Southern Bent-wing Bat - National Recovery Team

The Southern Bent-wing Bat (Miniopterus orianae bassanii) is considered Critically Endangered and only occurs in south-western Victoria extending into the south-east corner of South Australia.

Once numbering in the hundreds of thousands across southern Australia, the current population is estimated to comprise fewer than 60,000 individuals.

In 2019 a National Recovery Team was formed to prevent this species from becoming extinct.

 

 

Overview of the Southern Bent-wing Bat

The Southern Bent-wing Bat is one of three closely related but genetically and geographically distinct bats (Cardinal & Christidis 2000).

Southern Bent-wing Bat Miniopterus orianae bassanii  - (southern sub-species), distributed in south-western Victoria extending into the south-east corner of South Australia.

Eastern Bent-wing Bat Miniopterus orianae oceanensis - (eastern subspecies), also known as the or Large Bent-wing Bat occurring near coastal eastern Australia from the northern tip of Queensland down the east coast including New South Wales and extending to central Victoria.  

Northern Bent-wing Bat Miniopterus orianae orianae - (northern subspecies), found in the northern part of Western Australia and Northern Territory.

Source: Australian Faunal database (AFD 2019)

Identification

The Southern Bent-wing Bat has dark brown to redish brown fur above, grey-brown fur underneath and pale brown areas of bare skin. Head and body length 52mm-58mm, forearm 45-49mm. It has a short muzzle, high crowned head, broad rounded ears and small eyes. It has a long wing length being nearly two and half times longer than the head and body. A diagnostic feature of the wing structure is the terminal phalanx (segment) of the longest finger, which is more than three times as long as the previous phalanx (Dwyer 1983, Hall & Woodside 1989, Menkhorst. & Lumsden 1995).

 
Southern Bent-wing Bat Image Steve Bourne
Southern Bent-wing Bat. Wing length being nearly two and half times longer than the head and body. A diagnostic feature of the wing structure is the terminal phalanx (segment) of the longest finger, which is more than three times as long as the previous phalanx.

 

Identification caution

Unlike most other species of insectivorous bats the Southern Bent-wing Bat roosts exclusively in caves or cave like structures rather than trees. In particular, the Southern Bent-wing Bat is reliant on only two main caves for rearing young and a handful of caves for roosting.  Identification is best carried out by non-handling techniques and certainly not by entering known roost sites. A preferred method of identification is by recording and analysing call sequences as each bat species has its own frequency range.

 

Distribution of Southern Bent-wing Bat

Southern Bent-wing Bat Source Atlas of Living Australia
The  Southern Bent-wing Bat is only occurs in south-west Victoria and the south-east corner of South Australia. Source: ALA 2019.

Ecology & Habitat

The Southern Bent-wing Bat can live up to 20 years. Habitat preference is associated with the availability of foraging areas and proximity to suitable roosting caves, for example many of the roosting caves are located near coastal cliffs and rock caves.

Movement

Foraging areas include forested areas, agricultural land, Volcanic Plains,  wetlands and coastal vegetation including beaches. Southern Bent-wing Bats can forage over large distances, with current research showing individuals can fly at least 72 kilometres in just a few hours.

Breeding

Breeding takes place in late May and early June, however the onset of gestation is delayed through winter until August. By late November to December females congregate the maternity caves where a single young is born to each female. A strong bond is developed between mother and young, which persists beyond the first flight to when the young leave the cave 'en mass' (Hall & Woodside 1989).  By February to March the young are free flying. The adult females return to the same roosting caves each year.

Caves
Southern Bent-wing Bats clustered on roof of a roosting cave. Image: Gabrielle Lanman
Southern Bent-wing Bats clustered on roof of
a roosting cave. Image: Gabrielle Lanman

Research in Eastern Bent-wing Bats show that different caves are used depending on the season, reproductive stage and sex of the bat. For instance, adult males, pregnant females and juveniles tend to group at separate caves during particular times of the year. Females have specific requirements for giving birth and rearing young, which includes a dome type cave, which retains warmth and humidity for development of the young. These caves are referred to as maternity caves. In Southern Bent-wing Bats, both adult males and females are found at maternity caves.

