This collection of videos has been prepared over the period 2014 -2019 with an emphasis on difficult subjects such as bats and owls.
Greater Horseshoe Bats in the Thermal Infrared
Bryanston is an independent boarding school that owns 400 acres to the South of the River Stour near Blandford Forum. In the late 1700s a large country mansion was built overlooking the river valley on a wooded ridge. It was demolished in 1890, because of damp, except for one wing formerly used as a kitchen. Greater Horseshoe bats were discovered there 70 years ago and since then ‘The Kitchens’ have been developed as a year-round roost by digging out deep caves into the hillside. The site is currently managed by the Vincent Wildlife Trust. Summer bat counts exceed 400 which is testament to the success of the habitat improvement. A BBC 4 documentary “Inside the Bat Cave” will feature the roost and we were commissioned to do the night time filming using our thermal cameras. The roost is behind the thick foliage and protected by a high fence.
The video also includes a lot of other animals that we saw around Bryanston.
Hunting strategy of soprano pipistrelles
This video has been compiled from the Moot filming session on 13th August 2016 with emphasis on how soprano pipistrelles engage flying insects. In cruise flight at 3.5 m/s the echolocation calls synchronise with the wingbeat at 12.5 beats/second. When prey is located at about 2 metre range the frequency jumps to 2 calls per wingbeat. At about 50 cm the normal call (a chirp of 5.7 ms duration) changes to a short pulse of energy rising to 200 pulses per second (1 every 2.5 cm of flight). This distance is similar to the limit of normal echolocation. The problem is that bats dislocate their ear bones to avoid deafening themselves and it takes a time to restore hearing (thought to be 2-8 ms). So bats should not be able to echolocate when they get close to the prey. They seem to shift to another homing strategy which could be heterodyning the outward and inward signals but we do not know. The video hints that they also use their eyes for terminal guidance since they follow the prey accurately at very close range.
The following video was filmed with Peter and Sam Whieldon at Peters owl care sanctuary at Otterbourne in Hampshire. This video is full of new science. Note the dark eyes in the barn and tawny owls – we do not have an explanation for this yet. The stationary poses have very little contrast – the feathers block infrared to keep the bird warm. Everything about the attack of an owl is towards hiding its warm parts. The hot armpits are hidden behind the body, the facial disk (in the case of the barn owl) becomes cooler, the legs go cold and the eyes squint. Barn owls are masters of infrared concealment and this is one of the pieces of evidence that small mammals have infrared threat-warning. At the end of this video is an intriguing section that hints at hot-spot sensing in owls. They cannot use their eyes for this because the vitreous humor is opaque in the infrared. Dyson the barn owl has never hunted – he has always been in captivity. Yet he is transfixed by the hot hand warmer as though an instinct is telling him this is interesting.
The BLE roost in the attic of a farm house at Romsey has between 20 and 45 bats and is one of the largest in Hampshire. The entrance is a small open window. We filmed here 2 years ago but over winter we have modified the camera to run a 1/1000th second exposure and wanted to get back to capture the hovering action. The hot attic gives the essential warm background to see the membranes and the way the bats hover at the entrance provides a unique opportunity to study the wing patterns. Notice at the end of the video the bats that have been flying for a while have semi-transparent wings and cold limbs. It is as though the wings suffuse with blood while they sleep for repair and maintenance but this is pumped out during flight – just one theory!
Many thanks to Mr Edward Jewell the owner for permission to film and the team of myself, Peter Thorne, Nik Knight, Steve Page, Mike Pawling and Chloe Mockridge
Jim Park discovered this roost in a tree in Wildgrounds Nature Reserve. Our team was me, Peter Thorne, Natalie Boote and Jim. It was easy to locate the roost as it was so hot. There were other unoccupied holes. At 9.20 a mature bat poked her head out and seemed to survey the scene and sample the air. She was vocal – was she communicating potential threats or imminent rain? Emergence started at 9.25 with a lot of social calling from within the hole. From 10.30 bats started returning and performing at strange stalling manoeuvre in front of the hole. Some seemed to miss and land on the tree (are these young?). A slug wandered around the hole the whole evening. There was a bees nest with a lone guard in the next tree but when we shone lights more came out.
This video is a compilation of bat footage produced for the Bat Conservation Trust Annual Conference 2020. It mainly uses the Leonardo Merlin camera and the Horizon camera. The Merlin camera uses an optic with an F5.6 lens and 400mm focal length to produce a 2 degree field of view at maximum zoom. The focal plane is a 1024×768 array of HgCdTe photodiodes achieving a sensitivity of 0.02 degrees centigrade when cooled to liquid nitrogen temperature. Horizon has a 1280×786 array and 50Hz frame rate for capturing fast action. This video highlights the way bats co-exist with people with roosts of greater horseshoe, soprano pipistrelles, serotine and brown long-eared bats.
Barn owl hovering
This video is from 2014 and shows a barn owl hovering in a strong howling wind at Tidgrove Manor at the invitation of owner Mr Rayleigh Place. This started the interest in infrared adaptations in nature. In the howling wind it seems impossible the owl was using hearing and in complete darkness vision should have been poor yet it is staring at the ground. The owl was filmed with high contrast because everything was washed out by the light drizzle. This showed very dark wings (low thermal emission). There is only three states of matter in the infrared: transparent, reflecting or emitting. Owls wings have the property of low emission which requires microscopic engineering. This gave a hint that their prey was sensitive to infrared – something that was later discovered in the fur of mice, shrews and rabbits.
The longest filming session with nightjars was 30 minutes due to personal safety. The camera hums due to the cooling engine and the birds think this is a large male churring. So they tend to swoop and after a few experiences of seeing a nightjar in your face thats enough! So this video is hard won. Note the hot moth fly past the perched nightjar.