 63 individuals from an impressive 6 species; BLE’s, daubentons, common and soprano pips, brandt’s and natterer’s. This represents a great result capping a brilliant experience for me that was well worth the midge onslaught.

Brown Long eared bat – obvious large ears which join over the head allow quick identification to genus level (Plecotus)

Daubentons being held from above, this photo clearly shows the large feet characteristic of the species.

The best way to release the animals after surveying is to hold them up in the air and allow them to begin flapping to fly off with their own momentum.

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