Maternity caves

There are only two main maternity caves for this species; the Starlight cave near Warrnambool, Victoria and the Naracoorte Bat Cave in south-eastern South Australia. There is a third, smaller maternity site near Cape Bridgewater, Victoria.

Feeding

The Southern Bent-wing Bat is nocturnal and uses echolocation (emission of ultrasonic sounds) for navigation and feeding. Recording and analysing call sequences provides a means of identifying a particular species of bat as each species has its own frequency range. 

Feeding is carried out during flight and prey is caught in the tail or wing membrane and transferred to the mouth whilst in flight, sometimes insects are caught in the mouth (Hall & Woodside 1989).  Diet consists of a range of night flying insects eg. Order Diptera (mosquitos, midges), Lepidoptera (moths), Orthoptera (crickets), Coleoptera (beetles), Hemiptera (bugs), Hymenoptera (ants). Microchiroptera play a major role as the nocturnal predator of night flying insects, many of which are a nuisance to humans. It is estimated the Common Bent-wing Bat can consume about 25% of its 15 gram body weight per meal and may eat two or three meals per night, therefore a population of 1000 bats could consume several kilograms of insects per night.

Roosting

On return to their roosting caves the bats cluster at preferred microclimates within the cave. During this time they carry out grooming and rest by lowering their body temperature to the ambient temperature and go into a torpor state for a period of hours. Over the winter months when insect activity is low and food is not readily available they go into a deep hibernation and select the coolest caves and coolest part of the cave. During this time their body temperature can be down to 20C, which limits the loss of body fats through winter (Speakman & Thomas 2003). They are particularly prone to disturbance when at rest as valuable reserves of body fat are consumed if disturbed.

 

Threats

There has been a severe decline in the Southern Bent-wing Bat. The total population size was estimated to be approximately 200,000 several decades ago, now it is down to about 60,000 across its entire range. The winter count in 2010 only found about 25,000 although this low number may be attributed to the fact that further winter roosting sites are yet to be located.

Human disturbance

Probably the most severe impact on a population is caused through human disturbance at roost sites because it can result in abandonment of a cave. During a state of torpor and particularly over winter when the bats are in a deep hibernation human disturbance in the form of noise, lights and handling can cause a rise in body temperature, which uses up body reserves. If there is repeated exposure this can result in loss of body reserves beyond recovery and the bat is likely to die.

Disturbance at a maternity cave is considered a major threat because it can impact on the breeding success of the population.

Disturbance during summer has been identified as a problem in some areas where tourists and holidaymakers enter caves, which is likely to result in bats fleeing the cave in daylight and using valuable energy reserves and making them more prone to predators. Repeated disturbance can result in abandonment of the cave. Disturbance has been identified as an issue in some areas along the coastline where otherwise perfectly good caves have been rendered unsuitable, sometimes forcing bats to use much smaller caves and crevices that are marginal habitat, which can only support low numbers (Lumsden 1998).

The use of different caves is an integral part of the ecology of this species, therefore it should not be assumed that disturbance which leads to abandonment of a cave will result in the bats simply moving to another cave. It will most likely cause mortalities and have wider implications for the populations roosting, social organisation and mating behaviour.

Insecticides

The impact of insecticides on insect numbers and therefore food for the Southern Bent-wing Bat is unknown, however ingestion of insects treated with insecticides may cause secondary poisoning.  Direct ingestion of insecticides can occur particularly when grooming fur or wings that have been exposed to spray. 

Impacts on foraging habitats

Habitat change mainly through loss of trees and loss of connectivity between landscape elements may reduce range, as generally bats are reluctant to cross open landscapes (Racey & Entwistle 2003). Loss of wetlands through drainage can impact on feeding areas.

Drought

Low rainfall impacts on wetland condition and food availability for insectivorous bats. Low rainfall can also reduce the movement of water through caves, reducing the humidity in caves which has been observed to cause mortality of bats.

