Talk by Frank Box on Nestboxes

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Frank sells his nestboxes commercially – you can contact him at the Australian Nestbox Company, 81 Haig St, Gordon Park  4031 Ph:  3857.1086  email:

Sheryl has asked me to give a talk on using insectivorous bats to control the Fruit Spotting Bug and the Fruit Piercing Moth.  I am going to restrict my comments to my particular area of interest, which is animals that breed or roost in tree hollows.  Hollow-using birds may not have much to offer you in controlling caterpillars on trees or moths that are active at night.  Dollar birds are insectivorous but they are strictly hunters of the air.

There are two groups of hollow-users that would be an advantage.  The first group consists of the gliding possums.  Feathertail gliders, the smallest of the group, eat nectar, pollen, manna (an insect exudate), sap, small insects, foliage and blossoms.  They are communal animals that forage through foliage looking for food.  Imagine an army of these pouring over your trees every night devouring every insect that they find.  Feathertails are tiny and could probably fit through a hole the size of a 10 cent piece.  Roosts of up to 16 individuals have been found.  Larger gliders that also occur in south-east Queensland include the sugar glider and the squirrel glider.  Food is similar to the feathertail but the numbers of individuals living together is much smaller. The problem with gliders is that if you don’t already have them, there is not much that you can do about it.  But if you do have them they can be encouraged by providing roosting sites – gliders readily accept artificial hollows.

The second group of hollow-users offers more to a greater number of people and I am referring to the various species of insectivorous bat.  These are not flying foxes and they are definitely not fruit-eaters.  Nor are they anything like as big, with some being no larger than a 50 cent piece.  There are 23 species of microbats in our part of Australia and, because there are so few caves, most roost in tree hollows.  Some species fly high and fast, taking insects on the wing.  There are other species whose speciality is the middle or lower layers of a forest.  Some are adapted to rainforest where they flutter like butterflies.  Several pick insects off foliage while others even hunt on the ground.

Of the studies that have been done, moths can constitute a large part of their diet – up to 90% for some local species.  There are some reports of microbats being used in the USA for farm pest-control and I would like to read a part of an article in a Bat Conservation International newsletter about a farmer’s experience with bats controlling pests:  “The multitude of birds and the growing colony of bats at his farm have taken care of his original concern about overuse of pesticides in his orchards – he has gone from spraying 13 times a year to only twice and that being after the birds and bats have gone.  His hunch about bats helping with his corn ear-worm problem has paid off.  Where he used to have 1-4 of destructive moth larvae on an average ear of corn, he has had none for the last several years ever since his bat population began increasing.” 

So what can be done to encourage microbats?  There seem to be two types of bat roosts.  Males often roost singly and they can make do behind exfoliating bark and in a myriad of other places.  Females, on the other hand, form maternity colonies in spring which require largish hollows.  Most batboxes target these maternity colonies.

There are many different recommended designs for batboxes. There is one design from Wales with a V shape and a slot entrance at the base.  The Americans favour two very large sheets placed parallel with each other but separated by only a few centimetres.  In Europe, batboxes made from concrete are not uncommon.  We are presently using a British design made from ply which is chunky but quite small – about 12cm x 10cm x 9cm. Most boxes seem to use a slot in the base as entrance, typically 12mm to 30mm wide.  All batboxes need a rough surface inside for the bats to cling to. Some use parallel saw cuts as grooves.  Others tack shade cloth onto the interior walls.  I now scour the ply with a laminate cutter.

Another thing that most people agree on is that there should be no clutter around the box.  My theory is that foliage breaks up their echo-location and makes it harder for them to locate an entrance.  There are less guidelines about the temperature aspects of siting a box.  Where bats migrate, they typically go cold in winter and return to warmer areas in summer.  There is logic to this in that food is frequently scarce in winter and many bats need to go into torpor for their food reserves to last the distance.  Only when it is really cold can they do this.  In addition, there are many reports of bats in roofs and under metal caps on the top of electricity poles – very hot places which they seem not to mind at all.  How this influences the siting of batboxes in sub-tropical areas is still unclear.

That batboxes can make a difference is indicated by Australia’s only longitudinal study of batboxes.  The first boxes were installed in Organ Pipes National Park, outside Melbourne, about 10 years ago.  The park was declared because of a geological feature and contained no trees 30 years ago when the first plantings began, so there are no natural hollows.  It took 3 years for the first boxes to be occupied.  There are now 35 batboxes installed and a resident population of over 100 bats and about 6 different species. 

Unfortunately that is about the only real data available in Australia on batboxes.  A lot of individuals have tried batboxes with variable success but there seems to have been little attempt to distill some firm conclusions from their experiences.  My belief at present, based on what I have heard and seen, is that batbox shape may not be that important.  The width of the entrance slit probably is but perhaps not critical.  The main determinant of success could be the number of boxes involved.  At Organ Pipes the maternity colonies relocate between boxes nightly, and the most popular box is in the centre of a cluster.  This suggests that bats may need several roosts sites in a cluster and could explain why so many single batboxes aren’t successful.

As with any good idea, there are going to be problems.  For gliders, we now hide the entrance to the box at the back of the box to discourage competition from birds.  While this has been totally successful, it has made the box irresistible to the European honeybee – a feral animal.  We are trialling placing a piece of carpet on the inside of the lid but it is too early to say if this will work.  For batboxes, the main problem has been ants, the sort that weaves nests in leaves.  These are native ants but it makes for rather an expensive antbox.  Broadleaf trees are the worst, with least infestations where the boxes are mounted on smooth-bark eucalypts.  At Organ Pipes, ant infestations are treated with a liberal sprinkling of Johnsons Baby Powder.  Another idea that I have not yet tried is to place lengths of black pvc tubing in the tree to see if the ants prefer that instead. 

Article compiled by Sheryl Backhouse

 Some extra facts about Microbats by Jenny Awbery

  • Microbats (microchiropterans) are small bats found all over the world. 
  • The majority of bats are microbats. 
  • Most microbats eat insects – for example, up to 600 mosquitoes in one night. 
  • Microbats have very poor eyesight and use highly sophisticated echolocation, or radar, to find their food. That is why their ears are very large and their eyes are small. 
  • Microbats need a warm home – the temperature needs to be between 80 and 90 degrees F. That is why many bats live together in caves. 
  • Microbats also can live in trees, buildings and houses if the temperature is between 80 – 90 degrees F.