Evening emergence surveys.

I was surveying a set of buildings last night. Building surveys had found evidence of bats using the building. This had included droppings, individual and in small clusters, as well as butterfly wings. The butterfly wings indicated that bats may have been returning to the building to perch and eat some of their larger prey. In order to gather further evidence on the species and numbers of bats I initially arranged to visit the site and undertake an emergence survey.

Bats emerge in the evening to catch insects. Some species emerge around dusk, while others wait until it is nearly fully dark. Standing outside a building from just before sunset you can see any bats that roost in the building emerging. As it gets darker this becomes more difficult and the use of aids such as night vision binoculars or night shoot video recorders can help. The best aid to finding where bats are roosting, where they are going to eat and what species they are is a bat detector. One type of bat detector commonly used is the heterodyne bat detector. These detectors take the echolocation that the bats use to locate their prey and convert it into a sound that can be heard by humans. The differences in the calls, their frequency, repetition rate and tonal qualities can all be used to help distinguish different species.

During the survey last night I had Common Pipistrelle bats visiting from 20.27, 18 minutes after sunset. They continued to feed until around the building until 21.30 when they became more intermittent visitors to the site. The best part of the survey however was the arrival of a Myotid species of bats. These bats produce sounds on the heterodyne bat detector that are sharper ticks compared to the wetter slaps of the Pipistrelles. They continued feeding around the buildings until around 21.50 when they became intermittent. Their echolocation calls on the Pettersen D200 bat detector was a series of fast tiks, with a tonal quality similar to walking over straw. They are probably Natterer’s bats but I’ll have to return to site to gather more evidence before being sure.