Windfarm developments

The positioning of wind turbines near known or potential feeding areas and in bat flight lines has been raised an issue and is relevant to several proposed windfarms in the Southern Bent-wing Bat’s range. Wind turbines located near roosting caves (particularly near a maternity cave) raises the likelihood of contact between rotating blades and bats, particularly as there is a concentration of bats from around the region and there is frequent flying to and from the maternity cave when raising young. A full assessment of potential impacts on bats should be part of any development proposal, which includes assessment of flight paths between the maternity cave and roost sites, between maternity cave and feeding sites and between roost sites and feeding areas.

Introduced predators

In Victoria, predators include the introduced Black Rat, Feral Cat, and Red Fox.

Cave Management

There is a need to protect the microclimate inside maternity caves which under natural conditions maintains a constant temperature and level of humidity. Caves which are modified by enlarging or closing off vents can alter the microclimate. In some cases caves need to be protected from erosion and inappropriate use of heavy machinery which may cause a cave to collapse.

 

Recovery Team

Partners

 

Recovery Team objectives

A key part of Southern Bent-wing Bat recovery is to understand more about the population dynamics e.g. population numbers, breeding success and survival rates of the various age classes and demographic groups (i.e. males vs. females, juveniles vs. adults).

 

Current activities

 

 

Previous measures and findings at the Southern Bent-wing Bat maternity cave
2010

In 2010 the cave was fenced off to exclude heavy machinery causing cave collapse. Stock proof fence around cave entrances.

Results from Winter monitoring in 2010 found that during winter the bats migrate to a large number of caves.

In 2010, approximately 23,500 Southern Bent-wing Bats were observed in Victoria and South Australia. This is considerably less than the estimated 40,000 known from summer counts, suggesting further roosting sites are yet to be located.

2012

In 2012 two summer surveys were completed at the maternity cave using thermal imaging camera to count bats exiting the cave each night. These surveys focused on gaining estimates of the female population and also to determine the survival rate of pups to juveniles.

It is estimated that 10-15,000 adult Southern Bent-wing Bats used the cave.  Autumn migration monitoring was undertaken at the maternity cave using Anabat recordings to determine the timing of the migration to roosting caves.

A 3D image of the maternity cave roof was created to determine the approximate surface area of the roof. This information is necessary to determine estimates of adult bats and pups by counting bats over a small known area and extrapolating out to the surface area.

Monitoring of overwintering caves was carried out in Victoria and South Australia in July 2012 with 33 known Victorian habitat caves. Approximately 4200 bats were counted but more information is required on the location of winter roosting areas. Further searches for caves that support Southern Bent-wing Bats is needed. It is important to note the survey teams use passive recording techniques so as to minimise disturbance to overwintering bats.

The South Australian count only managed to locate approximately a third (15,478) of their estimated population during winter.

2013

A Management Plan for the maternity cave near Warrnambool was completed in 2013 with approved actions implemented to protect the only known Victorian breeding cave and maternity population.

The summer monitoring program consisted of two maternity cave monitoring events aimed at determining the female Southern Bent-wing bat population estimates as well as the rate of juvenile survival and species recruitment. A thermal imaging camera was used to film the evening exit of bats. 

Southern Bent-wing Bats leaving a cave are counted using a thermal imaging camera and sophisticated  automated counting system. Image: Kristen Lear
Southern Bent-wing Bats leaving a cave are counted using a thermal imaging camera and sophisticated  automated counting system. Image: Kristen Lear

Bat call recorders have been installed in the maternity cave and a significant non-breeding roost cave since March 2013 to better determine bat movements and cave usage. A further five non-breeding caves are being monitored monthly with count estimates and limited bat call recordings (since March / April).

2014

Two summer counts were undertaken. Surves to discover where the majority of the Southern bent-wing Bat population roosts over the winter months carried out.

Undertake research to determine if wind farms pose a potential threat to the Southern Bent-wing Bat.

 

References

 

Other resources

 

 

 

 

 

 

 

 

 

 

 

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