Trelissick A-Z: Bats
The A-Z of the Trelissick countryside continues with insight into bats - and what better time of year to do so. However, there's much more to these nocturnal creatures than their vampiric connotations.
What are bats?
Bats are mammals like us and many of the animals we have come to think of as pets. As such, they have warm blood, fur and a baby bat (known as a pup) feeds on its mother’s milk for at least a few weeks after being born. Bats are the only mammals that can truly fly – we’re not talking about that graceful gliding/slowly falling thing that ‘flying’ foxes do here but real, high-quality flying. If you were to examine a bat’s wing for some reason, you would find that it has remarkably similar bones to that of a human arm but with tremendously elongated finger bones and fine skin stretched between to form the wing membrane.
There are 18 species of bat in the UK, some of which have unfortunately become very rare. The smallest of these species is the pipistrelle which can weigh as little as 4grams and has a wingspan of 18-25cm. Our largest bat is the tree-nesting noctule bat which can weigh as much as 40grams and boasts a wingspan of 33-45cm. Although not the case if we were to discuss global populations, all British bats feed on insects such as moths, beetles and midges. Individual species hunt in their own characteristic way, often catching and eating prey on the wing or ‘hanging up’ to devour larger beasties like spiders. Flying about also uses up an incredible amount of energy and so all bats have large appetites – a tiny pipistrelle can eat over 3000 insects in one night!
Bats don’t build nests, but instead choose suitable locations – depending on the time of year – to roost. Some prefer hollowed out old trees whilst other will plump for a nice damp, dark cave. Many species’ favour sheltering in old buildings, where they hang out behind boarding or in roof spaces. Bats are also clean and sociable animals that spend many hours grooming themselves – a fine quality for communal living.
As many of you will be aware, our UK bat populations have declined dramatically in the past 100 years. Many rural areas that used to serve these night-flying mammals as feeding grounds and roosting sites have been cleared to make way for urban expansion and roads. Changes in farming practice and the widespread use of pesticides have also greatly diminished much of a bat’s insect prey and even directly killed some of the bats themselves. Within the UK, bats are one of (if not, the most) legally protected animals. Roosts are also protected by law which means that it is actually illegal to damage, disturb or destroy any roosting sites whether the bats are present or not.
Summer branch drop
Down on the woodland walks we had a few trees come down during early summer. Some may find this surprising because of the fine weather we enjoyed throughout this period, but it is actually a fairly common phenomenon called ‘summer branch drop’. Many tree experts (or dendrologists, as they are technically known) believe this behavior is caused by water stress in over-extended limbs during very hot weather. Throughout any kind of heat-wave, a tree will have to work much harder to move water from its roots up to its longest branches and this stress can cause branches to be suddenly dropped. Certain trees seem to be more susceptible than others, with beeches, oaks and horse chestnuts being prime culprits.
Part of the North Woodland Walk was closed for a week during June because a beech tree shed a large limb in this manner. The ranger team made the decision to close this area so we would have enough time to properly survey the tree for bat roosts before employing a tree surgeon to reduce the crown and make the tree safe for our visitors to walk beneath. Under normal circumstances, we would not instigate any tree work during the spring/summer owing to the bird nesting season but safety is paramount and situations such as this one demand to be dealt with quickly. As an additional concern, it is critical that bats are not disturbed during the summer months because any roost that we found would likely be a maternity roost. One such roost could house all the bats of a certain species from the surrounding area, having converged on a suitable location to rear their young and destruction of this habitat would devastate the bat population of Trelissick and beyond.
Bats and trees
Bats are surprisingly long-lived creatures, often reaching 20-30 years old but breed very slowly with only one pup per brood. To make sure our tree work had the minimum impact on our nocturnal friends, we employed a specialist bat ecologist and a tree surgeon to take an endoscope up into the canopy. This special type of camera could be inserted into cracks and crevices to make sure there were no bats inside before any cuts were made. After confirmation was given that no roosts were present, we ‘plugged’ the entrance holes to any potential roosts so that bats wouldn’t colonise them between the survey and the tree work being carried out. This beech tree has now been significantly reduced in stature and this part of the woodland walk is once again safe to explore.
Trees fulfill many of a bat’s needs and nowhere is this more relevant than Trelissick where – of course – we have a fair bit of woodland but also the only record of the tree-nesting barbastelle bat on the River Fal.
Thanks for reading and keep a look out for the next instalment of the A-Z, a series we have started to increase our visitors awareness of the wildlife and conservation work on the Trelissick Estate.
- The National Trust Ranger team, Trelissick and North Helford
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